Metabolism of Biomolecules PDF

METABOLISM OF BIOMOLECULES Learning Outcomes: At the end of the three hours of lecture – discussion on metabolism of biomolecules, the students will be able to: 1. describe the role of ATP in metabolism, 2. explain the catabolic processes in the body,...

METABOLISM OF BIOMOLECULES Learning Outcomes: At the end of the three hours of lecture – discussion on metabolism of biomolecules, the students will be able to: 1. describe the role of ATP in metabolism, 2. explain the catabolic processes in the body, a. glycolysis b. formation of acetyl coA c. citric acid cycle and d. oxidative phosphorylation 3. trace the pathway for the electron transport chain, 4. summarize the overall scheme of carbohydrate, lipid and protein metabolism. INTRODUCTION Energy is needed to perform work. As we need energy to walk, jump and think, the cell needs a ready supply of cellular energy for the many functions that support these activities. Cells need energy for active transport, to move molecules between the environment and the cell. Energy is also needed for biosynthesis of small metabolic molecules and production of macromolecules from these intermediates. Energy is also required for mechanical work, including muscle contraction and motility of the sperm cells. We need a supply of energy-rich food molecules that can be degraded or oxidized to provide this needed cellular energy. Our diet includes three major sources of energy: the carbohydrate, fats and proteins. Each of these types of large biological molecules must be broken down into its basic subunits before they can be taken into the cell and used to produce cellular energy. In this chapter we are going to study the steps of the ancient energy-harvesting pathway. This energy harvesting pathway involves many processes or chemical reactions by which food is converted to ATP: the cellular energy needed to sustain life. The degradation of fuel molecules and production of ATP for cellular energy-requiring functions, including anabolism is called catabolism. Anabolism is a metabolic process in which energy is used to make compounds and tissues from simple molecules. The sum total of all chemical reactions involved in maintaining the living state of the cells, and thus the organism is called metabolism. 8.1 ATP: The Cellular Energy Currency With a series of enzymes, biochemical pathways in the cell carry out a step-by-step oxidation of glucose. Small amount of energy are released at several points in the pathway and that energy is harvested and saved in the bonds of a molecule that has been called the universal energy currency. This molecule is adenosine triphosphate (ATP). ATP is a nucleotide, which means that it is a molecule composed of a nitrogenous base; a five-carbon sugar; and one, two or three phosphoryl group.  ATP serves as a “go-between” molecule that couples the exergonic (energy releasing) reactions of catabolism and the endergonic (energy requiring) reactions of anabolism. ATP Structure: A phosphoester bond joins the first phosphoryl group to the five-carbon sugar ribose. The next two phosphoryl groups are joined to one another by phosphoanhydride bonds. A phosphoanhydride bond is a high-energy bond. When it is broken or hydrolyzed, a large amount of energy is released. When the phosphoanydride bond of ATP is broken, the energy that is released can be used for cellular work. These high-energy bonds are indicated as squiggles (~). Hydrolysis of ATP yields adenosine diphosphate (ADP), an inorganic phosphate group (Pi), and energy. The energy released by this hydrolysis of ATP is then used to drive biological processes, for instance, the phosphorylation of glucose or fructose. An example of the way in which the energy of ATP is used can be seen in the first step of glycolysis, the anaerobic degradation of glucose top produce chemical energy. Figure 7.1 ATP Molecular Structure Adenosine triphosphate (ATP) 7.2 OVERVIEW OF CATABOLIC PROCESSES: Catabolic processes begin with a supply of nutrients. When we eat a meal, we are eating quantities of carbohydrates, fats, and proteins. From this point catabolic processes can be broken down into a series of stages: