Bài giảng Biochemistry 2/e - Chapter 28: Metabolic Integration and Unidirectionality of Pathways

Chapter 28Metabolic Integration and Unidirectionality of Pathwaysto accompanyBiochemistry, 2/ebyReginald Garrett and Charles GrishamAll rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777 Outline28.1 A Systems Analysis of Metabolism 28.2 Metabolic Stoichiometry 28.3 Unidirectionality 28.4 Metabolism in a Multicellular OrganismSystems Analysis of MetabolismCatabolic and anabolic pathways, occurring simultaneously, must act as a regulated, orderly, responsive whole See Figure 28.1 - catabolism, anabolism and macromolecular synthesis Just a few intermediates connect major systems - sugar-Ps, alpha-keto acids, CoA derivs, and PEP ATP & NADPH couple catabolism & anabolism Phototrophs also have photosynthesis and CO2 fixation systems 28.2 Metabolic StoichiometryThree types of stoichiometry in biological systems Reaction stoichiometry - the number of each kind of atom in a reaction Obligate coupling stoichiometry - the required coupling of electron carriers Evolved coupling stoichiometry - the number of ATP molecules that pathways have evolved to consume or produce - a number that is a compromise, as we shall seeThe Significance of 38 ATPsThe "ATP stoichiometry" has a large effect on the Keq of a reaction Consider the Keq for glucose oxidation (page 932) If 38 ATP are produced, cellular G is -967 kJ/mol and Keq = 10170, a very large number! If G = 0, 58 ATP could be made, but the reaction would come to equilibrium with only half as much glucose oxidized as we could have hadSo the number of 38 is a compromise! Significance of large KeqThe more ATP obtained, the lower the equilibrium constant, and the higher the level of glucose required If [glucose] is below this value, it won't be effectively utilized Large Keq means that this threshold level of glucose will be be very low Large Keq also means that the reaction will be far from equilibrium and can thus be regulated The ATP EquivalentWhat is the "coupling coefficient" for ATP produced or consumed?Coupling coefficient is the moles of ATP produced or consumed per mole of substrate converted (or product formed) Cellular oxidation of glucose has a coupling coefficient of 30-38 (depending on cell type) Hexokinase has a coupling coefficient of -1 Pyruvate kinase (in glycolysis) has a coupling coefficient of +1 The ATP Value of NADH vs NADPHThe ATP value of NADH is 2.5-3 The ATP value of NADPH is higher NADPH carries electrons from catabolic pathways to biosynthetic processes [NADPH]>[NADP+] so NADPH/NADP+ is a better e- donating system than NADH/NAD So NADPH is worth 3.5-4 ATP! Nature of the ATP EquivalentA different perspectiveG for ATP hydrolysis says that at equilibrium the concentrations of ADP and Pi should be vastly greater than that of ATP However, a cell where this is true is dead Kinetic controls over catabolic pathways ensure that the [ATP]/[ADP][Pi] ratio stays very high This allows ATP hydrolysis to serve as the driving force for nearly all biochemical processes Solvent Capacity of the CellThe capacity to keep all metabolites solvated What is the role of ATP in solvent capacity? Consider phosphorylation of glucose If done by Pi, the concentration of Pi would have to be 2700 M However, using ATP, and if [ATP] and [ADP] are equal, [G-6-P]/[G] is maintained at 850 ATP, an activated form of phosphate, makes it possible for cell to carry out reactions while keeping concentrations of metabolites low Substrate CyclesIf ATP c.c. for a reaction in one direction differs from c.c. in the other, the reactions can form a substrate cycle See Figure 28.2 The point is not that ATP can be consumed by cycling But rather that the difference in c.c. permits both reactions (pathways) to be thermodynamically favorable at all times Allosteric effectors can thus choose the direction!Unidirectionality of PathwaysA "secret" role of ATP in metabolism Both directions of any pair of opposing pathways must be favorable, so that allosteric effectors can control the direction effectively The ATP coupling coefficient for any such sequence has evolved so that the overall equilibrium for the conversion is highly favorable See Figure 28.4 for an illustration! ‘Energy Charge’Adenylates provide phosphoryl groups to drive thermodynamically unfavorable reactionsEnergy charge is an index of how fully charged adenylates are with phosphoric anhydridesIf [ATP] is high, E.C.1.0If [ATP] is low, E.C. 0Fueling the BrainBrain has very high metabolism but has no fuel reservesThis means brain needs a constant supply of glucoseIn fasting conditions, brain can use -hydroxybutyrate (from fatty acids), converting it to acetyl-CoA in TCAThis allows brain to use fat as fuel!Creatine Kinase in MuscleMuscles must be prepared for rapid provision of energyCreatine kinase and phosphocreatine act as a buffer system, providing additional ATP for contractionGlycogen provides additional energy, releasing glucose for glycolysisGlycolysis rapidly lowers pH, causing muscle fatigueMuscle Protein DegradationDuring fasting or high activity, amino acids degrade to pyruvate, which can be transaminated to alanineAlanine circulates to liver, where it is converted back to pyruvate - food for gluconeogenesisThis is a fuel of last resort for the fasting or exhausted organism