Carbohydrate Metabolism

Chapter 8 Carbohydrate Metabolism Overview Metabolism Section 8.1: Glycolysis Section 8.2: Gluconeogenesis Section 8.3: The Pentose Phosphate Pathway Section 8.4: Metabolism of Other Important Sugars Section 8.5: Glycogen Metabolism From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Metabolism and Jet Engines Carbohydrate Metabolism: Biochemical processes responsible for the formation, breakdown and interconversion of carbohydrates in living organisms Energy transforming pathways of carbohydrate metabolism include: glycolysis, Glycogenesis Glycogenolysis Gluconeogenesis pentose phosphate pathway From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Glycolysis occurs in almost every living cell Section 8.1: Glycolysis Figure 8.2 Major Pathways in Carbohydrate Metabolism Glycolysis occurs in almost every living cell Ancient process central to all life Splits glucose into two three-carbon pyruvate units Catabolic process that captures some energy as 2 ATP and 2 NADH From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

ATP Adenosine triphosphate (ATP) is a nucleoside triphosphate used in cells as a coenzyme to store and transport chemical energy

NAD+/NADH Nicotinamide adenine dinucleotide (NAD) is a coenzyme found in all living cells that transfers electrons between reactions in metabolism.

Nicotinamide adenine dinucleotide phosphate NADPH is the reduced form of NADP+. NADP+ differs from NAD+ in the presence of an additional phosphate group on the 2' position of the ribose ring that carries the adenine moiety. NADPH is a cofactor used in anabolic reactions, such as lipid and nucleic acid synthesis, which require NADPH as a reducing agent.

Glycolysis is an anaerobic process

Pyruvate The conjugate base of pyruvic acid It is a key intermediate in several metabolic pathways Made from glucose through glycolysis Converted back to carbohydrates (such as glucose) via gluconeogenesis, or to fatty acids through acetyl-CoA. Used to construct the amino acid alanine, and be converted into ethanol or lactic acid via fermentation. Pyruvic Acid Alanine

Section 8.1: Glycolysis The Fates of Pyruvate Figure 8.7 The Fates of Pyruvate The Fates of Pyruvate Under aerobic conditions, pyruvate is converted to acetyl-CoA for use in the citric acid cycle and electron transport chain From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.1: Glycolysis The Fates of Pyruvate Figure 8.7 The Fates of Pyruvate The Fates of Pyruvate Under anaerobic conditions pyruvate can undergo fermentation: alcoholic or homolactic From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.1: Glycolysis Energetics of Glycolysis Figure 8.9 Free Energy Changes during Glycolysis in Red Blood Cells Energetics of Glycolysis In red blood cells, only three reactions have significantly negative DG values Many reactions are reversible From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Regulation of Glycolysis Section 8.1: Glycolysis Regulation of Glycolysis The rate of the glycolytic pathway in a cell is controlled by the allosteric enzymes: Hexokinases I, II, and III Phosphofructokinase 1 (PFK-1) Pyruvate kinase Reactions catalyzed by these allosteric enzymes can be turned on or off by activator or inhibitor molecules From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Regulation of Glycolysis Continued Section 8.1: Glycolysis Regulation of Glycolysis Continued Pyruvate kinase (converts PEP to pyruvate) activated by high AMP concentrations, inhibited ATP PFK-1 is activated by fructose-2,6-bisphosphate, produced via hormone- induced covalent modification of PFK-2 From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.1: Glycolysis The peptide hormones glucagon and insulin also regulate glycolysis/gluconeogenesis pathways Glucagon is released by pancreatic alpha-cells when blood glucose is low; triggers a series of reactions that inhibit glycolysis and activate gluconeogenesis Insulin is released by pancreatic beta-cells when blood glucose is high; triggers a series of reactions that activate glycolysis and inhibit gluconeogenesis From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.2: Gluconeogenesis Gluconeogenesis is the formation of new glucose molecules from precursors in the liver Occurs when blood sugar levels are low and liver glycogen is depleted Precursor molecules include lactate, pyruvate, and a-keto acids Gluconeogenesis Reactions Reverse of glycolysis except the three irreversible reactions From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