Stage 1 - hydrolysis of macromolecules to subunits Stage II - conversion of subunits to a form that can be completely oxidized, usually acetyl CoA Stage III - complete oxidation of acetyl CoA and the production of ATP  FOOD Proteins Carbohydrates Fats Amino Acids Simple sugars Fatty acids & glycerol G L Y C O L Y S I S PYRUVATE Acetyl CoA Citric acid cycle Oxidative Phosphorylation Figure 7.2 Overview of Catabolism Processes Stage I: Hydrolysis of Macromolecules to subunits The purpose of the first stage of catabolism is to degrade large food molecules into their component subunits. These subunits – simple sugars, amino acids, fatty acids, and glycerol – are then taken into the cells of the body for use as an energy source. Stage II: Conversion of Subunits to a form That Can Be Completely Oxidized usually acetyl CoA The monosaccharide, amino acids, fatty acids and glycerol must now be assimilated into the pathways of energy metabolism. The two major pathways are glycolysis and citric acid cycle. Simple sugars enter the glycolysis pathway in the form of glucose or fructose. They are converted to acetyl CoA, which is a form that can be completely oxidized in the citric acid cycle. Amino groups are removed from amino acids, and the remaining carbon skeletons enter the catabolic processes at many steps of the citric acid cycle. Fatty acids are converted to acetyl CoA and enter the citric acid cycle in that form. Glycerol, produced by the hydrolysis of fats, is converted to glyceraldehydes-3-phosphate, one of the intermediates of glycolysis, and enters energy metabolism at that level. Stage III: The Complete Oxidation of Nutrients and the Production of ATP Acetyl CoA carries two-carbon remnant of the nutrients, acetyl groups, to the citric acid cycle. Acetyl CoA enters the cycle, and electrons and hydrogen atoms are harvested during the complete oxidation of the acetyl group to CO2. Coenzyme A is released and recycled to carry additional acetyl groups to the pathway. The electrons and hydrogen atoms that are harvested are used in the process of oxidative phosphorylation to produce ATP. 7.3 GLYCOLYSIS Glycolysis – is an anaerobic breakdown of glucose that results in a gain of two ATP. It is also known as the Embden-Meyerhof Pathway, The ten steps of glycolysis are outlined in the diagram below. The first substrate in the pathway is the hexose sugar glucose. Ten enzymes are needed to carry out the reaction of the pathway. The first reactions of glycolysis involve an energy investment. ATP molecules are hydrolyzed, energy is released, and phosphoryl groups are added to the hexose sugars. In the remaining steps of glycolysis, energy is harvested to produce a net gain of ATP. There are three major products of glycolysis: the chemical energy in the form of ATP, chemical energy in the form of NADH, and two three-carbon pyruvate molecules. Chemical Energy as ATP: Four ATP molecules are formed by the process of substrate-level phosphorylation. This means that a high-energy phosphoryl group from one of the substrates in glycolysis is transferred to ADP to form ATP. The two substrates involved in these transfer reactions are 1,3-bisphosphoglycerate and phosphoenolpyruvate (steps 7 and 10). Although four ATP molecules are produced during glycolysis, the net gain is only two ATP molecules because two ATP molecules are used early in the glycolysis.  Chemical energy in the form of reduced NAD+, NADH Nicotinamide adenine dinucleotide (NAD+) is a conenzyme derived from the vitamin niacin. The reduced form of NAD+, NADH, carries hydride anions, hydrogen atoms with two electrons, removed during the oxidation of one of the substrates, glyceraldehydes-3-phosphate (step 6). Under aerobic conditions the electrons and hydrogen atom are transported from the cytoplasm into the mitochondria. Here they enter an electron transport system for the generation of ATP by oxidative phosphorylation. Under anaerobic condition, NADH is used as a source of electrons in fermentation reactions. Two Pyruvate molecules. The end product of glycolysis is pyruvate, One molecule of six-carbon glucose is converted to two three-carbon pyruvate molecule. The fate of the