Carbohydrates Chemistry, Classification & Metabolism PDF

CARBOHYDRATES Chemistry, Classifications, Metabolism And Diabetes Mellitus Clinical Chemistry (Michael L. Bishop), Lippincotts Illustrated Reviews Biochemistry Stryer’s Biochemistry Harper’s Illustrated Biochemistry 1 This Chapter Explains abo...

CARBOHYDRATES Chemistry, Classifications, Metabolism And Diabetes Mellitus Clinical Chemistry (Michael L. Bishop), Lippincotts Illustrated Reviews Biochemistry Stryer’s Biochemistry Harper’s Illustrated Biochemistry 1 This Chapter Explains about:  The terms monosaccharide, disaccharide, oligosaccharide, and polysaccharide.  Different structures of glucose and other monosaccharides, and describe the various types of isomerism.  Describe the digestion, absorption of carbohydrates  Carbohydrate metabolism and reactions of the citric acid cycle to yield ATP.  Glucose metabolism (Glycogenesis, Glycogenolysis, Gluconeogenesis, Glycolysis)  Hormonal regulation of carbohydrate metabolism.  Diabetes Mellitus 2  What are CARBOHYDRATES? Are organic molecules They are widely distributed in plants and animals Are composed of carbon, hydrogen and oxygen Are called as SACCHARIDES (means SUGAR) Are ALDEHYDE or KETONE derivatives of POLYHYDRIC ALCOHOLS 3  BIOMEDICAL IMPORTANCE OF CARBOHYDRATES Carbohydrates are a dietary source of energy. Precursors of many organic compounds, such as fats and amino acids Glycoprotein and glycolipids in the cell membrane. Diseases such as , diabetes mellitus, galactosemia, glycogen storage disease (GSD) and lactose intolerance are due to defects in carbodydrate metabolism. 4  CLASSIFICATION of CARBOHYDRATES: Carbohydrates are classified into Four major groups: 1. Monosaccharides 2. Disaccharides 3. Oligosaccharides 4. Polysaccharides 5 1. Monosaccharides : They are simple sugars, containing ONE sugar unit. Composed of multiple hydroxyl groups. Based on the number of carbons atoms monosaccharide is called as a  triose (3 carbon)  tetrose (4 carbon)  pentose (5 carbon) eg.: ribose  hexose (six carbon), etc. Examples such as glucose, galactose, and fructose are known as HEXOSES. They are called as “ SINGLE SUGARS” or MONOSACCHARIDES 6  MONOSACCHARIDE may be an ALDOSE or KETOSE sugar 1 ALDOSES (example, GLUCOSE) have an aldehyde at one end (C#1). Therefore, called as ALDOHEXOSE KETOSES (example, FRUCTOSE) have a keto group, at C#2. Therefore called as KETOHEXOSE 2 7  Nomenclature (names) for stereoisomers: The central carbons of a carbohydrate are asymmetric (chiral)— means four different 1 1 groups are attached to the carbon atoms. This allows for various spatial arrangements around each asymmetric carbon forming molecules called stereoisomers. D and L designations are based on the configuration around the single asymmetric carbon in Glyceraldehyde. 8  Enantiomers Diastereomers Epimers O O O O O O H H H H H H C C C C C C HO C* H H C* OH HO C* H H C* OH H C* OH HO C* H H C* OH HO C* H HO C* H HO C* H HO C* H HO C* H HO C* H H C* OH H C* OH HO C* H H C* OH H C* OH HO C* H H C* OH H C* OH H C* OH H C* OH H C* OH CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH L-glucose D-glucose D-mannose D-galactose D-glucose D-mannose Enantiomers = two chemical structure that are mirror images to each other e.g.: L-Glucose and D-Glucose. Pairs of isomers that have opposite configurations at one or more chiral centers but are NOT mirror images are diastereomers Epimers = Two sugars that differ in configuration at only one chiral center  ISOMERS of GLUCOSE Compounds that have same chemical formula but have different arrangements of atoms are called as ISOMERS. Examples; FRUCTOSE and GALACTOSE are isomers of GLUCOSE to each other. 10  D or L designation refers to the asymmetric carbon farthest from the aldehyde or keto group. Most naturally occurring sugars are D isomers. D & L sugars are mirror images of one another. For example, D-glucose and L-glucose are shown at right side of the slide. D-glucose and L-glucose are ENANTIOMERS (non-superimposable 11 COMPLETE mirror images of each other). 