Introduction to Glucose Catabolism

OUTLINE: Catabolism of Glucose Glycogenolysis phosphorylase debranching enzyme phospho-gluco-mutase (PGM) Glycolysis Introduction & overview; 2 phases Phase I hexokinase phospho-gluco-isomerase (PGI) phospho-fructo-kinase (PFK-1) Aldolase triose-phosphate isomerase (TPI) Phase II CATABOLISM Glycogen-o-lysis Glyco-lysis Pyruvate Oxidation Krebs' Cycle First, let’s discuss how crucial glucose is for cellular metabolism Phosphorylation Oxidative 1 CATABOLISM Glucose Importance: • Glucose is an excellent fuel. – yields good amount of energy upon oxidation • −2840 kJ/mol glucose (–678 kcal/mol) – can be efficiently stored in the polymeric form – Many organisms and tissues can meet their energy needs on glucose only. • Glucose is a versatile biochemical precursor. – Many organisms can use glucose to generate: • • • • • all the amino acids membrane lipids nucleotides in DNA and RNA cofactors needed for the metabolism of EVERYTHING IOW, EVERYTHING!! CATABOLISM Glucose Utilization: • Storage – can be stored in the polymeric form (starch, glycogen) – used for long-term energy needs • Energy production – generates energy via oxidation of glucose – short-term energy needs • Production of NADPH and pentoses – generates NADPH for use in relieving oxidative stress and synthesizing fatty acids, amino acids, etc. (anabolism) – generates pentose phosphates for use in DNA/RNA biosynthesis • Structural carbohydrate production – used for generation of alternate carbohydrates used in cell walls of bacteria, fungi, and plants 2 CATABOLISM Glucose Utilization: Four Major Fates of Glucose What if you need glucose for cellular metabolism and energy? NADPH / Glucose is Stored for Later Use as Glycogen (animals) or Starch (plants) • Glycogen/starch are branched polymers of α(1à4)-linked glucose with α(1à6) linkages (glycogen every 8-12 glucose units; starch every 24-30 glucose units). • Glycogen storage occurs mainly in the liver and muscle. Digestion • Starch storage occurs mainly in the leaves and seeds. maltose maltase Digestion versus intra-cellular utilization glucose (isomaltase) 3 Glycogenolysis is performed mainly by Glycogen Phosphorylase intra-cellular utilization 1 1 2 3 2 3 a-D Glycogenolysis is performed mainly by Glycogen Phosphorylase 4 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 is performed mainly by Glycogen Phosphorylase Mechanism Carbocation (carbonium ion) intermediate 1– Vitamin B6 (Pyridoxal Phosphate) Acid catalysis a Base catalysis 5 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 6 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 7 Glycolysis: Overview • Two Phases/Four concepts – Preparatory phase • Phosphorylation by ATP • Cleavage – Payoff • Oxidation • Phosphorylation of ATP 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 (energy coupling). •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 8