pyruvate also depends on whether the reactions are occurring in the presence or absence of oxygen. Under aerobic conditions (presence of oxygen) it is used to produce acetyl CoA destined for the citric acid cycle and complete oxidation. Under anaerobic conditions (absence of oxygen) it is used as an electron acceptor in formation reactions. Figure 7.3 Summary of the Reactions of Glycolysis. These reactions occur in the cell cytoplasm GLUCOSE ATP 1 Six- carbon stage ADP Glucose-6-P (ATP-requiring) 2 Fructose-6-P ATP 3 ADP Fructose-1,6-bisphosphate 4 2 Glyceraldehyde-3-P Dihydroxyacetone-P 5 2 NAD+ 2 Pi 6 2 NADH + 2H+ 2 1,3-Bisphosphoglycerate 2 ADP 7 Three - Substrate-level phosphorylation carbon 2 ATP stage (ATP- 2 3-Phosphoglycerate generating 8 2 2-Phosphoglycerate 9 2 H2 O 2 Phosphoenolpyruvate 2 ADP 10 2 ATP Substrate-level phosphorylation 2 Pyruvate STEPS: 1. Glucose is phosphorylated at the expense of ATP to produce glucose-6-phosphate. Enzyme: Hexokinase 2. Glucose-6-phosphate is rearranged to produce fructose-6-phosphate. Enzyme: Phosphoglucose isomerase 3. Fructose-6-phosphate is phosphorylated to produce fructose-1,6-bisphosphate at the expense of another ATP. The expenditure of 2 ATP represents an energy investment to “activate” the glucose for its eventual oxidation. Enzyme: Phospho-fructokinase 4 and 5. Aldolase cleaves the six-carbon fructose-1,6-bisphosphate into two nonidentical three-carbon molecules, dihydroxyacetone phosphate and glyceraldehydes-3- phosphate. The dihydroxyacetone phosphate is converted to glyceraldehydes-3- phosphate by the enzyme triose phosphate isomerase. Enzyme (4): Aldolase 6. Glyceraldehyde-3-phosphate is oxidized and NADH is produced. An inorganic phosphate group is transferred to the carboxylate group to produce 1,3-bisphosphoglycerate. Enzyme: Glyceraldehyde-3-phosphate dehydrogenase 7. ATP is produced in the first substrate level phosphorylation in the pathway. The phosphoryl group is transferred from the substrate to ADP to produce ATP. Enzyme: Phosphoglycerate 8. The C – 3 phosphoryl group of 3-phosphoglycerate is transferred to the second carbon. Enzyme: Phosphoglycerate mutase 9. Dehydration of 2-phosphoglycerate generates the energy-rich molecule phosphoenolpyruvate. Enzyme: Enolase 10. The final substrate level phosphorylation produces ATP and pyruvate. Enzyme: Pyruvate kinase REGULATION OF GLYCOLYSIS Energy harvesting pathways, like glycolysis are responsive to the energy needs of the cell. Reactions of the pathway speed up when there is a demand for ATP. They slow down when there is abundant ATP to meet the energy requirements of the cell. Mechanism for the Control of the Rate of the Glycolytic Pathway: 1. Allosteric enzymes 2. Feedback inhibition 7.4 CONVERSION OF PYRUVATE TO ACETYL CoA Figure 7.4 The Decarboxylation and Oxidation of Pyruvate to produce Acetyl CoA H O NAD+ NADH O H – C – C – C + H – S – CoA CH3 + CO2 Pyruvate H O- dehydrogenase C=O complex S – CoA Pyruvate Coenzyme A Acetyl coenzyme A Aerobic Condition: The figure above show the reaction that converts pyruvate to acetyl CoA. First, pyruvate is decarboxylated, which means that it loses a carboxyl group that is released as CO 2. Next it is oxidized, and the hydride anion that is removed is accepted by NAD +. Finally, the remaining acetyl group, CH3CO - , is linked to coenzyme A by a thioester bond. This very complex reaction is carried out by three enzymes and five coenzymes that are organized together in a single bundle called the pyruvate dehydrogenase complex. Anaerobic Condition: During strenuous muscular activity, pyruvic acid is converted into lactic acid rather than acetyl CoA. During the resting period, the lactic acid is converted back to pyruvic acid. The pyruvic acid in turn is converted back to glucose by the process called gluconeogenesis (anabolism). If the glucose is not needed at that moment, it is converted into glycogen by glycogenesis. 