Principles of Intermediary Metabolism PDF

Chapter 2 Principles of Intermediary Metabolism Steven E. Raper Key Points 1 Intermediary metabolic pathways – the metabolic manipulation and balancing of ingested carbohydrate, fat, and protein – can process essentially all nutrients to acetyl coenzyme A (CoA) for energy production predominantly...

Chapter 2 Principles of Intermediary Metabolism Steven E. Raper Key Points 1 Intermediary metabolic pathways – the metabolic manipulation and balancing of ingested carbohydrate, fat, and protein – can process essentially all nutrients to acetyl coenzyme A (CoA) for energy production predominantly through aerobic glycolysis, the citric acid cycle, and oxidative phosphorylation. 2 Glucose must always be available for brain function; if not available directly from the diet, it can be mobilized for a brief period from glycogen stores and then derived from proteins in the liver and kidneys. 3 Free fatty acids are a direct source of energy for cardiac and skeletal muscles. 4 Hepatic protein synthesis, when excess amino acids are available, includes albumin, fibrinogen, and apolipoproteins and can reach 50 g/day. 5 The citric acid cycle includes a series of mitochondrial enzymes that transform acetyl CoA – itself derived from pyruvate or fatty acyl CoA – into water, carbon dioxide, and hydrogen-reducing equivalents. Each molecule of acetyl CoA that enters the citric acid cycle yields 12 molecules of adenosine triphosphate (ATP). 6 Oxidative phosphorylation converts the energy from NADH and FADH2 into ATP by the electron transport chain and ATP synthase with a process called the proton motive force. 7 Biotransformation of potentially toxic, often hydrophobic, compounds into hydrophilic, excretable compounds occurs mainly in the liver by the cytochromes P-450, the uridine diphosphate-glucuronyl (UDPglucuronyl) transferases, the glutathione (GSH) S-transferases, and the sulfotransferases. INTERMEDIARY METABOLISM: AN OVERVIEW Introduction Intermediary metabolism – derived from the Greek word for change – is predominantly the fate of dietary carbohydrate, fat, and protein in a series of life-sustaining cellular chemical transformations. Although admittedly intricate, all surgeons should be familiar with the basics of the biochemistry by which nutrients are converted to energy. Understanding the major biochemical pathways is a prerequisite to making use of the rapid – and exciting – expansion of medical knowledge directed at managing human metabolic derangement and improving health by beginning at the cellular level. 1 Intermediary metabolic pathways – the metabolic manipulation and balancing of ingested carbohydrate, fat, and protein – can process essentially all nutrients to acetyl coenzyme A (CoA) for energy production predominantly through aerobic glycolysis, the citric acid cycle, and oxidative phosphorylation. The major intermediary metabolites are glucose, fatty acids, glycerol, and amino acids. Glucose is metabolized to pyruvate and lactate by glycolysis. Aerobic metabolism allows conversion of pyruvate to acetyl CoA. Acetyl CoA enters the citric acid cycle resulting in carbon dioxide, water, and hydrogen-reducing equivalents (a major source of adenosine triphosphate [ATP]). In the absence of oxygen, glycolysis ends in lactate. Glucose can be stored as or created from glycogen. Glucose can also enter the phosphogluconate pathway, where it is converted to reducing equivalents for fatty acid synthesis and ribose fivecarbon sugars important in nucleotide formation. Glucose can be converted into glycerol for fat formation and pyruvate for amino acid synthesis. Gluconeogenesis allows synthesis of glucose from lactate, amino acids, and glycerol. With regard to lipid metabolism, long-chain fatty acids arise from dietary fat or synthesized from acetyl CoA. Fatty acids can be oxidized to acetyl CoA by the process of beta-oxidation or converted to acylglycerols (fat) for storage as the main energy reserve. In addition to the fats noted previously, acetyl CoA can be used as a precursor to cholesterol and other steroids and in the liver can form the ketone bodies, acetoacetate and 3hydroxybutyrate, which are critical sources of energy during periods of starvation. Proteins are degraded in two major ways: an energy independent path, usually in lysosomes, and an energy requiring path, usually through the ubiquitin pathway. About three-fourths of the amino acids generated in protein catabolism are reutilized for protein synthesis and one-fourth is deaminated to form ammonia and subsequently urea. Amino acids may be divided into nutritionally essential and nonessential. Nonessential amino acids require fewer enzymatic reactions from amphibolic intermediates or essential amino acids. Each day, humans turn over 1% to 2% of total body protein. CARBOHYDRATE METABOLISM The products of intestinal carbohydrate digestion are glucose (80%) and fructose and galactose (20%). Fructose and galactose are rapidly converted to glucose, and the body uses glucose as the primary molecule for transport and uptake of carbohydrates by cells throughout the body. Despite wide uctuations in dietary intake, blood glucose levels are tightly regulated by the liver. About 90% of portal venous glucose is removed from the blood by liver cells through carrier-facilitated di usion. Large numbers of carrier molecules on the sinusoidal surface of the hepatocyte are capable of binding glucose and transferring it to the cytoplasm. The rate of glucose transport is enhanced (up to 10-fold) by insulin. Given the critical role of glucose in survival, complex metabolic pathways have evolved for the storage of glucose in the fed state, the release of glucose from glycogen, and the synthesis of new glucose. Blood glucose is stored, primarily in liver and muscle, as glycogen. Glycogen is a complex polymer of glucose with an average molecular weight of 5 million. The liver can convert up to 100 g of glucose into glycogen per day by glycogenesis. The liver can also release glucose into the blood by glycogenolysis, (breakdown of glycogen), or gluconeogenesis, (formation of new glucose from substrates such as alanine, lactate, or glycerol). Hormones play a key role in the hepatic regulation of glycogen balance. Insulin, for example, stimulates glycogenesis and glycolysis; glucagon stimulates glycogenolysis and gluconeogenesis through cyclic adenosine monophosphate (AMP) and protein kinase A.1 Glycogenesis and Glycogenolysis 2 Glucose must always be available for brain function; if not available directly from the diet, it can be mobilized for a brief period from glycogen stores and then derived from proteins in the liver and kidneys. The rst step in glycogen storage is the transport of glucose through the plasma membrane. Once in the hepatocyte, glucose and ATP are converted by the enzyme glucokinase to glucose-6-phosphate (G6P), the rst intermediate in the synthesis of glycogen (Fig. 2-1). Because complete oxidation of one molecule of G6P generates 37 molecules of ATP, and storage uses only one molecule of ATP, the overall e ciency of glucose storage as glycogen is a remarkable 97%. Glycogenolysis does not occur by simple reversal of glycogenesis. Each glucose molecule on a glycogen chain is released by glycogen phosphorylase (Fig. 2-2). Eventually, G6P is reformed. G6P cannot exit from cells and must rst be converted back to glucose. The conversion of G6P to glucose is catalyzed by