Glycogenolysis Must Deal With Branch Points in Glycogen

OUTLINE: Glycogenolysis phosphorylase debranching enzyme phosphoglucomutase Glycolysis introduction & overview; 2 phases Phase I hexokinase PGI PFK-1 Aldolase TPI Glycogenolysis must deal with Branch Points in Glycogen • Glycogen phosphorylase works on nonreducing ends until it reaches four residues from an (a1® 6) branch point. • Debranching enzyme has two activities; a glycosyltransferase and a glycosidase – Debranching enzyme transfers a block of three residues to the nonreducing end of the chain. – Debranching enzyme hydrolyzes the single remaining (a1®6)-linked glucose, which becomes a free glucose unit (i.e., NOT glucose 1-phosphate). • The Glc enters glycolysis, but the Glc 1-P must be converted to the glycolytic intermediate Glc 6-P first. • How? 1 Glycogenolysis must deal with Branch Points in Glycogen • Glycogen phosphorylase works on nonreducing ends until it reaches four residues from an (a1® 6) branch point. • Debranching enzyme has two activities; a glycosyltransferase and a glycosidase – Debranching enzyme transfers a block of three residues to the nonreducing end of the chain. – Debranching enzyme hydrolyzes the single remaining (a1®6)-linked glucose, which becomes a free glucose unit (i.e., NOT glucose 1-phosphate). • The Glc enters glycolysis, but the Glc 1-P must be converted to the glycolytic intermediate Glc 6-P first. • How? Glycogenolysis produces Glucose 1-Phosphate, which must be converted to Glucose 6-Phosphate by Phosphoglucomutase for Metabolism a-D- H • Phosphoglucomutase must be phosphorylated at an active-site Ser. • This is performed by a Glc 1,6-P2 to initiate the active form. • During turnover, the bisphosphate flips within a “vestibule” 2 Glycogenolysis produces Glucose 1-Phosphate, which must be converted to Glucose 6-Phosphate by Phosphoglucomutase for Metabolism H a-D- • Phosphoglucomutase must be phosphorylated at an active-site Ser. • This is performed by a Glc 1,6-P2 to initiate the active form. • During turnover, the bisphosphate flips within a “vestibule” Glucose Utilization Four Major Fates of Glucose Catabolism Anabolism 3 Glycolysis Learning outcomes for each pathway in course: 1. Understand the logic for getting from starting compound to end product 2. Know the names, structures, enzymes of each step 3. Energetics 4. Control mechanisms – • Sequence of enzyme-catalyzed reactions by which glucose is converted into pyruvate • Pyruvate can be further aerobically oxidized. • Pyruvate can be used as a precursor in biosynthesis. • • In the evolution of life, glycolysis probably was one of the earliest energyyielding pathways. Research of glycolysis played a large role in the development of modern biochemistry. – – – – understanding the role of coenzymes discovery of the pivotal role of ATP development of methods for enzyme purification inspiration for the next generations of biochemists Glycolysis Oxidation at cleavage site – • It developed before photosynthesis, when the atmosphere was still anaerobic. • Thus, the task upon early organisms was how to extract free energy from glucose anaerobically. • Some of the free energy is captured in the synthesis of ATP and NADH. • The solution: – First: All intermediates are phosphorylated: keeps them in the cell and “Activates” them for chemical degradation. – Second: Need to split in middle to make pathway convergent – Third: Overall oxidation (loss of only 4 e–) – Fourth: Keto-enol tautomerization is key – Fifth: Collect energy from the high-energy metabolites. Oxidation state +14 4 Glycolysis: Overview • Two Phases/Four concepts – Preparatory phase • Phosphorylation by ATP • Cleavage – Payoff • Oxidation • Phosphorylation of ATP Glycolysis: Overview • Two Phases/Four concepts – Preparatory phase • Phosphorylation by ATP • Cleavage – Payoff • Oxidation • Phosphorylation of ATP 5 Glycolysis: Hexokinase First Phosphorylation •Rationale • traps glucose inside the cell • lowers intracellular (unphosphorylated) glucose concentration to allow further uptake by facilitative diffusion through GLUTS. P •This process uses the energy of ATP. •Multiple isoforms of hexokinase exist in organisms (e.g., hexokinase I, II, III, and IV (glucokinase)). •Nucleophilic oxygen at C6 of glucose attacks the last (g) phosphate of ATP. •ATP-bound Mg++ facilitates this process by shielding the negative charges on ATP. •Highly