3 Enzymes Involved: 1. Pyruvate dehydrogenase 2. Dihydrolipoyl transacetylase 3. Dihydrolipoyl dehydrogenase 5 Coenzymes Involved: 1. TPP – thiamine pyrophosphate 2. Acetyl CoA – pantothenic acid 3. NAD+ - nicotinamide 4. FAD – riboflavin 5. Lipoamide 7.5 THE CITRIC ACID CYCLE The citric acid cycle (also known as the tricarboxylic acid cycle, the TCA cycle, or the Krebs cycle) is a series of chemical reactions of central importance in all living cells that utilize oxygen as part of cellular respiration. In aerobic organisms, the citric acid cycle is part of a metabolic pathway involved in the chemical conversion of carbohydrates, fats and proteins into carbon dioxide and water to generate a form of usable energy. It is the second of three metabolic pathways that are involved in fuel molecule catabolism and ATP production, the other two being glycolysis and oxidative phosphorylation. The citric acid cycle also provides precursors for many compounds such as certain amino acids, and some of its reactions are therefore important even in cells performing fermentation. History The citric acid cycle is also known as the Krebs cycle after Sir Hans Adolf Krebs (1900- 1981), who proposed the key elements of this pathway in 1937 and was awarded the Nobel Prize in Medicine for its discovery in 1953. It is correctly written without a possessive apostrophe. Location of cycle and inputs and outputs The citric acid cycle takes place within the mitochondrial matrix in eukaryotes, and within the cytoplasm in prokaryotes. Figure 7.5 The Citric Acid Cycle Figure 7.6 Simplified diagram of Citric Acid Cycle Fuel molecule catabolism (including glycolysis) produces acetyl-CoA, a two-carbon acetyl group bound to coenzyme A. Acetyl-CoA is the main input to the citric acid cycle. Citrate is both the first and the last product of the cycle, and is regenerated by the condensation of oxaloacetate and acetyl-CoA Tabulated Results for the enzymes involved in TCA cycle. Reactants/ Products/ Molecule Enzyme Reaction type Coenzymes Coenzymes I. Citrate 1. Aconitase Dehydration H 2O II. cis-Aconitate 2. Aconitase Hydration H 2O 3. Isocitrate III. Isocitrate Oxidation NAD+ NADH + H+ dehydrogenase 4. Isocitrate IV. Oxalosuccinate Decarboxylation dehydrogenase 5. α-Ketoglutarate Oxidative NAD+ + NADH + H+ V. α-Ketoglutarate dehydrogenase decarboxylation CoA-SH + CO2 6. Succinyl-CoA GDP GTP + VI. Succinyl-CoA Hydrolysis synthetase + Pi CoA-SH 7. Succinate VII. Succinate Oxidation FAD FADH2 dehydrogenase VIII. Fumarate 8. Fumarase Addition (H2O) H 2O 9. Malate IX. L-Malate Oxidation NAD+ NADH + H+ dehydrogenase 10. Citrate X. Oxaloacetate Condensation synthase The sum of all reactions in the citric acid cycle is: Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 3 H2O → CoA-SH + 3 NADH + H+ + FADH2 + GTP + 2 CO2 + 3 H+ Two carbons are oxidized to CO2, and the energy from these reactions is stored in GTP , NADH and FADH2. NADH and FADH2 are coenzymes (molecules that enable or enhance enzymes) that store energy and are utilized in oxidative phosphorylation. A simplified view of the process: The process begins with the oxidation of pyruvate, producing one CO 2, and one acetyl- CoA. Acetyl-CoA reacts with the four-carbon carboxylic acid, oxaloacetate--to form the six carbon carboxylic acid, citrate. Through a series of reactions citrate is converted back to oxaloacetate. This cycle produces 2 CO2 and consumes 3 NAD+, producing 3NADH and 3H+. It consumes 3 H2O and consumes one FAD, producing one FADH+. 1st turn end= 1 ATP, 3 NADH, 1 FADH2 Since there are two molecules of Pyruvic acid to deal with, the cycle turns once more. The complete end result= 2 ATP, 6 NADH, 2 FADH2 Regulation Many of the enzymes in the TCA cycle are regulated by negative feedback from ATP when the energy charge of the cell is high. Such enzymes include the pyruvate dehydrogenase complex that synthesises the acetyl-CoA needed for the first reaction of the TCA cycle. Also the enzymes citrate synthase, isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase, that regulate the first three steps of the TCA cycle, are inhibited by high concentrations of ATP. This regulation ensures that the TCA cycle will not oxidise excessive amount of pyruvate and acetyl-CoA when ATP in the cell is plentiful. This type of negative regulation by ATP is by an allosteric mechanism. Several enzymes are also negatively regulated when the level of reducing