glucose-6-phosphatase, which exists only in hepatocytes, kidney, and intestinal epithelial cells. Brain and muscle both use glucose as a primary fuel source and do not contain the phosphatase enzyme. This lack of glucose-6-phosphatase ensures a ready supply of glucose for the energy needs of brain and muscle. Liver uses glucose primarily as a precursor for other molecules and not for fuel. Glycolysis Glycolysis is the mammalian cellular pathway by which glucose is converted to pyruvate or lactate (Fig. 2-3). The glycolytic pathway is interesting in that glucose can be metabolized in the presence (aerobic) or absence (anaerobic) of oxygen. Aerobic glycolysis is one of four stages in the oxidation of glucose and the only stage that occurs in the cytosol. As will be discussed below, stages II to IV occur in mitochondria; the citric acid cycle, electron transport generation of the proton motive force, and ATP synthase leading to generation of ATP. Figure 2-1. The chemical reactions of glycogenesis and glycogenolysis. Glucose-6-phosphatase allows hepatic glucose to be transported out of the hepatocyte for use in other tissues. Glucose-6-phosphate, in red, plays a central role in carbohydrate metabolism. Figure 2-2. Glucagon-stimulated enzyme cascade, responsible for the control of glycogen metabolism. Inactive forms are shown in black, active forms in blue. The aerobic conversion of glucose to pyruvate has three e ects: (a) a net gain of two ATP molecules, (b) generation of two reducing equivalents of the nicotinamide adenine nucleotide (NADH + H+), and usually, (c) conversion of pyruvate to acetyl CoA with subsequent conversion of acetyl CoA in the mitochondria to ATP. The conversion of glucose to pyruvate is regulated by three enzymes: hexokinase (glucokinase), phosphofructokinase, and pyruvate kinase, which are nonequilibrium reactions and as such, functionally irreversible. Under anaerobic conditions, NADH cannot be reoxidized by transfer of reducing equivalents through the electron transport chain to oxygen. Instead, pyruvate is reduced by NADH to lactate. Glycolysis takes place in the cytoplasm, in contrast to the citric acid cycle and oxidative phosphorylation which are mitochondrial processes. During times of glucose excess, as in the fed state, hepatic glycolysis can generate energy in the form of ATP, but the oxidation of ketoacids is a preferred energy source in liver. The conversion of lactate (through pyruvate) to glucose – a process possible only in the presence of oxygen – is an important means of preventing severe lactic acidosis. Active skeletal muscles and erythrocytes form large quantities of lactate. In patients with large wounds, lactate also accumulates. The liver is exceptionally e cient at converting lactate to pyruvate through the Cori cycle (Fig. 2-4). As a result, one would expect that only signi cant liver dysfunction would a ect the Cori cycle and lead to hyperlactatemia. However, lactate levels are now widely used to assess shock – septic and otherwise.2 The hypothesis is that circulatory hypoperfusion impairs tissue oxygen delivery with resultant mitochondrial hypoxia. In the absence of adequate oxygen, mitochondria switch to anaerobic glycolysis and oxidative phosphorylation stops. As a result, serum lactate concentrations appear proportional to ongoing tissue oxygenation de cits; thus improved lactate clearance can be used as a surrogate for success of sepsis therapy.3 Serum lactate can also be used to assess prognosis and triage patients to ICU level care.4 Figure 2-3. The glycolytic pathway. There is a net gain of two ATP molecules per glucose molecule. Phosphofructokinase is the key regulatory enzyme in this pathway; however, all the enzymes in red catalyze irreversible reactions. The pathway shown here is active only in the presence of aerobic conditions. In erythrocytes, a unique variant of glycolysis enhances oxyhemoglobin dissociation. The rst site in glycolysis for generation of ATP is bypassed, leading to the formation of 2,3-bisphosphoglycerate by an additional enzyme called bisphosphoglycerate mutase. Kinetics of the mutase present in erythrocytes allow the presence of high concentrations of 2,3bisphosphoglycerate to build up. The 2,3-bisphosphoglycerate displaces oxygen from hemoglobin, allowing a shift of the oxyhemoglobin dissociation curve to the right. Gluconeogenesis There is an absolute minimum requirement for glucose in humans. Below a certain blood glucose concentration, brain dysfunction causes coma and death. When glucose becomes scarce, as in the fasting state, glycogenolysis occurs. Once glycogen stores have been depleted, the liver and kidneys are capable of synthesizing new glucose by the process of gluconeogenesis. Glucagon is produced in response to low blood sugar levels and stimulates gluconeogenesis. Figure 2-4. The gluconeogenesis pathway. The irreversible nature of the glycolytic pathway means that a di erent sequence of biosyntheses is required for glucose production. The enzymes in red catalyze irreversible reactions that are different from those in glycolysis. In mammals, glucose cannot be synthesized from acetyl coenzyme A, only from cytosolic pyruvate. Gluconeogenesis is not a simple reversal of the glycolytic pathway. In glycolysis, as noted previously, the conversion of glucose to pyruvate is a one-way reaction. As a result, four separate, functionally irreversible enzyme reactions are required to convert pyruvate into glucose (Fig. 2-5). These enzymes are pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase. Other enzymes are shared with the glycolytic pathway. About 60% of the naturally occurring amino acids, glycerol, or lactate can also be used as substrates for glucose production. Alanine is the amino acid most easily converted into glucose. Simple deamination allows conversion to pyruvate, which is subsequently converted to glucose. Other amino acids can be converted into three-, four-, or ve-carbon sugars and then enter the phosphogluconate pathway (next section). Gluconeogenesis is enhanced by fasting, critical illness, and periods of anaerobic metabolism. Figure 2-5. The Cori cycle, an elegant mechanism for the hepatic conversion of muscle lactate into new glucose. Pyruvate plays a key role in this process. Phosphogluconate Pathway When glucose enters the liver, glycogen is formed until the hepatic glycogen capacity is reached (about 100 g). If excess glucose is still available, the liver converts it to fat by the phosphogluconate pathway (also known as the pentose phosphate pathway) (Fig. 2-6). The cytosolic phosphogluconate pathway can completely oxidize glucose, generating CO2 and nicotinamide adenine dinucleotide phosphate (NADPH) through what is known as the oxidative phase. Hydrogen atoms released in the phosphogluconate pathway combine with oxidized nicotinamide adenine dinucleotide phosphate (NADP+) to form reduced nicotinamide adenine dinucleotide phosphate (NADPH − H+).5 The oxidative phase is present only in tissues, such as the adrenal glands and gonads, that require reductive biosyntheses such as steroidogenesis or other forms of lipid synthesis. Essentially, all tissues contain the nonoxidative phase, which is reversible and produces ribose precursors for nucleotide synthesis. In erythrocytes, the phosphogluconate pathway provides reducing equivalents for the production of reduced glutathione by glutathione reductase. Reduced glutathione can remove hydrogen peroxide, which