11  Cyclization of sugar structure An aldehyde can react with an alcohol to form a hemiacetal. Similarly a ketone can react with an alcohol to form a hemiketal. 12 12  Cyclization of sugar structure Pentoses and hexoses can cyclize, as the aldehyde or keto group reacts with a hydroxyl group on one of the distal carbons. Example: glucose forms an intra- molecular hemiacetal by reaction of the aldehyde on C1 with the hydroxyl on C5. By this reaction six-member pyranose ring (pyran compound) is formed. The representations of the cyclic sugars at right are called Haworth projections. 13 13  If at C1 the OH group below the ring, it forms (α)-D-glucose. If at C1 the OH group above the ring, it forms (β) -D-glucose. 14 α-Glucoyranose 15 16  2.DISACCHARIDES: TWO monosaccharides are covalently linked together by a GLYCOSIDIC bonds to form a “DOUBLE sugar” or “Disaccharide”. There are THREE disaccharides: DISACCHARIDE DIETARY SOURCE MADE FROM SUCROSE TABLE SUGAR GLUCOSE + FRUCTOSE LACTOSE MAJOR SUGAR IN MILK GLUCOSE + GALACTOSE MALTOSE PRODUCT OF STARCH GLUCOSE + GLUCOSE DIGESTION 17  DISACCHARIDES are of TWO types: REDUCING DISACCHARIDE: Glucose Glucose is a disaccharide with FREE ALDEHYDE or KETO Group. Non-Reducing End Reducing End Examples : MALTOSE and LACTOSE NON-REDUCING Non- DISACCHARIDE: is a Non-Reducing End Reducing End disaccharide with NO FREE ALDEHYDE or KETO Glucose Fructose GROUP. Examples: SUCROSE 18  3.OLIGOSACCHARIDES: are formed when THREE(3) to TEN(10) monosaccharides are linked together by covalent glycosidic bonds. Example: TRISACCHARIDES 4.POLYSACCHARIDE: They are formed following linkage of many monosaccharide (glucose units) or disaccharide units through covalent glycosidic bonds 19 POLYSACCHARIDES: are A. Homopolysaccarides B. Hetropolysaccaharides A. Homopolysaccharides: The polysaccharides which are composed of same types of sugar units. (i) STARCH: A homopolymer of D-glucose units held by α- glycosidic bonds. Starch can solely be found in green plants and staple food such as oats, barley and potatoes. 20  STARCH has TWO forms: (a) Amylose (long unbranched chain), glucose units are held by α-(14) glycosydic bond α- (14) (a) Amylopectin (branched chain). It is linked by α-(14) bond in chain and α-(16) glycosydic bond at branch points. α- (16) 21 α- (14)  (ii) GLYCOGEN: is carbohydrate reserve (storage) in animals in their liver and muscle. Glucose is repeating unit in glycogen joined by α-(14) glycosidic bonds, and α-(16) glycosidic bonds at branching points. Glycogen α- (16) 22 α- (14) (iii) CELLULOSE: It is the most abundant type of polysaccharide in nature. It’s the major component of plant cell wall. Made up of linear unbranched chains of β-D- glucose units linked by β-1,4- glycosidic bond. It’s not digested by humans since we lack the enzyme cellulase to hydrolyze the β-1,4- glycosidic bond. 23 (a)  and  glucose ring structures  Glucose  Glucose (b) Starch: 1–4 linkage of  glucose monomers (c) Cellulose: 1–4 linkage of  glucose monomers 24 B- Heteropolysaccharides The polysaccharides which are composed of different types of sugars or derivatives are called as HETEROPOLYSACCHARIDES or HETEROGLYCANS. Example: MUCOPOLYSACCHARIDES commonly known as GLYCOSAMINOGLYCANS Are components of tissue structure (connective tissue-cartilage, skin, blood vessels, tendons) in collagen and elastin fiber 25 25  MUCOPOLYSACCHARIDES Important mucopolysaccharides are: (i) CHONDROITIN SULFATE: major constituent of mammalian tissues (bone, cartilage, tendons, heart valves, skin, cornea). It consists of polymers of disaccharides with D-glucuronic acid and N-acetyl D-galactoseamine sulfate (ii) HYALURONIC ACID: found in synovial fluid, skin. It is an important lubricant in the cells. It is made up of D-glucuronic acid and N-acetylglucosamine 26 26 (iii) HEPARIN: is an anticoagulant (prevents blood clotting) found in blood, lung, liver, kidney, spleen. It is composed of glucosamine and glucuronic acid 27 27 (iv) CHITIN Makes up the exoskeleton of insects and crustaceans. Provides structural support for the cell walls of many fungi (Leathery texture). It is made up of N-acetyl glucosamine containing β-1,4- glycosidic bond. It is used as a strong and flexible surgical thread that biodegrades as a wound heals. Used to waterproof paper, and in cosmetics and lotion to retain moisture. 