equivalents in a cell are high (high ratio of NADH/NAD+). This mechanism for regulation is due to substrate inhibition by NADH of the enzymes that use NAD+ as a substrate. This includes both the entry point enzymes pyruvate dehydrogenase and citrate synthase. Major metabolic pathways converging on the TCA cycle Most of the body's catabolic pathways converge on the TCA cycle, as the diagram shows. Reactions that form intermediates of the cycle are called anaplerotic reactions. The citric acid cycle is the second step in carbohydrate catabolism (the breakdown of sugars). Glycolysis breaks glucose (a six-carbon-molecule) down into pyruvate (a three-carbon molecule). In eukaryotes, pyruvate moves into the mitochondria. It is converted into acetyl-CoA and enters the citric acid cycle. In protein catabolism, proteins are broken down by protease enzymes into their constituent amino acids. These amino acids are brought into the cells and can be a source of energy by being funnelled into the citric acid cycle. In fat catabolism, triglycerides are hydrolyzed to break them into fatty acids and glycerol. In the liver the glycerol can be converted into glucose via dihydroxyacetone phosphate and glyceraldehyde-3-phosphate by way of gluconeogenesis. In many tissues, especially heart tissue, fatty acids are broken down through a process known as beta oxidation which results in acetyl-CoA which can be used in the citric acid cycle. Sometimes beta oxidation can yield propionyl CoA which can result in further glucose production by gluconeogenesis in liver. The citric acid cycle is always followed by oxidative phosphorylation. This process extracts the energy from NADH and FADH2, recreating NAD+ and FAD, so that the cycle can continue. The citric acid cycle itself does not use oxygen, but oxidative phosphorylation does. 7.6 OXIDATIVE PHOSPHORYLATION (Electron Transport Chain) The majority of the energy conserved during catabolism reactions occurs near the end of the metabolic series of reactions in the electron transport chain. The electron transport or respiratory chain gets its name from the fact electrons are transported to meet up with oxygen from respiration at the end of the chain. The overall electron chain transport reaction is: 2 H+ + 2 e+ + 1/2 O2 ---> H2O + energy Notice that 2 hydrogen ions, 2 electrons, and an oxygen molecule react to form as a product water with energy released in an exothermic reaction. This relatively straight forward reaction actually requires eight or more steps. The energy released is coupled with the formation of three ATP molecules per every use of the electron transport chain. Pre-Initiation of Electron Transport Chain: The electron transport chain is initiated by the reaction of an organic metabolite (intermediate in metabolic reactions) with the coenzyme NAD + (nicotinamide adenine dinucleotide). This is an oxidation reaction where 2 hydrogen atoms (or 2 hydrogen ions and 2 electrons) are removed from the organic metabolite. (The organic metabolites are usually from the citric acid cycle and the oxidation of fatty acids). The reaction can be represented simply where M = any metabolite. MH2 + NAD+ -----> NADH + H+ + M: + energy One hydrogen is removed with 2 electrons as a hydride ion (H -) while the other is removed as the positive ion (H+). Usually the metabolite is some type of alcohol which is oxidized to a ketone. NAD+ is a coenzyme containing the B-vitamin, nicotinamide, shown on a previous page. The purpose of the other seven steps in the electron transport chain is threefold: 1) to pass along 2H+ ions and 2e- to eventually react with oxygen; 2) to conserve energy by forming three ATP's; and 3) to regenerate the coenzymes back to their original form as oxidizing agents. Initiation of Electron Transport Chain: Once the NADH has been made from a metabolite in the citric acid cycle inside of the mitochondria, it interacts with the first complex 1 enzyme, known as NADH reductase. This complex 1 contains a coenzyme flavin mononucleotide (FMN) which is similar to FAD. The sequence of events is that the NADH, plus another hydrogen ion enter the enzyme complex and pass along the 2 hydrogen ions, ultimately