3 2 1 Gluconeogenesis Reactions Three bypass reactions: 1. Synthesis of phosphoenolpyruvate (PEP) via the enzymes pyruvate carboxylase and pyruvate carboxykinase 2. Conversion of fructose-1,6-bisphosphate to fructose-6-phosphate via the enzyme fructose-1,6-bisphosphatase 3. Formation of glucose from glucose-6-phosphate via the liver and kidney-specific enzyme glucose-6-phosphatase 2 1

Section 8.2: Gluconeogenesis Gluconeogenesis Regulation Rate of gluconeogenesis is affected primarily by: substrate availability allosteric effectors Hormones (e.g., cortisol and insulin) From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.2: Gluconeogenesis Cori Cycle: Lactate produced by anaerobic glycolysis in the muscles moves to the liver and is converted to glucose, which then returns to the muscles and is metabolized back to lactate. Gluconeogenesis Substrates Three of the most important substrates for gluconeogenesis are: 1. Lactate—released by skeletal muscle following exercise After transfer to the liver lactate is converted to pyruvate, then to glucose 2. Glycerol—a product of fat metabolism. Must be converted to their intermediate glyceraldehyde 3-phosphate to produce glucose. 3. Alanine—generated from pyruvate in exercising muscle Alanine is converted to pyruvate and then glucose in the liver From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Allosteric Regulation allosteric effectors may activate (+) or inhibit (-) steps along the metabolic pathways GLUCONEOGENESIS  GLYCOLYSIS 

Hormonal Regulation The peptide hormones glucagon and insulin also regulate glycolysis/gluconeogenesis pathways Glucagon is released by pancreatic alpha-cells when blood glucose is low; triggers a series of reactions that inhibit glycolysis and activate gluconeogenesis Insulin is released by pancreatic beta-cells when blood glucose is high; triggers a series of reactions that activate glycolysis and inhibit gluconeogenesis

Pentose Phosphate Pathway Alternate glucose metabolic pathway Products are NADPH and ribose-5-phosphate Two phases: oxidative and nonoxidative

