BIO 608: Carbohydrate Metabolism

• BIO 608 - Advanced Biochemistry covers carbohydrate metabolism in Topic 7.
• Dr. Carine SEBAALY presents the topic.

Outline of Carbohydrate Metabolism

• Glycogen metabolism occurs in the muscle and liver.
• The regulation of glycogen metabolism is important.
• Gluconeogenesis and its reciprocal regulation are key processes.
• The pentose phosphate pathway is another important metabolic route.

Glycogen Details

• Glycogen is an energy storage polysaccharide found in animals and microbes.
• Glycogen is similar to amylopectin but has a higher molecular weight.
• Glycogen has shorter and more frequent branch points compared to amylopectin.
• Glycogen is composed of linear glucose molecules linked via alpha 1,4 bonds, with branches formed by alpha 1,6 glycosidic bonds.

Glycogen Utilization

• Glycogen phosphorylase cleaves α(1→4) bonds through phosphorolysis, producing α-D-glucose-1-phosphate.
• The glycogen is shortened by one glucose residue each time.
• The process repeats repetitively until four glucose residues remain near a branch point.
• α-D-glucose-1-phosphate converts to α-D-glucose-6-phosphate by phosphoglucomutase to enter glycolysis or other pathways.

Debranching Process

• The bifunctional (α1,4→α1,4)-glucantransferase catalyzes two reactions.
• Transferase activity shifts three of the remaining four glucose residues from a limit branch to another nonreducing end using a new α(1→4) linkage.
• α(1→6)-glucosidase activity then removes the final glucose molecule at the branch point.

Glycogen Metabolism and Glycogenolysis

• Glycogenolysis results in releasing large amounts of glucose 1-phosphate.
• Glucose 1-phosphate is converted to glucose 6-phosphate by phosphoglucomutase.
• Glucose 6-phosphate can be further processed into glucose by glucose 6-phosphatase, but this process is only available in liver cells.
• Finally glucose is released into the bloodstream.

Glycogen Synthesis from UDP-Glucose

• Glycogenesis begins with an active form of glucose: UDP-glucose.
• UDP-glucose is an activated form of glucose for glycogen synthesis.
• Glycogen synthase is the essential enzyme.
• A primer of at least four glucose residues is needed.
• Glycogenin, a small protein, serves as this primer for glycogen synthase; it catalyzes the initial addition of glucose to itself at Tyrosine-194.
• Glycogenin remains covalently attached to the reducing end of the glycogen molecule.
• Amylo-(1,4→1,6) transglycosylase is responsible for adding glycogen branches and transfers 6-7 residues from a branch terminus of at least 11 residues.

Regulation of Glycogen Metabolism

• A hormone binds to a cell surface receptor.
• A G-protein is released, which activates adenylate cyclase.
• Adenylate cyclase synthesizes cAMP, which binds to the regulatory subunit of protein kinase A (PKA), then releasing the active C subunit of PKA.
• Active PKA phosphorylates phosphorylase b kinase.
• Active kinase converts inactive phosphorylase b to active phosphorylase a, which then catalyzes glycogen breakdown.

Glycogen Phosphorylase Activity Control

• Phosphorylase exists primarily in a less active T-state when unphosphorylated.
• In a phosphorylated form, it exists primarily in a more active R-state.
• Allosteric effectors can alter the T ⇌ R equilibrium.

Glycogen Synthase Activity

• Glycogen synthase is active in its unphosphorylated a form.
• A number of kinases can phosphorylate glycogen synthase, converting it to the inactive b form.
• Dephosphorylation reactivates the enzyme.
• Glucose-6-phosphate acts as an allosteric activator.
• Glucose-6-phosphate shifts equilibrium from T to R state.

Glycogen Metabolism and Human Disease

• There are congenital defects of glycogen metabolism.