28 29 Digestion and Absorption of Carbohydrate 30  Digestion of Carbohydrate Daily Intake of Carbohydrate: A minimum daily intake of 50-100 g CHO is recommended in adult humans to prevent loss of muscle protein. The sites of digestion of CHO are mouth and intestinal lumen. Digestion begins in the mouth. The dietary polysaccharides are of animal (glycogen) and plant origin (starch). During mastication (chewing) of food, salivary α- amylase acts briefly on starch in a random manner, 31 breaking some α-(1→4) glycosidic bonds.  Digestion contd… The digestive product in the mouth are mixer of smaller, branched oligosaccharide molecules. Further digestion occur in small intestine. The pancreatic amylase, oligosaccharidases, disaccharidases hydrolyze the CHO. The intestinal mucosa secretes a group of three disaccharidases Maltase digest maltose to produce glucose and glucose. Sucrase digest sucrose to produce glucose and fructose Lactase digest lactose to produce galactose and glucose. 32  Absorption of Carbohydrates The monosaccharides are absorbed by the intestinal mucosa: glucose and galactose by active transport and fructose by passive transport. The monosaccharides are then transported by the portal vein to the liver. 33  Absorption of Carbohydrates contd.. The monosaccharides are transported into the intestinal cells by active transport (energy requiring process) that involves specific transport proteins. Na ions are involved and co-transported in some glucose transporters. 34  Glucose is the only carbohydrate to be directly used for energy or stored as glycogen. Galactose and fructose must be converted to glucose before they can be used. 35  METABOLISM OF CARBOHYDRATES 36  Metabolism Metabolic pathways can be catabolic (degradative) or anabolic (synthetic) i. Catabolic pathways break down complex molecules into simple molecules such as CO2 , NH3 and water. ii. Anabolic pathways form complex molecules from simple molecules. E.g. Protein synthesis from amino acids. 37 Three major biochemical pathways are involved: 1) Glycolysis pathway and Krebs Cycle 2) Hexose monophosphate shunt pathway (HMP shunt) or pentose phosphate pathway 3) Glycogenesis 38  1- Glycolysis Glycolysis occurs in the cytosol of all tissues. It is the breakdown of glucose to provide energy in the form of ATP, also some intermediate molecules for other metabolic pathways. Conversion of glucose into pyruvate, then to lactate is anaerobic glycolysis, because it occurs in the absence of oxygen. Conversion of glucose into pyruvate, then to acetyl CoA is aerobic glycolysis, because, it occurs in presence of oxygen. 39  Glycolysis contd… The conversion of glucose into pyruvate occurs in two stages, First 5 reactions consume energy, latter 5 reactions generate energy. 40  Glycolysis Glucose 3 regulated steps 10 steps No O2 Pyruvate Lactate Energy (ATP) and metabolites 41  Glycolysis Triose phosphate isomerase Hexokinase (regulatory) Glyceraldehyde 3- phosphate dehydrogenase Phosphoglucose isomerase Phosphoglycerat e kinase Phosphofructo Kinase-1 (regulatory) Phosphoglycerate mutase Aldolase A Enolase Pyruvate Kinase (regulatory) 42 Glucose +2 NAD+ +2 ADP +2 Pi 2 pyruvate+2 NADH + 2 ATP G-6-P is important compound for many metabolic pathways – glycolysis, gluconeogenesis, pentose phosphate pathway (PPP), glycogenesis and glycogenolysis. In this form glucose is trapped inside the cell.  Regulation of glycolysis The Glycolysis pathway is regulated by control of 3 enzymes (Irreversible steps): Hexokinase Phosphofructokinase I Pyruvate Kinase 44  Reduction of pyruvate to lactate Lactate formed by the action of lactate dehydrogenase is the final product of anaerobic glycolysis. Skeletal muscles ferment glucose to lactate during exercise, when the exertion is brief and intense. Cell membranes contain carrier proteins that facilitate transport of lactate.  