Carbohydrate Metabolism Notes

NOTES NOTES CARBOHYDRATE METABOLISM CITRIC ACID CYCLE (KREBS CYCLE) osms.it/citric-acid-cycle ▪ Generates energy in the form of GTP, NADH, and FADH2 ▪ Occurs in mitochondria ▪ Starts with acetyl-CoA → CO2 Process ▪ Acetyl-CoA + oxaloacetate (via citrate synthase) → citrate + CoA ▪ Citrate (via aconitase) → isocitrate ▪ Isocitrate + NAD+ (via isocitrate dehydrogenase) → α-ketoglutarate + NADH + CO2 ▫ Rate-limiting step ▪ α-ketoglutarate + NAD+ + CoA-SH (α-ketoglutarate dehydrogenase) → succinyl-CoA + NADH + CO2 ▫ Requires five cofactors: thiamine, lipoic acid, CoA, FAD+, NAD+ ▫ See mnemonic ▪ Succinyl-CoA + phosphate + GDP (via succinate thiokinase) → succinate + GTP ▪ Succinate + FAD+ (via succinate dehydrogenase) → fumarate + FADH2 ▪ Fumarate + H2O (via fumarase) → malate ▪ Malate + NAD+ (via malate dehydrogenase) → oxaloacetate + NADH ▫ Oxaloacetate then enters next cycle ▪ Generates one GTP molecule, three NADH molecules, one FADH2 molecule MNEMONIC: T-rex Loves and Cares For Nachos Five required cofactors Thiamine Lipoic acid CoA FAD+ NAD+ Figure 1.1 Mnemonic for the five cofactors required by α-ketoglutarate dehydrogenase. OSMOSIS.ORG 1

Figure 1.2 The citric acid (Krebs) cycle. Each acetyl-CoA molecule generates 12 ATP. ELECTRON TRANSPORT CHAIN & OXIDATIVE PHOSPHORYLATION osms.it/etc-and-oxidative-phosphorylation Oxidative phosphorylation ▪ Generates energy as ATP ▪ Occurs in inner mitochondrial membrane Electron transport chain ▪ Series of proteins, lipids, metals that facilitates electron movement → proton gradient used to create ATP ▪ Starts with electron donors NADH, FADH2 ▫ NADH from cytoplasm comes through malate-aspartate shuttle 2 OSMOSIS.ORG ▫ FADH2 from cytoplasm comes through glycerol-3-phosphate shuttle ▫ NADH donates electron to complex I (contains flavin mononucleotide, ironsulfur centers) → NAD+ ▫ FADH2 donates electron to complex II (i.e. succinate dehydrogenase) → FAD ▫ Electrons from either complex flow into coenzyme Q (ubiquinone) ▫ Coenzyme Q passes electrons to cytochromes (proteins with heme

Chapter 1 Biochemistry: Carbohydrate Metabolism groups — Fe3+ + e- ↔ Fe2+): complex III (cytochromes b and c1) → cytochrome c → complex IV (cytochrome oxidase: cytochromes a, a3) → oxygen ▪ Movement of electrons → electrical current → complexes I, III, IV use this energy to pump protons across inner mitochondrial membrane ▪ Protons can move back into mitochondria through F0 → proton gradient forms, powering F1: ADP → ATP ▫ Collectively called complex V ▪ An ADP/ATP antiport pumps ATP into cytoplasm of the cell, supplies complex V with new ADP Figure 1.3 The flow of electrons through the electron transport chain, which takes place in the inner mitochondrial membrane. OSMOSIS.ORG 3