increases the conversion of oxyhemoglobin to methemoglobin and subsequent hemolysis. LIPID METABOLISM Lipid Transport Lipid transport throughout the body is made complicated by the fact that lipids are insoluble in water. To overcome this physicochemical incompatibility, dietary triglycerides are rst split into monoglycerides and fatty acids by the action of intestinal lipases. After absorption into small intestinal cells, triacylglycerols are reformed and aggregate into chylomicrons, which then enter the bloodstream by way of lymph. Chylomicrons are removed from the blood by the liver and adipose tissue. The capillary surface of the liver contains large amounts of lipoprotein lipase, which hydrolyzes triglycerides into fatty acids and glycerol. The fatty acids freely di use into hepatocytes for further metabolism. Similar to chylomicrons, very low-density lipoproteins (VLDLs) are synthesized by the liver and are the main vehicle for transport of triacylglycerols to extrahepatic tissues. The intestines and liver are the only two tissues capable of secreting lipid particles. In addition to chylomicrons and VLDLs, there are two other major groups of plasma lipoproteins: low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs). LDLs and HDLs contain predominantly cholesterol and phospholipid. Figure 2-6. The phosphogluconate pathway. One of the major purposes of this pathway is to generate reduced nicotinamide adenine dinucleotide, which can serve as an electron donor and allow the liver to perform reductive biosynthesis. Glucose-6-phosphate, in red, plays a central role in carbohydrate metabolism. The structure of all classes of lipoproteins is similar. There is a core of nonpolar lipids, either triacylglycerols or cholesteryl esters, depending on the particular lipoprotein. This nonpolar core is coated with a surface layer of amphipathic phospholipid or cholesterol oriented so that the polar ends are in contact with the plasma. A protein component is also present. The A apolipoproteins occur in chylomicrons and HDLs. The B apolipoproteins come in two forms: B-100 is the predominant apolipoprotein of LDLs, whereas the shorter B-48 is located in chylomicrons. The C apolipoproteins can transfer between VLDLs, LDLs, and HDLs. Apolipoproteins D and E also exist. Apolipoproteins have several functions in lipid transport and storage. Some, such as the B apolipoproteins, are an integral part of the lipoprotein structure. Other apolipoproteins are enzyme cofactors, such as C-II for lipoprotein lipase. Lastly, the apolipoproteins act as ligands for cell surface receptors. As an example, both B-100 and E serve as ligands for the LDL receptor.6 Plasma variations in LDL cholesterol, HDL cholesterol, and triglycerides a ect risk for atherosclerotic cardiovascular disease. As dyslipidemias are being identi ed and studied, new therapeutic approaches are needed. A convergence of human genetics and functional biology has led to recent advances in the study of lipoprotein metabolism. Genome-wide association studies have identi ed about 100 genes associated with plasma lipid phenotypes – many of which were not previously known to be associated with lipids. These genes are now being functionally validated through human genetic analysis such as deep targeted resequencing of kindreds with Mendelian lipid abnormalities or gene manipulation (over- or underexpression) in cultured cells and animal models.7 FATTY ACID METABOLISM Most human fatty acids in plasma are long-chain acids (C-16 to C-20). Because long-chain fatty acids are not readily absorbed by the intestinal mucosa, they must rst be incorporated into chylomicrons. In contrast, short-chain and medium-chain fatty acids are absorbed directly into the portal circulation and are avidly taken up by the liver. Free fatty acids in the circulation are noncovalently bound to albumin and are transferred to the hepatocyte cytosol by way of fatty acid–binding proteins. Fatty acidCoA esters are synthesized in the cytosol after hepatic uptake of fatty acids. These fatty acid-CoA esters can be converted into triglyceride, transported into mitochondria for the production of acetyl CoA and reducing equivalents, or stored in the liver as triglycerides. The rate-limiting step in the synthesis of triglyceride is the conversion of acetyl CoA to malonyl CoA. Malonyl CoA, in turn, inhibits the mitochondrial uptake of fatty acidCoA ester, favoring triglyceride synthesis. The liver also contains dehydrogenases that can unsaturate essential dietary fatty acids. Structural elements of all tissues contain signi cant amounts of unsaturated fats, and the liver is responsible for the production of these unsaturated fatty acids. As another example, dietary linoleic acid is elongated and dehydrogenated to the prostaglandin precursor arachidonic acid. Figure 2-7. Diagram of hepatic fatty acid metabolism. Both dietary and newly synthesized fatty acids are esteri ed and subsequently degraded in the mitochondria for energy, rst as reducing equivalents, then adenosine triphosphate via the electron transport chain. Acetyl CoA, in red, plays a central role in lipid metabolism. 3 Free fatty acids are a direct source of energy for cardiac and skeletal muscles and under basal conditions, most free fatty acids are catabolized for energy. Under conditions of adipocyte lipolysis, the liver can take up and metabolize fatty acids. Although fatty acid synthesis occurs in the cytosol, fatty acid oxidation occurs in the mitochondria. Fatty acid-CoA esters bind carnitine, a carrier molecule, and in the absence of cytosolic malonyl CoA, they enter the mitochondria, where they undergo betaoxidation to acetyl CoA and reducing equivalents (Fig. 2-7). Acetyl CoA can then take one of the following routes: (a) enter the tricarboxylic acid cycle and be degraded to carbon dioxide, (b) be converted to citrate for fatty acid synthesis, or (c) be converted into 3-hydroxy-3-methylglutaryl CoA (HMG-CoA), a precursor of cholesterol and ketone bodies. The mitochondrial hydrolysis of fatty acids is a source of large quantities of ATP. The conversion of stearic acid to carbon dioxide and water, for instance, generates 136 molecules of ATP and demonstrates the highly e cient storage of energy as fat. By a process called beta-oxidation, acetylCoA molecules are cleaved from fatty acids. The acetyl CoA is then metabolized through the citric acid cycle under normal circumstances. In times of signi cant lipolysis – starvation, uncontrolled diabetes, or other conditions of triglyceride mobilization from adipocyte stores – the predominant ketone bodies 3-hydroxybutyrate and acetoacetate are formed in hepatic mitochondria from free fatty acids and are a source of energy for extrahepatic tissues. Ketogenesis is regulated primarily by the rate of mobilization of free fatty acids. Once in the liver mitochondria, the relative proportion of acyl CoA destined to undergo beta-oxidation is limited by the activity of an enzyme, carnitine palmitoyltransferase-1. Lastly, there are mechanisms that keep the levels of acetyl CoA entering the citric acid cycle constant, so that only at high mitochondrial levels will acetyl CoA be converted to ketone bodies. Even the brain, in times of starvation, can use ketone bodies for half of its energy requirements. At some point, however, the ability of liver to perform beta-oxidation may be inadequate. Under such circumstances, hepatic storage of triglyceride or fatty in ltration