Cori Cycle Lactate is released into the blood by exercising muscle and by cells that lack mitochondria such as RBCs. This lactate is taken up by the liver and converted to glucose which is released back into the circulation. This cycle is known as Cori cycle.  Energy production from Glycolysis Anaerobic glycolysis produces 2X2 =4 ATP when glucose is converted to lactate at the substrate level. Since two ATP are consumed, the net production of ATP is 2. Aerobic glycolysis produces 2 net ATP when glucose is converted to pyruvate and 2 NADH. Each NADH gives 3 ATP when oxidized in electron transport chain. So net production of ATP is 8.  Glycolysis: Energy balance sheet Hexokinase: - 1 ATP (taking) Phosphofructokinase: -1 ATP (taking) NADPH: +2 NADPH (3ATP from each NADPH) = 6ATP (giving) Phsophoglycerate kinase: +2 ATP (giving) Pyruvate kinase: +2 ATP (giving) Total ATP/ molecule of Glucose: +2 ATP, +2 NADH = 2 + 6 = 8ATP 48 Recall: Location Glycolysis occurs in the cytoplasm (cytosol). Krebs cycle occurs in the matrix. The end product of glycolysis (pyruvate) must make its way into the mitochondria. 49 Oxidative Decarboxylation 50  Oxidative decarboxylation of pyruvate Pyruvate, the end product of glycolysis, must be transported into the mitochondria before it enters the TCA cycle. In the mitochondrial matrix pyruvate is oxidized and decarboxylated to acetyl CoA by pyruvate dehydrogenase complex.  TCA cycle The tricarboxylic acid cycle, also called Krebs cycle or citric acid cycle, that plays several important roles in metabolism. It is the key metabolic pathway that connects Carbohydrate, fat and protein metabolism. The reactions of the cycle are carried out by eight enzymes that completely oxidize acetate (acetyl-CoA), into two molecules each of carbon dioxide and water. Oxidation phosphorylation of NADH and FADH2 provides energy in form of ATP.  TCA contd… The cycle occurs mainly in the mitochondria, it is close to the electron transport chain, which oxidize the reduced coenzymes (NADH) produced by the cycle. The TCA cycle is thus a aerobic pathway because O2 is required as the final electron acceptor.  Reaction of TCA cycle In TCA cycle, oxaloacetate is first condense with acetyl group of acetyl CoA, and then is regenerated at the end as the cycle is completed  Reactions of TCA cycle 1. Oxaloacetate + Acetyl CoA = Citrate catalyzed by citrate synthase 2. Citrate → Isocitrate , catalyzed by aconitase 3. Isocitrate + NAD = α ketoglutarate + NADH + H+ CO2 catalyzed by isocitrate dehydrogenase 4. α ketoglutarate + NAD = Succinyl CoA+ NADH +H + CO2 catalyzed by α ketoglutarate dehydrogenase complex  Reactions of TCA cycle 5. Succinyl CoA +GDP+ Pi = Succinate + GTP + CoA catalyzed by Succinate thiokinase 6. Succinate +FAD = Fumarate + FADH2 catalyzed by succinate dehydrogenase 7. Fumarate + H2O = Malate, catalyzed by fumarase 8. Malate + NAD = Oxaloacetate + NADH+H, by malate dehydrogenase The Net Equation Acetyl CoA + 3 NAD + FAD + ADP + HPO4-2 ————> 2 CO2 + CoA + 3 NADH+ + FADH+ + GTP In each cycle two molecules of CO2 , three NADH, one FADH2 & one GTP is produced. NADH & FADH2 are oxidized in the electron transport chain to produce ATP. Multiple these by 2 because for every 1 glucose we get 2 pyruvate molecules and thus 2 acetyl CoA molecules. 57  Activators and Inhibitors of TCA cycle Activated by Ca, ADP (low energy) Inhibited by ATP, NADH (high energy), Citrate. Control enzymes 1. Citrate synthase 2. Isocitrate dehydrogenase 3. α ketoglutarate dehydrogenase complex  PENTOSE PHOSPHATE PATHWAY The pentose phosphate pathway (PPP) also called hexose monophosphate shunt (HMP). It is alternate glucose metabolic pathway It takes place in the cytosol.  The pentose phosphate pathway has two main functions: 1. Generation of NADPH (reductant) which is required for many biosynthetic pathways and for detoxification of reactive oxygen species. 2. Production of ribose sugar for nucleotide and nucleic acid synthesis. 60  It reconnects with glycolysis because two of the end products of the pentose pathway are glyceraldehyde 3-P and fructose 6-P; two intermediates further down in the glycolytic pathway. 