Figure 1.4 Oxidative phosphorylation. The passing of electrons along the electron transport chain generates an electrical current, which provides the energy that allows complexes I, III, and IV to pump protons into the space between the inner and outer mitochondrial membranes. This creates a gradient across the inner mitochondrial membrane. The protons use proton channel F0 to flow down the gradient, back into the mitochondrial matrix. F0 is attached to enzyme F1, an ATP synthase, which uses the proton gradient to phosphorylate ADP → ATP. GLUCONEOGENESIS osms.it/gluconeogenesis ▪ Synthesis of glucose from noncarbohydrate substrates ▫ E.g. amino acids, lactate, glycerol ▪ Occurs primarily in liver cells; also in epithelial cells of kidney, intestine ▫ Inside cytoplasm, mitochondria, endoplasmic reticulum ▪ Starts with glycogenolysis after glucose depletion Process ▪ Like backwards glycolysis, with three exceptions ▪ Obtaining pyruvate ▫ Lactate (via lactate dehydrogenase) → pyruvate ▫ Amino acids (not leucine, lysine); e.g. alanine (via alanine transaminase) → 4 OSMOSIS.ORG ▪ ▪ ▪ ▪ ▪ ▪ pyruvate Obtaining ATP, glycerol ▫ Triacylglyceride breakdown → fatty acids and glycerol → acetyl CoA + ATP (β-oxidation) Pyruvate (via pyruvate carboxylase) → oxaloacetate Oxaloacetate (malate dehydrogenase) → malate Malate leaves mitochondria; malate (via malate dehydrogenase) → oxaloacetate Oxaloacetate (via PEPCK) → phosphoenolpyruvate (PEP) PEP undergoes reversed glycolysis reactions until dihydroacetone-phosphate (DHAP) ▫ Alternatively, glycerol (via glycerol kinase) → glycerol-3-phosphate;

Chapter 1 Biochemistry: Carbohydrate Metabolism glycerol-3-phosphate (via glycerol-3phosphate dehydrogenase) → DHAP ▪ DHAP (via aldolase) → fructose-1,6bisphosphate ▪ Fructose-1,6-bisphosphatase → fructose6-phosphate ▫ Rate-limiting step ▪ Fructose-6-phosphate (via isomerase) → glucose-6-phosphate ▪ Glucose-6-phosphate (via glucose-6phosphatase) → glucose Figure 1.5 The process of gluconeogenesis. OSMOSIS.ORG 5

GLYCOGEN METABOLISM osms.it/glycogen-metabolism ▪ Polymer of glucose molecules linked by glycosidic bonds ▪ Stores energy in skeletal muscle, liver Glycogen synthesis ▪ Glucose + phosphate (via hexokinase) → glucose-6 phosphate ▪ Glucose-6 phosphate (via phosphoglucomutase) → glucose-1phosphate + energy (UTP) ▪ Glucose-1-phosphate + UTP (via UDPglucose pyrophosphorylase) → UDPglucose ▪ UDP-glucose added (via glycogen synthase) to glycogen branch/glycogenin (→ alpha-1,4-glycosidic bond) ▪ Branching enzyme cuts off part of glucose chain, creates branch (→ alpha-1,6glycosidic bond) Glycogen breakdown, AKA glycogenolysis ▪ Glucagon → liver breakdown of glycogen ▪ Epinephrine → skeletal muscle breakdown of glycogen ▪ Glycogen phosphorylase cleaves alpha-1,4 bonds on branches; catalyzes phosphate transfer to glucose residue → one glucose1-phosphate is released at a time ▫ Repeats until branch is only 4 glucose units long ▪ Debranching enzyme: 4-alphaglucanotransferase moves 3 glucose units off branch, onto main chain; alpha-1,6glucosidase cleaves last remaining glucose 6 OSMOSIS.ORG ▪ Cleaved glucose-1-phosphate (via phosphoglucomutase) → glucose-6phosphate ▫ With glucose-6-phosphate ▪ In liver cells, glucose-6-phosphatase removes phosphate → free glucose into blood ▪ In skeletal muscle, glucose-6-phosphate → glycolysis pathway Regulation ▪ Principles ▫ Glycogen synthase: active without phosphate ▫ Glycogen phosphorylase: active with phosphate ▪ Hormones ▫ Insulin: binds to membrane tyrosine kinase receptors → protein phosphatase removes phosphates → glycogen synthase activates, glycogen phosphorylase deactivates ▫ Glucagon: binds to membrane G-protein coupled receptors (in liver) → ATP (adenylyl cyclase) → cAMP → kinase A → adds phosphates → glycogen phosphorylase activates, glycogen synthase deactivates

