Biochemistry 2e Garrett Grisham Chapter 3 Thermodynamics of

Biochemistry 2/e - Garrett & Grisham Chapter 3 Thermodynamics of Biological Systems 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 • Basic Thermodynamic Concepts • Physical Significance of Thermodynamic Properties • p. H and the Standard State • The Effect of Concentration • Coupled Processes • High-Energy Biomolecules Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Basic Concepts • The system: the portion of the universe with which we are concerned • The surroundings: everything else • Isolated system cannot exchange matter or energy • Closed system can exchange energy • Open system can exchange either or both Copyright © 1999 by Harcourt Brace & Company

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

Biochemistry 2/e - Garrett & Grisham The First Law The total energy of an isolated system is conserved. • E (or U) is the internal energy - a function that keeps track of heat transfer and work expenditure in the system • E is heat exchanged at constant volume • E is independent of path • E 2 - E 1 = E = q + w • q is heat absorbed BY the system • w is work done ON the system Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Enthalpy • • • A better function for constant pressure H = E + PV If P is constant, H = q H is the heat absorbed at constant P Volume is approx. constant for biochemical reactions (in solution) So H is approx. same as E Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham The Second Law • Systems tend to proceed from ordered to disordered states • The entropy change for (system + surroundings) is unchanged in reversible processes and positive for irreversible processes • All processes proceed toward equilibrium - i. e. , minimum potential energy Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Entropy • • A measure of disorder An ordered state is low entropy A disordered state is high entropy d. Sreversible = dq/T Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham The Third Law • The entropy of any crystalline, perfectly ordered substance must approach zero as the temperature approaches 0 K • At T = 0 K, entropy is exactly zero • For a constant pressure process: Cp = d. H/d. T Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Free Energy • Hypothetical quantity - allows chemists to asses whether reactions will occur • G = H - TS • For any process at constant P and T: G = H - T S • If G = 0, reaction is at equilibrium • If G < 0, reaction proceeds as written Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham G versus Go’ • How can we calculate the free energy change for rxns not at standard state? • Consider a reaction: A + B C + D • Then: G = Go’ + RT ln ([C][D]/[A][B]) Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Energy Transfer A Crucial Biological Need • Energy acquired from sunlight or food must be used to drive endergonic (energy-requiring) processes in the organism • Two classes of biomolecules do this: – Reduced coenzymes (NADH, FADH 2) – High-energy phosphate compounds - free energy of hydrolysis larger than -25 k. J/mol) Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham High-Energy Biomolecules • • Study Table 3. 3! Note what's high - PEP and 1, 3 -BPG Note what's low - sugar phosphates, etc. Note what's in between - ATP! Note difference (Figure 3. 8) between overall free energy change - noted in Table 3. 3 - and the energy of activation for phosphoryl-group transfer! Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham ATP An Intermediate Energy Shuttle Device • PEP and 1, 3 -BPG are created in the course of glucose breakdown • Their energy (and phosphates) are transferred to ADP to form ATP • But ATP is only a transient energy carrier - it quickly passes its energy to a host of energy-requiring processes Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Phosphoric Acid Anhydrides Why ATP does what it does! • ADP and ATP are examples of phosphoric acid anhydrides • Note the similarity to acyl anhydrides • Large negative free energy change on hydrolysis is due to: – electrostatic repulsion – stabilization of products by ionization and resonance – entropy factors Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Phosphoric-Carboxylic Anhydrides • These mixed anhydrides - also called acyl phosphates - are very energy-rich • Acetyl-phosphate: G°´ = -43. 3 k. J/mol • 1, 3 -BPG: G°´ = -49. 6 k. J/mol • Bond strain, electrostatics, and resonance are responsible Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Enol Phosphates • Phosphoenolpyruvate (PEP) has the largest free energy of hydrolysis of any biomolecule • Formed by dehydration of 2 -phosphoglycerate • Hydrolysis of PEP yields the enol form of pyruvate - and tautomerization to the keto form is very favorable Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham Ionization States of ATP • ATP has five dissociable protons • p. Ka values range from 0 -1 to 6. 95 • Free energy of hydrolysis of ATP is relatively constant from p. H 1 to 6, but rises steeply at high p. H • Since most biological reactions occur near p. H 7, this variation is usually of little consequence Copyright © 1999 by Harcourt Brace & Company

Biochemistry 2/e - Garrett & Grisham The Effect of Concentration • • Free energy changes are concentration dependent We will use the value of -30. 5 k. J/mol for the standard free energy of hydrolysis of ATP But at non-standard-state conditions (in a cell, for example), the G is different! Equation 3. 12 is crucial - be sure you can use it properly In typical cells, the free energy change for ATP hydrolysis is typically -50 k. J/mol Copyright © 1999 by Harcourt Brace & Company