of the liver can be signi cant, leading to the development of nonalcoholic steatohepatitis. Triglyceride storage by itself does not appear to be a cause of hepatic brosis, but fatty in ltration may be a marker for the derangement of normal processes by alcohol or drug toxicity, diabetes, or long-term total parenteral nutrition. A speci c type of microvesicular fatty accumulation is also seen in a variety of diseases, such as Reye syndrome, morbid obesity, and acute fatty liver of pregnancy. Figure 2-8. The LDL receptor, an example of a transmembrane receptor that participates in receptormediated endocytosis. The LDL receptor speci cally binds lipoproteins that contain apolipoprotein B- 100 or E. Once internalized, the lipoproteins are degraded. AA, amino acids; EGF, epidermal growth factor. As noted above, fatty acids are critical elements of all mammalian cells; as energy substrate, in cellular structure, and for intracellular signaling. Evolutionarily, storage of excess fat in adipose tissue mitigated starvation. But in most modern societies the ready availability of calorie-dense foods has led to an epidemic of obesity as is discussed in detail in other chapters. In terms of intermediary metabolism, excess dietary fatty acids are now known to cause insulin resistance in muscle through intramyocellular triglyceride content leading to type II diabetes. This e ect is likely due to intracellular perturbations in active lipid metabolites such as diacylglycerols or ceramides. Other studies have documented mitochondrial abnormalities possibly through interference with serine kinases.8 CHOLESTEROL METABOLISM Cholesterol is an important regulator of membrane uidity and is a substrate for bile acid and steroid hormone synthesis. Cholesterol may be available by dietary intake or by de novo synthesis. In mammals, mostly new cholesterol is synthesized in the liver from its precursor, acetyl CoA. Dietary cholesterol intake can suppress endogenous synthesis by inhibiting the rate-limiting enzyme in the cholesterol biosynthetic pathway, HMGCoA reductase. A competitive antagonist, lovastatin, can also block HMGCoA reductase and e ectively lower plasma cholesterol by blocking cholesterol synthesis, stimulating LDL receptor synthesis, and allowing an increased hepatic uptake and metabolism of cholesterol-rich LDL lipoproteins. The structure of the LDL receptor is known and serves as a model for the structure and function of other cell membrane receptors (Fig. 2-8). Cholesterol is lipophilic and hydrophobic, and most plasma cholesterol is in lipoproteins esteri ed with oleic or palmitic acid. The liver can process cholesterol esters from all classes of lipoproteins. Hepatocytes can also take up chylomicron remnants containing dietary cholesterol esters. Abnormally elevated levels of cholesterol in VLDLs or LDLs are associated with atherosclerosis, whereas high HDL levels are protective. Newly synthesized hepatic cholesterol is also used to synthesize bile acids for further intestinal absorption of dietary fats. A large proportion of the bile acids secreted by the liver into bile are returned to the liver via the enterohepatic circulation (Fig. 2-9). Phospholipids The three major classes of phospholipids synthesized by the liver are lecithins, cephalins, and sphingomyelins. Although most cells in the body are capable of some phospholipid synthesis, the liver produces 90%. Phospholipid formation is controlled by the overall rate of fat metabolism and by the availability of choline and inositol. The main role of phospholipids of all types is to form plasma and organelle membranes. The amphiphilic nature of phospholipids makes them essential for reducing surface tension between membranes and surrounding uids. Phosphatidylcholine, one of the lecithins, is the major biliary phospholipid and is important in promoting the secretion of free cholesterol into bile. Thromboplastin, one of the cephalins, is needed to initiate the clotting cascade. The sphingomyelins are necessary for the formation of the myelin nerve sheath. Figure 2-9. The enterohepatic circulation of bile acids. The primary bile acids, cholic acid, and chenodeoxycholic acid, are synthesized in the liver from cholesterol. Deoxycholic acid and lithocholic acid are formed in the colon (blue lines) during bacterial degradation of the primary bile acids. All four bile acids are conjugated with glycine or taurine in the liver. Most of the lithocholic acid is also sulfated, which decreases reabsorption and increases fecal excretion. Bile acids are absorbed passively in the epithelium of the small and large intestine and actively in the distal ileum. PROTEIN METABOLISM Formation and Catabolism of Plasma Proteins 4 Hepatic protein synthesis, when excess amino acids are available, includes albumin, brinogen, and apolipoproteins and can reach 50 g/day. Of the total hepatic protein synthesized, 75% is destined for export in plasma. Most newly synthesized proteins are not stored in the liver, and the rate of protein synthesis is primarily determined by the intracellular levels of amino acids. The tertiary structure of many proteins undergoes posttranslational modi cation after they have been synthesized in the liver’s rough endoplasmic reticulum (ER). Glycosylation, or the addition of carbohydrate moieties, occurs in the smooth ER. Sialation, or the addition of sialic acid, occurs in the Golgi. Glycosylation is important in allowing some proteins to bind with speci c receptors for subsequent hepatic uptake and processing. Removal of sialic acid residues, or desialation, from the terminal galactose molecules of glycoproteins allows them to bind to the asialoglycoprotein (ASGP) receptor in the liver and undergo degradation. Desialation, therefore, is important in the clearance of senescent proteins from the plasma. Intracellular proteases hydrolyze proteins into peptides, and the peptides are in turn hydrolyzed by peptidases. Ultimately, free amino acids are generated. Unlike carbohydrate and lipids, excess amino acids are degraded if they are not immediately reincorporated into new proteins. Protein degradation occurs primarily by one of two routes. ASGPs are internalized into lysosomes via receptor-mediated endocytosis. The lysosomal enzymes do not require ATP and are nonselective in their activities; more than 20 known hydrolytic enzymes are present in lysosomes. A second pathway involves the covalent attachment of ubiquitin, named for the fact that it exists in all mammalian cells, targeting proteins for destruction. This pathway is ATP dependent and generally is used for proteins with shorter half-lives.9 Amino Acid Synthesis Essentially, all the end products of dietary protein digestion are amino acids, which are absorbed by the enterocytes into the portal circulation in an ionized state. Liver amino acid uptake occurs by one of several active transport mechanisms. Amino acids are not stored in the liver but are rapidly used in the production of plasma proteins, purines, heme proteins, and hormones. Under certain conditions, the amine group is removed from amino acids, and the carbon chain is used for carbohydrate, lipid, or nonessential amino acid synthesis.10 Ten nutritionally essential amino acids must be obtained from dietary intake (Table 2-2). However, human tissues contain transferases, which convert the α-keto acids of leucine, valine, and isoleucine so that the corresponding α-keto acids can be used as dietary