Glycogen Metabolism, Gluconeogenesis, Photosynthesis PDF

# Glycogen Metabolism ## Glycogen Structure - homopolymer of a-D-glucopyranose ## Muscle and Liver Glycogen - liver glycogen is degraded for - distribution to other organs through the blood - maintaining proper blood glucose levels - muscle glycogen is degraded for its use in energy pro...

# Glycogen Metabolism ## Glycogen Structure - homopolymer of a-D-glucopyranose ## Muscle and Liver Glycogen - liver glycogen is degraded for - distribution to other organs through the blood - maintaining proper blood glucose levels - muscle glycogen is degraded for its use in energy production ## Chemical Structure of Glycogen # Glycogen Breakdown: Glycogenolysis ## Enzymes Involved 1. glycogen phosphorylase - catalyzes phosphorolysis (bond cleavage by substitution of phosphate group) forming glucose 1-P - essential cofactor is pyridoxal phosphate - derived from Vitamin B6 2. debranching enzyme - aka oligo(a-1,6 to a-1,4) glucantransferase - removes branches (transferase activity) and cleaves an a-1,6 glycosidic bond (a-1,6 glucosidase activity) 3. phosphoglucomutase - converts glucose 1-P to glucose 6-P ## Bioenergetics of the Phosphorolysis Reaction - ΔG of reaction will be from -5.3 to -8.3 kJ/mol - reaction is exergonic: high [P;] in cells makes glycogenolysis thermodynamically favorable ## 1. Release of Glucose as Glucose-1-P - glucose released is already phosphorylated without consuming ATP - glucose-1-P cannot leave the cell and go to the bloodstream - it readily enters the glycolytic pathway once it's converted to glucose-6-P by phosphoglucomutase ## 2. Re-modeling of Glycogen - re-modeling prior to further degradation occurs near the branch point ## 3. De-branching - a-1,6 glycosidic bond is cleaved by an a-1,6-glucosidase - released glucose from de-branching is converted to glucose-6-P and can enter glycolysis ## 4. Further Release of Glucose-1-P - glycogen phosphorylase ## Enzymes Involved 1. UDP-glucose pyrophosphorylase 2. glycogen synthase 3. branching enzyme ## Overview - UDP-Glucose is the activated form of glucose that serves as glucose donor for glycogen synthesis - a sugar nucleotide - UTP - uridine triphosphate # Glycogen Synthesis: Glycogenesis ## 1. Formation of Glucose-6-P ## 2. Conversion of Glucose-6-P to Glucose-1-P ## 3. Formation of UDP-Glucose ## Bioenergetics of Formation of UDP-glucose - reaction proceeds because of rapid hydrolysis of PP; to 2P; ## 4. Addition of Glucose to Glycogen ## Glycogen Synthase - it cannot simply link together 2 glucose molecules; it can only extend an already existing a-1,4 glucan chain - requires a primer of 4 or more glucose molecules in length - synthesized by the enzyme glycogenin - UDP-glucose attaches first to OH group of Tyr194 residue - when glycogen primer has 9 glucose residues, glycogen synthase will take over ## 5. Branching # Hormonal Regulation of Glycogen Metabolism ## Reciprocal Regulation - glycogenolysis and glycogenesis are reciprocally regulated - conditions that promote synthesis inhibit degradation and vice versa - regulation is employed in response to the blood glucose level - normal blood glucose level: 70-100 mg/100 mL ## Hormones for Regulation ### Insulin - stimulates uptake of glucose from the bloodstream - promotes glycogenesis and inhibits glycogenolysis ### Epinephrine (in muscle) and Glucagon (in liver) - stimulates glycogen breakdown to glucose # Gluconeogenesis - synthesis of glucose from non-carbohydrate precursors: - pyruvate, lactate, glycerol, some amino acids, TCA cycle intermediates - occurs when glycogen reserves are already depleted - NOT the exact reverse of glycolysis - differs in the irreversible steps - enzymes that catalyze the reversible steps are the same ## Gluconeogenesis ### Major Site: - Liver - Also occurs in the cortex of the kidney # Gluconeogenesis versus Glycolysis ## Why not a futile cycle? If GNG and glycolysis are allowed to proceed at high rate simultaneously, it will result to a futile cycle - large amount of energy produced will only be dissipated as heat ## Why not a futile cycle? - NOT THE CASE since... - in glycolysis: reactions catalyzed by hexokinase, PFK-1 and pyruvate kinase has a a large (-) ΔG value - essentially irreversible - in GNG, these were bypassed by a separate set enzymes - reactions are sufficiently exergonic - at the expense of hydrolysis of 4 ATP and 2 GTP molecules ## 1a. Pyruvate to OAA - occurs in the mitochondrial matrix ## 1b. OAA to Malate - once formed, malate is then transported across the mitochondrial membrane to the cytosol ## 1c. OAA to PEP ## 2. Frc-1,6-bP to Frc-6-P ## 3. Glc-6-P to Glucose ## Overall Reaction: ## Reciprocal Regulation # Cori Cycle - occurs under hypoxia (e.g. intense muscular activity) - lactate produced by the muscle goes to bloodstream, then to the liver - in the liver, lactate is converted to glucose via gluconeogenesis - glucose then goes back to the bloodstream, then to the muscle for glycogenesis (muscle activity stopped) or it enters glycolysis (muscle activity continues) - prevents lactic acidosis in the muscle tissues, which may lead to muscle cramps ## Cori Cycle - Cori cycle cannot be sustained indefinitely - glycolysis part produces 2 ATP - gluconeogenesis part consumes 6 ATP - each iteration of the Cori cycle consumes a net total of 4 ATP - Cori cycle shifts the metabolic burden from the muscles to the liver # Overview of Photosynthesis ## Photosynthesis - the process by which autotrophic organisms convert CO₂ into sugars with the use of energy coming from sunlight - basically involves conversion of light energy into chemical energy ## Two Phases of Photosynthesis ### 1. Light Reactions - light energy is converted to chemical energy - ATP and NADPH are produced - occurs in the thylakoid ### 2. Dark Reactions - CO₂ is fixed and reduced to form sugars - uses ATP and NADPH - occurs in the stroma ## Chloroplasts - site of photosynthesis ## Light Reactions – Z Scheme - linear; NADPH and ATP are both produced (along with O₂) ## Reaction-Center Complex - Each photosystem has a reaction center complex which transduce light into chemical energy - Each reaction center complex contains: - special pair of chlorophyll a molecules - primary electron acceptor ## Electron Transport Chain - ETC components: - plastoquinone - cytochrome complex - plastocyanin - ETC generates a proton gradient that drives ATP synthesis ## 2. Electron Transport Chain ## Cyclic Electron Flow - occurs when NADP+ is not available - e from Fd is transferred back to ETC between PS II and PS I - only ATP is generated ## Cyclic Electron Flow