62 The key enzyme in the pentose phosphate pathway Glucose-6-Phosphate dehydrogenase (G6PD) Catalyzing the first reaction in the pathway which converts glucose-6P to 6-phosphogluconolactone. This reaction is the commitment step in the pathway (Regulatory enzyme). It is feedback-inhibited by NADPH. Medically related enzyme. 63  Four major terms are used to describe what occurs in glucose metabolism. 1. Glycogenesis 2. Glycogenolysis 3. Gluconeogenesis 4. Glycolysis 64  1. Glycogenesis (glycogen synthesis) Glycogenesis is the process of glycogen synthesis, in which glucose molecules are added to the chains of glycogen for storage. This process is activated during rest periods and also activated by insulin in response to high glucose levels, for example after a carbohydrate- containing meal. 65 Glycogenesis (glycogen synthesis) cont...  2. Glycogenolysis This is the process of glycogen breakdown to form glucose molecules. Glycogenolysis occurs in the cytoplasm and is stimulated by glucagon and adrenaline hormones. 67  The two steps of glycogenolysis are; i) strand- shortening - during which glycogen polymer breaks into short strands via phosphorolysis ii) branch removal, during which free glucose is produced by debranching of glycerol. The enzymes required for this process are glycogen phosphorylase, debranching enzyme, and amylo-α-1, 6-glucosidase. 68 Glycogenolysis contd…  3. Gluconeogenesis Gluconeogenesis is the process of producing glucose from non-carbohydrate sources. Tissues such as the brain, RBC, Kidney medulla, lens, cornea and exercising muscle, require continuous supply of glucose as a metabolic fuel. Liver glycogen can meet these needs for only 10 to 18 hours in the absence of dietary intake of CHO.  During prolonged fast, hepatic glycogen stores are depleted and glucose is formed from lactate, pyruvate, glycerol (from TAG) and α ketoacids (from AA). After overnight fast, 90% gluconeogenesis occur in liver and 10% in the kidneys. However, during prolonged fasting, the kidneys become major producers, providing 40% of glucose. 71  Hormonal regulation of carbohydrate metabolism Several hormones from different glands work together to regulate blood glucose levels. Insulin is the most important hormone that lowers blood glucose when glucose levels are elevated. Glucagon, increases levels when blood glucose concentrations are too low, as occurs during fasting or between meals. These two groups of hormones are counter regulatory, meaning they prevent each other from getting out of control. 72  Hormonal regulation glycogen metabolism contd… Insulin increases glycogen synthesis and decreases glycogen degradation. Lack of insulin action results in the hyperglycemia and cause diabetes mellitus and other pathological disorders. Glucagon (in liver) and Epinephrine ( in muscle and liver) does the opposite means it decrease glycogen synthesis and increase glycogen degradation through cAMP.  Blood glucose regulation by Insulin Insulin is a polypeptide hormone produced by the β cells of the islets of Langerhans of the pancreas in response of hyperglycemia. Insulin secretion is closely coordinated with the release of glucagon by pancreatic α cells. 74  The β islet cells are freely permeable to glucose via the GLUT 2 transporter, and the glucose is phosphorylated by glucokinase. Substances causing release of insulin from the pancreas include amino acids, free fatty acids, ketone bodies, glucagon, and the sulfonylurea drugs tolbutamide and glyburide.  Insulin lowers blood glucose immediately by enhancing glucose transport into adipose tissue and muscle by recruitment of glucose transporters (GLUT 4) from the interior of the cell to the plasma membrane. Cell 76  Glucagon Opposes the Actions of Insulin Glucagon is the hormone produced by the α-cells of the pancreatic islets. Its secretion is stimulated by hypoglycemia. In the liver, it stimulates glycogenolysis by activating phosphorylase. Glucagon also enhances gluconeogenesis from amino acids and lactate. Both hepatic glycogenolysis and gluconeogenesis contribute to the hyperglycemic effect of glucagon, 77 whose actions oppose those of insulin. Carbohydrate metabolic disorder Diabetes mellitus (DM)  Diabetes mellitus Diabetes mellitus (DM) is a group of metabolic disorder characterized by high blood sugar (glucose) levels that result from defects in insulin secretion, or insulin action, or both. In patients with diabetes, the absence or insufficient production of insulin causes hyperglycemia. There are three types of diabetes mellitus: (i) Type I DM (ii) Type II DM (iii) Gestational diabetes 1- Type 1 diabetes It is also called insulin dependent diabetes mellitus (IDDM), or juvenile onset diabetes mellitus. In type 1 diabetes, the beta cells of pancreas undergoes an autoimmune attack by the body itself, and hence destruction of these cells causes absolute lack of insulin. Type 1 diabetes tends to occur at very young age usually before 30 years of age. 2- Type 2 diabetes Non-insulin dependent diabetes mellitus (NIDDM), or adult onset diabetes mellitus (AODM). In type 2 diabetes, patients can still produce insulin, but it is relatively inadequately for their body's needs, particularly because of insulin resistance. A major feature of type 2 diabetes is a lack of sensitivity to insulin by the cells of the body (particularly adipose tissue and muscle cells).  The release of insulin by the pancreas may also be defective and suboptimal. There is a known steady decline in beta cell production of insulin so require insulin therapy. Liver in these patients continues to produce glucose through (gluconeogenesis). 82  3- Gestational diabetes It occurs temporarily during pregnancy. Significant hormonal changes occurs during pregnancy that can lead to blood sugar elevation in genetically predisposed individuals. Blood sugar elevation during pregnancy is called gestational diabetes. Gestational diabetes usually resolves once the baby is born. However, 25%-50% of women with gestational diabetes will eventually develop type 2.  What are Symptoms of Diabetes Elevated blood sugar Loss of glucose in the urine Increased urine output (polyuria) Dehydration causing increased thirst and water consumption. Weight loss Fatigue Nausea and vomiting. Extremely elevated glucose levels can lead to lethargy and coma.  How is diabetes diagnosed? 1- Measurement of The fasting blood glucose level. After the person has fasted overnight (at least 8 hours). Normal fasting plasma glucose levels should be between 70 – 110 mg/dl. Fasting plasma glucose levels of more than 126 mg/dl on two or more tests on different days indicate diabetes. 2- Random blood glucose test: This can also be used to diagnose diabetes. A blood glucose level of 200 mg/dl or higher indicates diabetes. 3- Oral glucose tolerance test (OGTT): It is not routinely used. the person must be in good health (not have any other illnesses, active, not be taking medicines). Patient is given 75 gram of glucose and glucose measured every 30 min. for 5 hours. Interpretation of the OGTT curve result  Normal response: A person is said to have a normal response when the 2-hour glucose level is less than 140 mg/dl, and all values between 0 and 2 hours are less than 200 mg/dl. Impaired glucose tolerance: A person is said to have impaired glucose tolerance when the fasting plasma glucose is less than 126 mg/dl and the 2-hour glucose level is between 140 and 199 mg/dl. Diabetes: A person has diabetes when two diagnostic tests done on different days show that the blood glucose level is high. Gestational diabetes: A woman has gestational diabetes when she has any two of the following: a 100g OGTT, a fasting plasma glucose of more than 95 mg/dl, a 1-hour glucose level of more than 180 mg/dl, a 2-hour glucose level of more than 155 mg/dl, or a 3-hour glucose level of more than 140 mg/dl.  Lactose intolerance It is the inability to metabolize lactose, because Lactase is absent in the intestinal system or its availability is lowered. It is estimated that 75% of adults show some decrease in lactase activity during adulthood worldwide 89  References 1- Harper’s Biochemistry. Twenty-fifth edition. Eds: RK Murray, DK. Granner, PA. Mayes and VW Rodwell. 2- Lippincott’s Illustrated Reviews, Biochemistry, Fourth edition. Eds: Richard A. Harvey, Pamela C. Champe. Lippincott Williams & Wilkins. 90