supplements. The remaining nutritionally nonessential amino acids can be synthesized in one to three enzyme-catalyzed reactions. Hydroxyproline and hydroxylysine do not have a corresponding tRNA and arise by posttranslational modi cation of proline or lysine by mixed function oxidases. Glutamate, glutamine, and proline are derived from the citric acid cycle intermediate α-ketoglutarate. Aspartate and asparagine are synthesized from oxaloacetate. Serine and glycine are synthesized from the glycolysis intermediate 3phosphoglycerate. Cysteine and tyrosine are formed from essential amino acids (methionine and phenylalanine, respectively).11 Table 2-1 Amino Acids Required by Adult Humans Catabolism of Amino Acid Nitrogen Ammonia, derived largely from the deamination of amino acids, is toxic to all mammalian cells. The ammonia formed as a result of the deamination of amino acids is detoxi ed by one of two routes.12 The most important pathway involves the conversion of ammonia to urea by enzymes of the Krebs–Henseleit, or urea cycle, which occurs only in the liver (Fig. 2-10). A second route of ammonia metabolism involves synthesis of L-glutamine from ammonia and glutamate by renal glutamine synthetase. CELLULAR ENERGY GENERATION Overview and Stage I 5 The citric acid cycle includes a series of mitochondrial enzymes that transform acetyl CoA – itself derived from pyruvate or fatty acyl CoA – into water, carbon dioxide, and hydrogen-reducing equivalents. Each molecule of acetyl CoA that enters the citric acid cycle yields 12 molecules of ATP. The fundamental mechanism by which mammalian cells generate energy is the aerobic conversion of sugars and fatty acids into ATP. There are four stages with stage I – glycolysis – (see glycolysis above) beginning in the cytosol, converting glucose into two molecules of pyruvate. Also, cytosolic fatty acids are converted to fatty acyl CoA. Pyruvate and fatty acyl CoA are transported to the mitochondrial matrix and converted to acetyl CoA; generating the electron carriers NADH or FADH2 as well as CO2. In stage II, mitochondrial acetyl CoA enters the citric acid cycle further generating NADH, FADH2, additional CO2, and GTP. In stage III, oxygen is reduced to water via the electron transport chain using previously generated molecules of NADH and FADH2 as electron donors. The electron transport chain causes hydrogen ions to move from the mitochondrial matrix to the intermembrane space generating a proton motive force. Lastly, in stage IV, ATP synthase uses energy generated by the proton motive force to generate large amounts of ATP.13 Stage II: The Citric Acid Cycle: Integration of Metabolic Pathways and Oxidation of Acetyl CoA One major function of the citric acid cycle (also known as the Krebs cycle or the tricarboxylic acid cycle) is to act as a common pathway for the oxidation of carbohydrate, lipid, and protein and generate energy in the form of ATP. Conversely, the citric acid cycle is important in gluconeogenesis, lipogenesis, and amino acid metabolism. In the fed state, a large proportion of ingested energy from foodstu s is converted to glycogen or fat. The metabolism of sugars, fats, and proteins, then, allows adequate fuels for all tissue types under conditions from fed to fasting to starvation. The body accomplishes production of fuel substrates for organs and regulates intestinally absorbed nutrients for tissue consumption or storage by integrating three key metabolites: G6P, pyruvate, and acetyl CoA (Fig. 2-11). Each of these three simple chemical molecules can be extensively modified to allow a large number of metabolites. Figure 2-10. The urea cycle. Ammonia entering the urea cycle is derived from protein and amino acid degradation in tissues (endogenous) and the colonic lumen (exogenous). G6P can be stored as glycogen or converted into glucose, pyruvate, or ribose-5-phosphate (a nucleotide precursor). Pyruvate can be converted into lactate, alanine (and other amino acids), and acetyl CoA, or it can enter the tricarboxylic acid cycle by conversion to oxaloacetate. Acetyl CoA is converted to HMG-CoA (a cholesterol and ketone body precursor) or citrate (for fatty acid and triglyceride synthesis), or it is degraded to carbon dioxide and water for energy. In humans, acetyl CoA cannot be converted into pyruvate due to the irreversible reaction of pyruvate dehydrogenase. Thus, lipids cannot be converted into either carbohydrates or glucogenic amino acids. Probably the most important citric acid cycle function is to oxidize acetyl CoA into CO2. The citric acid cycle is composed of a series of enzyme reactions that occur in the mitochondrial matrix and inner membrane. Here, acetyl CoA combines with oxaloacetate to form citrate. Through a series of subsequent enzymatic reactions involving both dehydrogenases and decarboxylases, citrate is catabolized to result in the generation of hydrogen-reducing equivalents and carbon dioxide (Fig. 2-12). With each revolution of the citric acid cycle, a molecule of acetyl CoA generates three molecules of the reduced coenzyme NADH, one molecule of the reduced coenzyme FADH2, and one molecule of GTP. The reduced coenzymes NADH and FADH2 are charged with high-energy electrons that subsequently drive the electron transport chain of stage III to generate a hydrogen ion gradient. These reducing equivalents are transported to the inner mitochondrial membrane and the electron transport chain to generate more ATP. Each molecule of NADH is oxidized to yield three molecules of ATP, and each molecule of FADH2 is oxidized to yield two molecules of ATP. One molecule of ATP is generated at the substrate level in the conversion of succinyl CoA to succinate; thus, the total molecule of ATP generated per molecule of acetyl CoA is 12. Stages III and IV: Oxidative Phosphorylation 6 Oxidative phosphorylation converts the energy from NADH and FADH2 into ATP by the electron transport chain and ATP synthase with a process called the proton motive force. The covalent bond energy in glucose and fatty acids is transferred into high-energy electrons in stages I (glycolysis) and II (citric acid cycle). Flow of these high-energy electrons from NADH and FADH2 to oxygen, creating water, is coupled to transport of protons across the mitochondrial inner membrane from the matrix to the intermembrane space. Originally called Mitchell’s chemiosmotic hypothesis, many researchers now refer to this process as the proton motive force. The voltage gradient caused by the transport of these protons, and the ATP subsequently generated is the process known as oxidative phosphorylation. Below is an admittedly simpli ed explication of a complex process, and the stoichiometry of the reactions has been modi ed for clarity. Anyone interested in the precise biochemistry should consult the relevant sources (Fig. 2-13). Two high-energy electrons carried by NADH or FADH2 pass through three of four major multiprotein complexes: I – NADH-CoQ reductase; II – succinate-CoQ reductase; III – CoQH2-cytochrome c reductase; and, IV – cytochrome c oxidase. The paths of NADH and FADH2 are di erent initially. NADH electrons are transferred through complex I – NADH-CoQ reductase – to avin mononucleotide (FMN), seven iron–sulfur clusters (FeS), and then coenzyme Q (CoQ) to create CoQH2. Four protons are transported from the mitochondrial matrix as a result of the actions of complex I. Unlike NADH, FADH2 is oxidized by complex II – succinate-CoQ reductase. The two FADH2 