thermodynamically favorable/irreversible • DG°’ = –4 kcal/mol • regulated mainly by product inhibition Glycolysis: Phosphoglucose isomerase (PGI) Setting up Second Phosphorylation • Rationale – Convert C1 from aldehyde to hydroxyl which is easier to phosphorylate • P Slightly thermodynamically unfavorable/reversible – DG°’ = +0.4 kcal/mol – product concentration kept low by pairing with favorable next step to drive reaction forward P 6 Glycolysis: Phosphoglucose Isomerase (PGI) Mechanism a-D- • The isomerization is catalyzed by the active-site glutamate via general acid/base catalysis. • An aldose (glucose) can isomerize into a ketose (fructose) via an enediol intermediate. Glycolysis: Phosphoglucose isomerase (PGI) Mechanism a-D- • The isomerization is catalyzed by the active-site glutamate via general acid/base catalysis. • An aldose (glucose) can isomerize into a ketose (fructose) via an enediol intermediate. 7 Glycolysis: Phosphofructokinase-1 Second Phosphorylation •Rationale – Symmetrical phosphorylation to set up cleavage to 3-carbon sugar •First committed step of glycolysis – fructose 1,6-bisphosphate (Fru 1,6-P2) is committed to become pyruvate and yield energy P •This process uses the energy of ATP. •Highly thermodynamically favorable/irreversible (DG°’ = –3.4 kcal/mol) •Phosphofructokinase-1 is highly regulated. P – by ATP, fructose 2,6-bisphosphate, and other metabolites – do not burn glucose if there is plenty of ATP Glycolysis: Fructose-1,6-bisphosphate Aldolase POCH2CH2 (GAP) • Rationale | OH CH2OP POCH2CH2 | OH CH2OP Aldol Cleavage/ condensation – Six-carbon sugar is cleaved into two three-carbon phosphorylated sugars. • The reverse process is the familiar aldol condensation. • Multiple isoforms of aldolase exist; aldolase A, B, C. • Multiple mechanisms to result in same product (i.e., convergent evolution): P P – Animal and plant aldolases employ covalent catalysis. – Fungal and bacterial aldolases employ metal ion catalysis. • Thermodynamically unfavorable/reversible (DG°’ = +5.7 kcal/mol) – product (GAP) concentration kept low to pull reaction forward. P P 8 Glycolysis: Fructose-1,6-bisphosphate aldolase Mechanism Covalent Catalysis (Class I enzymes) b-D- Acid catalysis H Glu Glu Asp Asp Glycolysis: Fructose-1,6-bisphosphate aldolase Mechanism Covalent Catalysis (Class I enzymes) b-D- Acid catalysis H Glu Asp Glu Asp 9 Base Catalysis Glycolysis: Fructose-1,6-bisphosphate aldolase Mechanism Loss of water produces Schiff base Glu Glu Asp Asp Base Catalysis Glycolysis: Fructose-1,6-bisphosphate aldolase Mechanism Loss of water produces Schiff base Glu Glu Asp Asp 10 Base Catalysis Glycolysis: Fructose-1,6-bisphosphate aldolase Mechanism Critical base catalysis Glu Glu Asp Asp Glycolysis: Fructose-1,6-bisphosphate aldolase Mechanism Acid catalysis protonates the carbanion Glu Glu Asp Asp Enamine is in resonance with the carbanion. Protonation of the carbanion resolves this to a dihydroxyacetone phosphate (DHAP) triose in Schiff-base attachment to the active-site Lysine. 11 Glycolysis: Fructose-1,6-bisphosphate aldolase Mechanism Product Release is the slow step Glu After product release, protonation state of the acid/base residues returns to starting point. Glu Asp H2O Asp Glycolysis: Fructose-1,6-bisphosphate aldolase Mechanism Summary Mechanism of Class I Aldolases 12 Glycolysis: Triose-phosphate isomerase (TIM) • Aldolase creates two triose phosphates: – glyceraldehyde-3-phosphate (GAP) – an aldose – dihydroxyacetone phosphate (DHAP) – a ketose • P Rationale: – allows convergence to a single chemical pathway (the payoff) – Need to oxidize C1 of GAP (formerly C3 & C4 of glucose) to carboxylate • Completes preparatory phase of glycolysis • Uses same enediol intermediate mechanism as PGI • Thermodynamically unfavorable/reversible (DG°’ = +1.8 kcal/mol) – GAP concentration kept low to pull reaction forward P Glycolysis: Triose-phosphate isomerase Mechanism TIM caps enediol with loop around Pi so that its not a good leaving group, and the elimination reaction to form methyl glyoxal is limited. 13 Glycolysis: Triose-phosphate isomerase Mechanism TIM caps enediol with loop around Pi so that its not a good leaving group, and the elimination reaction to form methyl glyoxal is limited. Glycolysis: Overview • Two Phases/Four concepts – Preparatory phase • Phosphorylation by ATP • Cleavage – Payoff • Oxidation • Phosphorylation of ATP 14