to an interspace in the mitochondria. These hydrogen ions, acting as a pump, are utilized by ATP synthetase to produce an ATP for every two hydrogen ions produced. Three complexes (1, 3, 4) act in this manner to produce 2 hydrogen ions each, and thus will produce 3 ATP for every use of the complete electron transport chain. In addition, NADH passes along 2 electrons to first FMN, then to an iron-sulfur protein (FeS), and finally to coenzyme Q. The net effect of these reactions are to regenerate coenzyme NAD+. This regeneration of reactants occurs in many of the reactions so that a cycling effect occurs. The NAD+ is ready to react further with metabolites in the citric acid cycle. Coenzyme Q, which also picks up an additional 2 hydrogen ions to make CoQH 2, is soluble in the lipid membrane and can move through the membrane to come into contact with enzyme complex 3. In summary, the very first enzyme complex in the electron transport chain is coupled with the formation of ATP. The coupled reaction may be written as: a) MH2 + NAD+ ---> NADH + H+ + M + energy b) ADP + P + energy ---> ATP + H2O Electron Transport - Enzyme Complex 3: Coenzyme QH2 carrying an extra 2 electrons and 2 hydrogen ions now starts a cascade of events through enzyme complex 3, also known as cytochrome reductase bc. Cytochromes are very similar to the structure of myoglobin or hemoglobin. The significant feature is the heme structure containing the iron ions, initially in the +3 state and changed to the +2 state by the addition of an electron. The CoQH 2 (yellow)passes along the 2 electrons first to cytochrome (blue) b1 heme (magenta), then b2 heme , then to an iron-sulfur protein (green), then to cytochrome c1 (red with black heme), and finally to cytochrome c (not shown). Co Q is represented by the inhibitor antimycin (yellow) in the graphic. In the meantime the 2 hydrogen ions are channeled to the interspace of the mitochondria for ultimate conversion into ATP. Complex 4: Refer to the middle graphic: Cytochrome c is a small molecule which is also able to move in the lipid membrane layer and diffuses toward cytochrome a complex 4. At this time it continues the transport of the electrons, and provides the third and final time that 2 hydrogen ions are channeled to the interspace of the mitochondria for ultimate conversion into ATP. ATP synthetase is also found at numerous locations in the bilayer membrane of the mitochondria. The function of this ATP enzyme is found in an earlier page. Three ATP are produced by the pumping action of the re-entry of the hydrogen ions through the ATP synthetase. Finally, oxygen has diffused into the cell and the mitochondria for the finally reaction of metabolism. Oxygen atom reacts with the 2 electrons and 2 hydrogens to produce a water molecule. Electron Transport Chain Diagram  THE NUMBER OF ATP PRODUCED BY THE COMPLETE OXIDATION OF ONE MOLECULE OF GLUCOSE Glycolysis Substrate-level phosphorylation ……………………………………… 2 ATP 2NADH X 2 ATP/cytoplasmic NADH ………………………………... 4 ATP Conversion of 2 pyruvate molecules to 2 acetyl CoA molecules: 2 NADH X 3 ATP/NADH ………………………………………………. 6 ATP Citric acid cycle (two turns): 2 GTP X 1 ATP/GTP …………………………………………………… 2 ATP 6 NADH X 3 ATP/NADH ………………………………………………. 18 ATP 2 FADH2 X 2 ATP/FADH2 ………………………………...…………... 4 ATP TOTAL : 36 ATP METABOLISM OF CARBOHYDRATES, LIPIDS, AND PROTEIN 7.7 CARBOHYDRATE METABOLISM Carbohydrates made up of carbon, hydrogen, and oxygen atoms are classified as mono-, di-, and polysaccharides, depending on the number of sugar units they contain. The monosaccharides—glucose, galactose, and fructose—obtained from the digestion of food are transported from the intestinal mucosa via the portal vein to the liver. They may be utilized directly for energy by all tissues; temporarily stored as glycogen in the liver or in muscle; or converted to fat, amino acids, and other biological compounds. Carbohydrate metabolism plays an important role in both types of diabetes mellitus. The entry of glucose into most tissues—including heart, muscle, and adipose tissue—is dependent upon the presence of the hormone insulin. Insulin controls the uptake and metabolism of glucose