electrons are transferred to succinate dehydrogenase-bound FAD when succinate is oxidized to fumarate, then an iron–sulfur cluster, and finally to CoQ creating CoQH2. Figure 2-11. Summation of the key regulatory molecules used by the liver during diverse metabolic functions. Essentially, any compound found in the body can be synthesized in the liver from glucose- 6-phosphate, acetyl coenzyme A, or pyruvate. As a consequence of the inability of mammalian liver to convert acetyl coenzyme A to pyruvate, fats cannot be converted to carbohydrates. Figure 2-12. The citric acid cycle. Reduced nicotinamide adenine dinucleotide and reduced avin adenine dinucleotide, formed in the citric acid cycle, are subsequently oxidized in mitochondria by means of the electron transport chain to generate ATP. Acetyl CoA plays a key role. CoQH2 from NADH or FADH2 is shuttled to complex III – CoQH2cytochrome c reductase – within the inner membrane. The two electrons are further transferred to cytochromes bL and bH, resulting in pumping of two additional hydrogen ions into the intermembranous space. Electrons are further transferred to another iron–sulfur cluster, cytochrome c1, and ultimately the intermembranous space protein cytochrome c, pumping two additional hydrogen ions. Complex II catalyzes the conversion of fatty acyl CoA to acetyl CoA for further metabolism through the citric acid cycle. Cytochrome c shuttles the two electrons through the intermembranous space to complex IV – cytochrome c oxidase – reoxidizing the cytochrome c molecule, transferring the electrons to copper containing Cua, the heme moiety of cytochrome a3, Cub, cytochrome a3, and ultimately to oxygen, yielding water. The net result of the electron transport chain is the pumping of ten H+ ions into the intermembranous space for two electrons owing from NADH to O2, and six H+ ions for each two electrons from FADH2 to O2. This generates the proton motive force; a voltage gradient across the inner mitochondrial membrane that directly provides energy for ATP generation in stage IV. ATP synthase harnesses the voltage gradient of protons across the inner membrane by interconverting the chemical potential energy into phosphoanhydride bonds of ATP. ATP synthase is composed of two main complexes: F0,, consisting of three types of membrane proteins, and F1, a five-polypeptide complex protruding into the matrix. Alternative Fuels Regardless of the fed state of the human body, there is a requirement for glucose utilization. The nervous system and erythrocytes have an absolute requirement for glucose. Glucose is a source of glycerol-3-phosphate for adipose tissue, and most other tissues for integrity of the citric acid cycle. To maintain adequate glucose for survival, other fuels can be used depending on environmental conditions. Under conditions of carbohydrate shortage, ketone bodies and free fatty acids are utilized to spare oxidation of glucose in muscle. These alternate fuels increase intracellular citrate, which inhibits both phosphofructokinase and pyruvate dehydrogenase. In starvation, fatty acid oxidation results in the production of glycerol, which, along with gluconeogenesis from amino acids, is the only source of the required glucose. Ultimately, even the brain can substitute ketone bodies for about half of its energy requirements. The preferred energy substrates for liver are ketoacids derived from amino acid degradation even in well-fed states. This is designed to allow the consumption of glucose by obligate tissues. Glucose produced by the dephosphorylation of G6P rapidly diffuses out of the cell and is taken up by the brain, muscles, and other organs. Hepatic glycolysis is used primarily for the production of intermediates of metabolism and not for energy. Hepatic fatty acid degradation for energy is also inhibited under most circumstances and occurs only during adipocyte lipolysis. By way of clinical relevance, alterations in liver mitochondrial function are now known to be important in two common diseases. Patients with type 2 diabetes have reduced ATP synthesis and abnormal ATP repletion in response to substrate-induced ATP depletion. Nonalcoholic fatty liver disease is also associated with lowered ATP repletion in response to oxidative stress.14 Most short-, medium-, and long-chain fatty acids (C8 to C20) are metabolized by mitochondria to generate ATP. Mitochondria cannot metabolize fatty acids with acyl chains greater than 20, and these very long-chain fatty acids are metabolized in peroxisomes, predominantly in the liver. Peroxisomes do not contain elements of the citric acid cycle or electron transport chain and thus do not generate ATP. Most of the energy is released as heat.15 Adipocyte Phenotype, Thermogenesis, and Obesity Epidemic obesity has created a surge of interest in the adipocyte, or fat cell. There are three basic types of adipocytes; white, brown, and beige. White adipocytes di erentiate for the storage of fat and contain few mitochondria. The traditional view is that adult human fat is comprised predominantly of white adipocytes. These adipocytes accumulate lipid in times of over feeding leading to obesity and insulin resistance.8 Brown adipocytes are so named because of the brown color created by a high density of cellular mitochondria. Unlike the mitochondria of other organs, the mitochondria of brown adipocytes contain uncoupling protein-1 which can catalyze a protein leak across the inner membrane uncoupling electron transport from ATP synthesis. This results in the dissipation of energy as heat, or thermogenesis. Brown adipocytes were thought to be present only in human infants, and rare circumstances such as cold-climate outdoor workers or pheochromocytoma patients. Recently, the identi cation of “beige” adipocytes has led to the realization that the adipocyte may be a target for therapy of obesity. Beige adipocytes appear capable of switching from a white phenotype to brown, and the hope that adipocytes could be switched to utilize excess calories for thermogenesis rather than storage as fat.16 Figure 2-13. The electron transport chain. A: NADH pathway. Electrons ow through complex I to complex III via the lipid soluble molecule CoQ. From complex III, the electrons are transported through the intermembrane space by cytochrome c to complex IV. For every two electrons that ow from NADH to O2, 10 protons are pumped to the intermembranous space. B: FADH2-succinate pathway. Electrons ow through complex II to complex III via the lipid soluble molecule Coenzyme Q (CoQ). From complex III, the electrons are transported through the intermembranous space by cytochrome c to complex IV. For every two electrons that ow from FADH2 and succinate to O2, six protons are pumped to the intermembranous space. BIOTRANSFORMATION Biotransformation is de ned as the intracellular metabolism of endogenous organic compounds (e.g., heme proteins and steroid hormones) and exogenous compounds (e.g., drugs and environmental compounds). Most biotransformation occurs chie y in the liver, which contains enzyme systems that can expose functional groups, such as hydroxyl ions (phase I reactions), or alter the size and solubility of a wide variety of organic and inorganic compounds by conjugation with small polar molecules (phase II reactions). A general strategy is to convert hydrophobic, potentially toxic compounds into hydrophilic conjugates that can then be excreted into bile or urine. 