Biochemistry 2e Garrett Grisham Chapter 28 Metabolic Integration

Biochemistry 2/e - Garrett & Grisham Chapter 28 Metabolic Integration and Unidirectionality of Pathways to accompany Biochemistry, 2/e by Reginald Garrett and Charles Grisham All 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 Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Outline • • 28. 1 A Systems Analysis of Metabolism 28. 2 Metabolic Stoichiometry 28. 3 Unidirectionality 28. 4 Metabolism in a Multicellular Organism Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Systems Analysis of Metabolism • • Catabolic 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, Co. A derivs, and PEP ATP & NADPH couple catabolism & anabolism Phototrophs also have photosynthesis and CO 2 fixation systems Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham 28. 2 Metabolic Stoichiometry Three 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 see Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham The Significance of 38 ATPs The "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 k. J/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 had • So the number of 38 is a compromise! Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Significance of large Keq The 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 Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham The ATP Equivalent • • What 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 Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham The ATP Value of NADH vs NADPH • The 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! Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Nature of the ATP Equivalent A 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 Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Solvent Capacity of the Cell • • • The 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 Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Substrate Cycles If 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! Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Unidirectionality of Pathways A "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! Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham ‘Energy Charge’ • Adenylates provide phosphoryl groups to drive thermodynamically unfavorable reactions • Energy charge is an index of how fully charged adenylates are with phosphoric anhydrides • If [ATP] is high, E. C. 1. 0 • If [ATP] is low, E. C. 0 Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Fueling the Brain • Brain has very high metabolism but has no fuel reserves • This means brain needs a constant supply of glucose • In fasting conditions, brain can use hydroxybutyrate (from fatty acids), converting it to acetyl-Co. A in TCA • This allows brain to use fat as fuel! Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Creatine Kinase in Muscle • Muscles must be prepared for rapid provision of energy • Creatine kinase and phosphocreatine act as a buffer system, providing additional ATP for contraction • Glycogen provides additional energy, releasing glucose for glycolysis • Glycolysis rapidly lowers p. H, causing muscle fatigue Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Muscle Protein Degradation • During fasting or high activity, amino acids degrade to pyruvate, which can be transaminated to alanine • Alanine circulates to liver, where it is converted back to pyruvate - food for gluconeogenesis • This is a fuel of last resort for the fasting or exhausted organism Copyright © 1999 by Harcourt Brace & Company