Gluconeogenesis Overview

• Gluconeogenesis is the production of new glucose using non-carbohydrate precursors.
• The human brain requires 120 grams of glucose per day out of 160 grams needed by the entire body.
• Glycogen reserves supply ~190 g, and body fluids supply ~20 g of glucose.
• The body contains about one day’s glucose supply.
• When glucose is depleted (fasting or prolonged exercise), the body synthesizes it through gluconeogenesis from other sources.
• Gluconeogenesis refers to the de novo synthesis of glucose.
• Glucose is synthesized from non-carbohydrate precursors.
• Gluconeogenesis is like glycolysis in reverse.
• Gluconeogenesis requires reversing three irreversible reactions in glycolysis.

Glycolysis and Gluconeogenesis Reactions

• The three irreversible reactions in glycolysis are those catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase.
• Gluconeogenesis bypasses these with glucose-6-phosphatase, fructose-1,6-bisphosphatase, and pyruvate carboxylase/phosphoenolpyruvate carboxykinase.

Gluconeogenesis Bypass Reactions

• Bypass 1 involves pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK).
• Bypass 2 uses fructose 1,6-bisphosphatase.
• Bypass 3 uses glucose 6-phosphatase.
• The Cori Cycle involves the liver being the most active gluconeogenic tissue.
• Glucose-Alanine Cycle also occur in the liver and peripheral tissues.

Energy costs of Gluconeogenesis

• The energy cost for gluconeogenesis is 2 Pyruvate + 4ATP + 2GTP + 2NADH + 2H+ + 4H2O → glucose + 4ADP + 2GDP + 6Pi + 2NAD+ with ΔG°' = -42.7 kJ/mol.
• The energy cost for reversal of glycolysis is 2 Pyruvate + 2ATP + 2NADH + 2H+ + 2H2O → glucose + 2ADP + 2Pi + 2NAD+ with ΔG°' = +79.9 kJ/mol.

Regulation Details

• Glycolysis and gluconeogenesis are reciprocally controlled to avoid futile cycles.
• Regulation accounts for needed pools of intermediates for other biosynthetic purposes.
• Louis Pasteur observed that anaerobic yeast exposed to oxygen decreased glucose utilization.
• Following oxygenation, the cellular amounts of glycolytic intermediates pointed to controlling the conversion of F6P to F1,6BP.
• Phosphofructokinase (PFK) is inhibited by ATP and activated by ADP and AMP.
• Glycolysis depends on adenylate energy charge.
• Fructose 2,6-bisphosphate activates glycolysis and inhibits gluconeogenesis.
• The bifunctional enzyme PFK-2/FBPase-2 synthesizes F2,6BP.
• Unphosphorylated enzyme increases kinase activity and decreases phosphatase activity.
• This leads to increased F2B, promoting glycolysis (increasing glucose) and decreasing gluconeogenesis activity.

Pentose Phosphate Pathway

• The pentose phosphate pathway oxidizes glucose.
• The PPP or HMP (hexose monoP shunt) is required to supply reducing equivalents, NADPH, for reductive biosynthesis (FA, steroids, cholesterol synthesis).
• The PPP or HMP is needed for ribose-5-phosphate (R5P) to synthesize nucleotides and nucleic acid.

Four Stages of the PPP

• Stage 1: two oxidations (glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase) generates NADPH and CO2.
• Stage 2: ribose 5-phosphate is produced
• Other Stages
• The oxidative phase generates 2 NADPH + H+, 1 CO2, and 1 pentose phosphate from 1 glucose 6-phosphate and 2 NADP+.
• Ribulose 5-phosphate can also be converted to the pentose sugar xylulose 5-phosphate (X5P) by ribulose 5-phosphate epimerase.
• The nonoxidative phase converts R5P (a C5 sugar) and X5P (C5) into glyceraldehyde 3-phosphate (GAP; C3) and sedoheptulose 7-phosphate (S7P; C7) by transketolase.
• Metabolized to fructose 6-phosphate (F6P; C6) and erythrose 4-phosphate (E4P; a C4 sugar) by transaldolase
• Moreover, E4P and X5P can be altered to GAP and F6P in a second transketolase reaction