7 Biotransformation of potentially toxic, often hydrophobic, compounds into hydrophilic, excretable compounds occurs mainly in the liver by the cytochromes P-450, the uridine diphosphate-glucuronyl (UDP-glucuronyl) transferases, the GSH S-transferases, and the sulfotransferases. Biotransforming enzymes are not distributed uniformly within the cells of the hepatic lobule. This heterogeneity may account for the ability of some drugs to cause damage preferentially in zone 3 hepatocytes (those nearest the central venule). Cytochromes P-450 The cytochromes P-450 are named for their ability to absorb light maximally at 450 nm in the presence of carbon monoxide. These enzymes are bound to the ER and collectively catalyze reactions by using NADPH and oxygen. The P-450 isozymes present in mammalian liver catalyze reactions such as oxidation, hydroxylation, sulfoxide formation, oxidative deamination, dealkylation, and dehalogenation. Such reactions allow further phase II conjugation with polar groups such as glucuronate, GSH, and sulfate. The cytochromes P-450 can also create potentially toxic metabolites. Drugs such as acetaminophen, isoniazid, halothane, and the phenothiazines can be converted into reactive forms that cause cellular injury and death. The cytochromes also are responsible for the formation of organic free radicals, reactive metabolites that can directly attack and injure cellular components or act as haptens in the generation of an autoimmune response. Several of the most potent known carcinogens are aromatic hydrocarbons, which are modified by cytochromes P-450. Uridine Diphosphate-glucuronyl Transferases Glucuronidation is the conjugation of UDP-glucuronic acid to a wide variety of xenobiotics by either ester (acyl) or ether linkages. The transferases catalyzing these reactions reside in the ER. Many common compounds are metabolized in this way, including bilirubin, testosterone, aspirin, indomethacin, acetaminophen, chloramphenicol, and oxazepam. Clinically signi cant loss of activity can occur with acute ethanol exposure or acetaminophen overdose, when formation of UDP-glucuronic acid from UDP-glucose is outstripped by use. Some acyl linkages lead to the generation of electrophilic centers that can react with other proteins. The covalent linkage of conjugated bilirubin to albumin is believed to occur by this mechanism. Glutathione S-transferases The GSH transferases are more selective in the biotransformations they perform. GSH conjugation occurs only with compounds that have electrophilic and potentially reactive centers. The role of GSH conjugation catalyzed by the GSH S-transferases is demonstrated by acetaminophen. In metabolism of this drug, cytochromes P-450 create an electrophilic center that reacts with protein thiol groups or GSH.17 The presence of GSH Stransferase allows the preferential detoxi cation of acetaminophen rather than its potentially injurious binding to thiol groups. A class of GSH Stransferases, known as ligandins, appears to facilitate the uptake and intracellular transport of bilirubin, heme, and bile acids from plasma to liver. In addition to the detoxi cation of potential toxins, GSH is a substrate for GSH peroxidase, an enzyme important in the metabolism of hydrogen peroxide. Sulfotransferases The sulfotransferases catalyze the transfer of sulfate groups from 3'phosphoadenosine-5'-phosphosulfate (PAPS) to compounds such as thyroxine, bile acids, isoproterenol, α-methyldopa, and acetaminophen. They are located primarily in the cytosol. Although many P-450 derivatives can be further conjugated by either the sulfotransferases or the glucuronyl transferases, a limited ability of the liver to synthesize PAPS makes glucuronidation the predominant mechanism. HEME AND PORPHYRIN METABOLISM Heme is formed from glycine and succinate and is the functional ironcontaining center of hemoglobin, myoglobin, cytochromes, catalases, and peroxidases. From glycine and succinate precursors, δ-aminolevulinic acid (δ-ALA) is synthesized by the rate-limiting enzyme ALA synthase. The porphyrinogens are intermediates in the pathway from δ-ALA to heme, and porphyrins are oxidized forms of porphyrinogen (Fig. 2-14). Inherited enzyme defects in the heme synthetic pathway cause the overproduction of various porphyrinogens, which can in turn cause clinical manifestations known as the porphyrias.18 Acquired porphyria can be caused by heavy metal intoxication, estrogens, alcohol, or environmental exposure to chlorinated hydrocarbons. Bilirubin IXa is the predominant heme degradation product in humans and is derived mostly from hemoglobin. The enzyme heme oxygenase, located in cells of the reticuloendothelial system, is primarily responsible for this conversion. Heme oxygenase resides in the ER and requires NADPH as a cofactor. Hepatic processing of bilirubin is further detailed in the section on bile formation. METAL METABOLISM Iron uptake appears to occur by two distinct processes: (a) receptormediated endocytosis of iron–transferrin complexes and (b) facilitated di usion across the plasma membrane. More iron is taken up and stored by the liver than by any other organ, with the exception of the bone marrow. Transferrin is synthesized in the liver and has speci c plasma membrane receptors on a number of di erent tissues. After endocytosis, the transferrin and iron dissociate and the transferrin and transferrin receptors return to the cell surface for recycling. A pathway appears to involve the dissociation of iron and transferrin at the plasma membrane and subsequent internalization by carrier-mediated di usion. Once internalized, iron is stored and forms a complex with apoferritin. Each apoferritin molecule is capable of storing several thousand iron molecules. The iron– apoferritin complex, called ferritin, is responsible for iron storage under physiologic conditions. Iron storage in a protein-bound form is essential because free iron can catalyze free radical formation, leading to cell injury.19 Figure 2-14. The heme biosynthetic pathway. Inherited defects of each of the heme biosynthetic enzymes except δ-aminolevulinic acid synthase have been described and lead to the clinical disorders known as the porphyrias. Copper is transported to the liver bound to albumin or histidine and enters the hepatocytes by a process of facilitated di usion. Once inside the cell, copper can bind to several intracellular proteins for storage or as a necessary enzyme cofactor. Copper-binding proteins include metallothionein, monoamine oxidase, cytochrome c oxidase, and superoxide dismutase. Ceruloplasmin is a liver-derived protein that binds hepatic copper for transport to other tissues. The low levels of ceruloplasmin seen in patients with Wilson disease suggest a pathogenetic defect. Zinc is taken up by and competes for the same binding sites as copper. In hepatocytes, zinc binds predominantly to metallothionein and is excreted into bile, in which it enters the enterohepatic circulation. Other metals, usually found in trace amounts, are lead, cadmium, selenium, mercury, and nickel. These metals are usually bound to metallothionein or GSH, and intoxication is associated with free radical formation and liver injury. SUMMARY The broad brush-stroke fundamentals of intermediary metabolism have been known for years, however, knowledge on the details expands at an ever-increasing rate. A working understanding of the fundamental biochemical reactions by which substrates are metabolized is important for all surgical disciplines. Advanced knowledge of the genetics, cellular biology, bioenergetics, and molecular biology is being exploited to support and perhaps enhance general and specialized cellular function, combat disease, and improve health. References 1. Lodish H, Berk A, Kaiser CA, et al., eds. Molecular Cell Biology. 