Chapter 1 Biochemistry: Carbohydrate Metabolism Figure 1.6 The process of glycogen synthesis. OSMOSIS.ORG 7

Figure 1.7 Glycogen breakdown. The process is completed differently in the liver and skeletal muscles due to the respective presence and absence of glucose-6-phosphatase in each. 8 OSMOSIS.ORG

Chapter 1 Biochemistry: Carbohydrate Metabolism Figure 1.8 The role of insulin in the regulation of glycogen levels. Figure 1.9 The role of glucagon in the regulation of glycogen levels. GLYCOLYSIS osms.it/glycolysis ▪ Energy-producing breakdown of glucose into pyruvate ▪ Occurs in cytoplasm of all cells PROCESS ▪ Glucose transporter (GLUT) carries glucose into cell ▪ Kinases (hexokinase, glucokinase) phosphorylate glucose → conformational change, i.e. glucose can’t diffuse out) → glucose-6-phosphate ▫ Uses one ATP molecule ▪ Glucose-6-phosphate (via phosphoglucoisomerase) → fructose-6phosphate ▪ Fructose-6-phosphate (via phosphofructokinase-1) → fructose-1,6bisphosphate ▫ Rate-limiting step ▫ Uses one ATP molecule OSMOSIS.ORG 9

Enzyme activation ▪ Fructose-6-phosphate (via phosphofructokinase-2) → fructose-2,6bisphosphate ▫ Up-regulated by insulin; downregulated by glucagon ▫ Fructose-2,6-bisphosphate activates phosphofructokinase-1 ▪ Fructose-1,6-bisphosphate (via aldolase) → glyceraldehyde 3-phosphate (G3P) + dihydroacetone-phosphate (DHAP) ▫ DHAP (via isomerase) → G3P → 2x G3P molecules per glucose ▪ G3P (via G3P-dehydrogenase) → 1,3-diphosphoglycerate (1,3-BPG); H+ + NAD+ → NADH (x2) ▫ 2x NADH enter electron transport chain ▪ 1,3-BPG + ADP (via phosphoglycerate kinase) → 3-phosphoglycerate + ATP (x2) ▫ Creates two ATP molecules ▪ 3-phosphoglycerate (via mutase) → 2-phosphoglycerate (x2) ▪ 2-phosphoglycerate (via enolase) → phosphoenolpyruvate (PEP) + H2O (x2) ▪ PEP + ADP (via pyruvate kinase) → pyruvate + ATP (x2) ▫ Creates two ATP molecules ▫ Up-regulated by fructose-1,6bisphosphate (feed-forward regulation) ▫ Down-regulated by ATP, alanine ▪ In total, process generates two ATP molecules ▪ In cells with oxygen, pyruvate enters citric acid cycle, electron transport chain to make more ATP ▫ 30–32 in total PENTOSE PHOSPHATE PATHWAY osms.it/pentose-phosphate-pathway ▪ Synthesis of ribose, NADPH from unused glucose ▪ Occurs in cytoplasm of all cells Irreversible oxidative phase ▪ Glucose-6-phosphate + NADP+ (via glucose-6-phosphate dehydrogenase) → 6-phosphogluconate + NADPH ▫ Rate-limiting step ▪ 6-phosphogluconate + NADP+ (6-phosphogluconate dehydrogenase) → ribulose-5-phosphate + NADPH + CO2 10 OSMOSIS.ORG Reversible non-oxidative phase ▪ Two options: ▫ Ribulose-5-phosphate (via isomerase) → ribose-5-phosphate ▫ Ribulose-5-phosphate (via epimerase) → xylulose-5-phosphate

Chapter 1 Biochemistry: Carbohydrate Metabolism Figure 1.10 Glycolysis. OSMOSIS.ORG 11

Figure 1.11 Pentose phosphate pathway. 12 OSMOSIS.ORG