Section 8.3: Pentose Phosphate Pathway Alternate glucose metabolic pathway Most active in cells where large quantities of lipids are synthesized (e.g., adipose tissue, adrenal cortex, mammary glands, liver) Two phases: oxidative and nonoxidative Oxidative phase: Produces ribulose-5-phosphate and two NADPH (Nicotinamide adenine dinucleotide phosphate) NADPH is a reducing agent used in anabolic (synthesis) processes (e.g., to produce lipids) From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Oxidation Hydrolysis Pentose Phosphate Pathway – Oxidative Phase Glucose-6-phosphate dehydrogenase Gluconolactonase Hydrolysis Figure 8.15a The Pentose Phosphate Pathway (oxidative) From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Oxidation Decarboxylation Section 8.3: Pentose Phosphate Pathway 6-phosphogluconate dehydrogenase Oxidation Decarboxylation Figure 8.15a The Pentose Phosphate Pathway (oxidative) From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.3: Pentose Phosphate Pathway Pentose Phosphate Pathway: Nonoxidative Produces important intermediates for nucleotide biosynthesis and glycolysis Ribose-5-phosphate Glyceraldehyde-3-phosphate Fructose-6-phosphate Figure 8.15b The Pentose Phosphate Pathway (nonoxidative) From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.3: Pentose Phosphate Pathway If the cell requires more NADPH than ribose molecules, products of the nonoxidative phase can be shuttled into glycolysis Figure 8.16 Carbohydrate Metabolism: Glycolysis and the Phosphate Pathway From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.4: Metabolism of Other Important Sugars 22% 70% Fructose, mannose, and galactose also important sugars for vertebrates Most common sugars found in oligosaccharides besides glucose From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Fructose Metabolism Section 8.4: Metabolism of Other Important Sugars Second to glucose in the human diet Found in fruit, honey, sucrose, high fructose corn syrup Can enter the glycolytic pathway in two ways: Through the liver (multi-enzymatic process) Muscle and adipose tissue (hexokinase) From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.4: Metabolism of Other Important Sugars Figure 8.17 Carbohydrate Metabolism: Other Important Sugars From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.5: Glycogen Metabolism Glycogenesis Synthesis of glycogen, the storage form of glucose, occurs after a meal Requires a set of three reactions (1 and 2 are preparatory and 3 is for chain elongation): 1. Synthesis of glucose-1-phosphate (G1P) from glucose-6-phosphate by phosphoglucomutase 2. Synthesis of uridine diphosphate (UDP)-glucose from G1P by UDP-glucose phosphorylase 3. Synthesis of Glycogen from UDP-glucose Requires two enzymes: Glycogen synthase to grow the chain Amylo-α(1,46)-glucosyl transferase which creates the α(1,6) linkages for branching From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.5: Glycogen Metabolism Figure 8.18 Glycogen Synthesis Glycogen synthase Glycogenesis Continued Glycogen synthase catalyzes addition of glucose onto existing oligosaccharide by transferring glucosyl group of UDP-glucose to non-reducing end of glycogen From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.5: Glycogen Metabolism Branching enzyme Glycogenesis Continued Branching enzyme amylo-a(1,41,6)-glucosyl transferase creates a(1,6) linkages for branches that occur ~10 residues apart a(1,6) Glycosidic Linkage is formed Figure 8.18 Glycogen Synthesis From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Glycogenolysis Section 8.5: Glycogen Metabolism Glycogen degradation requires two reactions: Removal of glucose from nonreducing ends (glycogen phosphorylase), which stops when four glucose units remain at the branch point (further degradation to the branch point results in a limit dextrin) 2. Hydrolysis of the a(1,6) glycosidic bonds at branch points by amylo-a(1,6)-glucosidase (debranching enzyme) From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.5: Glycogen Metabolism Glycogen phosphorylase uses organic phophate to cleave the a(1,4) linkages on the outer branches From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Glycogenolysis Cont. Section 8.5: Glycogen Metabolism amylo-a(1,6)-glucosidase first transfers the outer 3 glucose residues to a nearby non-reducing end, then uses water to hydrolyze the a(1,6) glycosidic bonds and remove the last glucose at the branch point Amylo-a(1,6)-glucosidase Amylo-a(1,6)-glucosidase From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.5: Glycogen Metabolism Amylo-a(1,6)-glucosidase Glucose-1-phosphate, the major product of glycogenolysis, is diverted to glycolysis in muscle cells to generate energy for muscle contraction. In hepatocytes, G1P is converted to glucose and released into the blood. From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.5: Glycogen Metabolism Regulation of Glycogen Metabolism Carefully regulated by synthesis to maintain consistent energy levels Regulation involves insulin, glucagon, epinephrine, and allosteric effectors Figure 8.22 Major Factors Affecting Glycogen Metabolism From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Section 8.5: Glycogen Metabolism Glucagon activates glycogenolysis Insulin inhibits glycogenolysis and activates glycogenesis Epinephrine release activates glycogenolysis and inhibits glycogenesis Figure 8.22 Major Factors Affecting Glycogen Metabolism From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press

Regulation of Glycogen Metabolism Glucagon is released from pancreas when blood glucose levels drop (e.g., after a meal). It binds to hepatocyte receptors, triggering a cascade of reactions that initiate glycogenolysis, releasing glucose into the bloodstream. Insulin inhibits enzymes of glycogenolysis, and activates glycogenesis enzymes, increasing rate of glucose uptake. Epinephrine, released due to emotional or physical stress, promotes glycogenolysis and inhibits glycogenesis; provides massive glucose release for flight-or-fight response.