7th ed. New York, NY: WH Freeman; 2013:699–704. 2. The ProCESS Investigators; Yealy DM, Kellum JA, Huang DT, et al. A randomized trial of protocol-based care for early septic shock. N Engl J Med 2014;370:1683–1693. 3. Jones AE, Shapiro NI, Trzeciak S, et al. Lactate clearance vs central venous oxygen saturation as goals of early sepsis therapy. JAMA 2010;303(8):739–746. 4. Whittaker SA, Fuchs BD, Gaieski DF, et al. Epidemiology and outcomes in patient with severe sepsis admitted to the hospital wards. J Crit Care 2015;30(1):78–84. Available from: http://dx.doi.org/10.1016/j.jcrc.2014.07.012. Accessed May 27, 2016. 5. Berg JM, Tymoczko JL, Stryer L, et al., eds. Biochemistry. 7th ed. New York, NY: WH Freeman; 2012:601–605. 6. Botham KM, Mayes PA. Lipid transport and storage. In: Murray RK, Bender DA, Botham KM, et al., eds. Harper’s Illustrated Biochemistry. 29th ed. New York, NY: McGraw-Hill; 2012:237–249. 7. Bauer RC, Stylianou IM, Rader DJ. Functional validation of new pathways in lipoprotein metabolism identified by human genetics. Curr Opin Lipidol 2011;22:123–128. 8. Turner N, Cooney GJ, Kraegen EW, et al. Fatty acid metabolism, energy expenditure and insulin resistance in muscle. J Endo 2014;220:T61–T79. 9. Römisch K. Endoplasmic reticulum-associated degradation. Annu Rev Cell Dev Biol 2005;21:435–456. 10. Brosnan JT. Interorgan amino acid transport and its regulation. J Nutr 2003;133(6 suppl 1):2068S–2072S. 11. Berg JM, Tymoczko JL, Stryer L, et al., eds. Biochemistry. 7th ed. New 12. 13. 14. 15. 16. 17. 18. 19. York, NY: WH Freeman; 2012:711–722. Ah Mew N, Lanpher BC, Gropman A, et al. Urea Cycle Disorders Overview. In: Pagon RA, Adam MP, Ardinger HH, et al., eds. GeneReviews® [Internet]. Seattle, WA: University of Washington; 2003:1993–2014. Available from: http://www.ncbi.nlm.nih.gov/books/NBK1217/. Accessed May 27, 2016. Lodish H, Berk A, Kaiser CA, et al., eds. Molecular Cell Biology. 7th ed. New York, NY: WH Freeman; 2013:517–552. Koliaki C, Roden M. Hepatic energy metabolism in human diabetes mellitus, obesity, and non-alcoholic fatty liver disease. Mol Cell Endo 2013;379:35–42. Chapter 12: cellular energetics. In: Lodish H, Berk A, Kaiser CA, et al., eds. Molecular Cell Biology. 7th ed. New York, NY: WH Freeman; 2013:531–532. Rosen ED, Spiegelman BM. What we talk about when we talk about fat. Cell 2014;156:20–44. Riddick DS. Drug biotransformation. In: Kalant H, Grant DM, Mitchell J, eds. Principles of Medical Pharmacology. 7th ed. Toronto: SaundersElsevier; 2007. Bonkovsky HL, Guo JT, Hou W, et al. Porphyrin and heme metabolism and the porphyrias. Compr Physiol 2013;3(1):365–401. Winter WE, Bazydlo LA, Harris NS. The molecular biology of human iron metabolism. Lab Med 2014;45(2):92–102. Chapter 3 Surgical Nutrition and Metabolism George Kasotakis Key Points 1 Starvation and systemic in ammatory response result in erosion of the fat-free mass and body weight (malnutrition) and are indicators for nutrition support if present. 2 Inflammation increases energy utilization and alters the metabolism of glucose, protein, fat, and trace minerals. 3 Hypermetabolism is seen in numerous disease states, and not merely in trauma and sepsis. 4 Numerous methods exist to aid assessment of patients’ nutritional status, each with its own advantages and disadvantages. 5 A strong relation between protein depletion and postoperative complications has been demonstrated in nonseptic, nonimmunocompromised patients undergoing elective major gastrointestinal surgery. 6 The main goal of perioperative or posttraumatic nutritional support is repletion or maintenance of protein, energy stores, and other nutrients to allow rapid and full recovery from illness. 7 Whenever providing nutritional support, supply caloric intake in the form of carbohydrate and fat in a 2:1 ratio, if no contraindications exist. 8 Enteral nutritional support is always preferable than parenteral nutrition, in the presence of a functioning gastrointestinal tract. 9 The maintenance of an intact brush border and intercellular tight junctions prevents the movement of toxic substances into the intestinal circulation and minimizes bacterial translocation. These functions may be affected in critical illness. Enteral nutrition helps restore them. 10 Allow hypocaloric enteral feedings in the acute phase of critical illness for up to 5 to 7 days in previously well-nourished patients. Start as early as feasible. 11 Routine glutamine supplementation is not supported during critical illness. 12 Supply micronutrients to prevent refeeding syndrome, and monitor electrolytes, liver function tests, and triglyceride levels as needed. 13 The adequacy of nutritional support should be reassessed frequently and adjustments made as needed until full convalescence. INTRODUCTION Patients undergoing gastrointestinal procedures with evidence of malnutrition at baseline are more likely to su er postoperative morbidity, mortality, and require longer hospital stays compared to their wellnourished counterparts.1,2 The problem is greater than most realize, with up to 14% of patients scheduled for elective gastrointestinal tract procedures found to be malnourished and up to 40% of those with gastrointestinal disease found to be at risk for malnutrition.3 It has been shown that poor nutritional status can detrimentally a ect postoperative outcomes,4 and in a consensus review of Enhanced Recovery After Surgery, it was recommended that patients receive carbohydrate loading 24 hours preoperatively and nutritional supplements, from the day of surgery, until oral intake is achieved.5 In addition to patients undergoing elective surgery, severe injury and critical illness are associated with a hypermetabolic state that can increase energy expenditure dramatically and complicate the resuscitation and recovery of the critically ill or severe injury victim. Over the last few years, aggressive nutritional support has been underlined as an integral part of the caring of the critically ill surgical patient, and the Society for Critical Care Medicine (SCCM) and the American Society for Parenteral and Enteral Nutrition (ASPEN) have made recommendations toward providing early, aggressive nutritional support that enables patients to preserve their immune function, maintain lean body mass, and minimize metabolic complications.6,7 This chapter addresses the areas of nutritional assessment and management of the surgical patient, reviewing key metabolic principles and providing an overview of pertinent literature. BASIC METABOLIC PRINCIPLES Body Composition Total body mass is composed of aqueous and nonaqueous components, with the former constituting approximately 55% to 60% of total body mass (total body water, TBW). Mineralized bone and adipose tissue make up the majority of the nonaqueous component. The relationship between total body mass and TBW is relatively constant for an individual and typically re ects the amount of body fat. Solid organs and muscle contain a higher proportion of water than bone and fat, therefore young lean males have a higher proportion of their total body mass as water, compared to obese or elderly individ