in these cells and plays a major role in regulating the blood glucose concentration. The reactions of carbohydrate metabolism cannot take place without the presence of the B vitamins, which function as coenzymes. Phosphorous, magnesium, iron, copper, manganese, zinc and chromium are also necessary as cofactors. Carbohydrate metabolism begins with glycolysis, which releases energy from glucose or glycogen to form two molecules of pyruvate, which enter the Krebs cycle (or citric acid cycle), an oxygen-requiring process, through which they are completely oxidized. Before the Krebs cycle can begin, pyruvate loses a carbon dioxide group to form acetyl coenzyme A (acetyl- CoA). This reaction is irreversible and has important metabolic consequences. The conversion of pyruvate to acetyl-CoA requires the B vitamins. The hydrogen in carbohydrate is carried to the electron transport chain, where the energy is conserved in ATP molecules. Metabolism of one molecule of glucose yields thirty-one molecules of ATP. The energy released from ATP through hydrolysis (a chemical reaction with water) can then be used for biological work.  Only a few cells, such as liver and kidney cells, can produce their own glucose from amino acids, and only liver and muscle cells store glucose in the form of glycogen. Other body cells must obtain glucose from the bloodstream. Under anaerobic conditions, lactate is formed from pyruvate. This reaction is important in the muscle when energy demands exceed oxygen supply. Glycolysis occurs in the cytosol (fluid portion) of a cell and has a dual role. It degrades monosaccharides to generate energy, and it provides glycerol for triglyceride synthesis. The Krebs cycle and the electron transport chain occur in the mitochondria. Most of the energy derived from carbohydrate, protein, and fat is produced via the Krebs cycle and the electron transport system. Glycogenesis is the conversion of excess glucose to glycogen. Glycogenolysis is the conversion of glycogen to glucose (which could occur several hours after a meal or overnight) in the liver or, in the absence of glucose-6-phosphate in the muscle, to lactate. Gluconeogenesis is the formation of glucose from noncarbohydrate sources, such as certain amino acids and the glycerol fraction of fats when carbohydrate intake is limited. Liver is the main site for gluconeogenesis, except during starvation, when the kidney becomes important in the process. Disorders of carbohydrate metabolism include diabetes mellitus, lactose intolerance, and galactosemia. 7.8 PROTEIN METABOLISM Proteins contain carbon, hydrogen, oxygen, nitrogen, and sometimes other atoms. They form the cellular structural elements, are biochemical catalysts, and are important regulators of gene expression. Nitrogen is essential to the formation of twenty different amino acids, the building blocks of all body cells. Amino acids are characterized by the presence of a terminal carboxyl group and an amino group in the alpha position, and they are connected by peptide bonds. Digestion breaks protein down to amino acids. If amino acids are in excess of the body's biological requirements, they are metabolized to glycogen or fat and subsequently used for energy metabolism. If amino acids are to be used for energy their carbon skeletons are converted to acetyl CoA, which enters the Krebs cycle for oxidation, producing ATP. The final products of protein catabolism include carbon dioxide, water, ATP, urea, and ammonia. Vitamin B6 is involved in the metabolism (especially catabolism) of amino acids, as a cofactor in transamination reactions that transfer the nitrogen from one keto acid (an acid containing a keto group [-CO-] in addition to the acid group) to another. This is the last step in the synthesis of nonessential amino acids and the first step in amino acid catabolism. Transamination converts amino acids to L-glutamate, which undergoes oxidative deamination to form ammonia, used for the synthesis of urea. Urea is transferred through the blood to the kidneys and excreted in the urine. The glucose-alanine cycle is the main pathway by which amino groups from muscle amino acids are transported to the liver for conversion to glucose. The liver is the main site of catabolism for all essential amino acids, except the branched-chain amino acids, which are catabolized mainly by muscle and the kidneys. Plasma amino-acid levels are affected by dietary carbohydrate through the action of insulin, which lowers plasma amino-acid levels (particularly the branched-chain amino acids) by promoting their entry into the muscle. Body proteins are broken down when dietary supply of energy is inadequate during illness or prolonged starvation. The proteins in the liver are utilized in preference to those of other tissues such as the brain. The gluconeogenesis pathway is present only in liver cells and in certain kidney cells. Disorders of amino acid metabolism include phenylketonuria, albinism, alkaptonuria, type 1 tyrosinaemia, nonketotic hyperglycinaemia, histidinaemia, homocystinuria, and maple syrup urine disease. 7.9 FAT (LIPID) METABOLISM Fats contain mostly carbon and hydrogen, some oxygen, and sometimes other atoms. The three main forms of fat found in food are glycerides (principally triacylglycerol [triglyceride], the form in which fat is stored for fuel), the phospholipids, and the sterols (principally cholesterol). Fats provide 9 kilocalories per gram (kcal/g), compared with 4 kcal/g for carbohydrate and protein. Triacylglycerol, whether in the form of chylomicrons (microscopic lipid particles) or other lipoproteins, is not taken up directly by any tissue, but must be hydrolyzed outside the cell to fatty acids and glycerol, which can then enter the cell. Fatty acids come from the diet, adipocytes (fat cells), carbohydrate, and some amino acids. After digestion, most of the fats are carried in the blood as chylomicrons. The main pathways of lipid metabolism are lipolysis, betaoxidation, ketosis, and lipogenesis. Lipolysis (fat breakdown) and beta-oxidation occurs in the mitochondria. It is a cyclical process in which two carbons are removed from the fatty acid per cycle in the form of acetyl CoA, which proceeds through the Krebs cycle to produce ATP, CO2, and water. Ketosis occurs when the rate of formation of ketones by the liver is greater than the ability of tissues to oxidize them. It occurs during prolonged starvation and when large amounts of fat are eaten in the absence of carbohydrate.  Lipogenesis occurs in the cytosol. The main sites of triglyceride synthesis are the liver, adipose tissue, and intestinal mucosa. The fatty acids are derived from the hydrolysis of fats, as well as from the synthesis of acetyl CoA through the oxidation of fats, glucose, and some amino acids. Lipogenesis from acetyl CoA also occurs in steps of two carbon atoms. NADPH produced by the pentose-phosphate shunt is required for this process. Phospholipids form the interior and exterior cell membranes and are essential for cell regulatory signals. DEFINITION OF TERMS: 1. Anabolism – a metabolic process in which energy is used to make a compounds and tissue from simple molecules. 2. ATP (adenosine triphosphate) – nucleotide with three phosphate groups; the energy currency 3. Catabolism (Metabolism) – the production of energy through the conversion of complex molecules into simple ones. 4. Cellular Respiration – metabolic reactions that use the energy primarily from carbohydrates but also from fatty acid or amino acid breakdown to produce ATP molecules. 5. Citric Acid Cycle – cycle of reactions in mitochondria that begins with citric acid; it breaks down an acetyl group as CO2, ATP, NADH, and FADH2 are given off; also called the Krebs cycle. 6. Gluconeogenesis – the production of glucose, especially in the liver, from amino acids, fats, and other substances that are not carbohydrates. 7. Glycogenolysis – the breakdown of glycogen to glucose. 8. Glycogenesis – the formation of glycogen from glucose. 9. Glycolysis – anaerobic breakdown of glucose that results in a gain of two ATP. 10. Lipogenesis – the formation of fatty acids and other lipids in the body. 11. Oxidative phosphorylation – the production of ATP from ADP or phosphate in the final stages of aerobic respiration. 12. Transamination – the formation of one amino acid from another. METABOLISM SUMMARY