Bài giảng Biochemistry 2/e - Chapter 3: Thermodynamics of Biological Systems

Chapter 3Thermodynamics of Biological Systemsto 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 OutlineBasic Thermodynamic ConceptsPhysical Significance of Thermodynamic PropertiespH and the Standard StateThe Effect of ConcentrationCoupled ProcessesHigh-Energy BiomoleculesBasic ConceptsThe system: the portion of the universe with which we are concernedThe surroundings: everything elseIsolated system cannot exchange matter or energyClosed system can exchange energyOpen system can exchange either or bothThe First LawThe 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 systemE is heat exchanged at constant volumeE is independent of pathE2 - E1 = E = q + wq is heat absorbed BY the systemw is work done ON the system EnthalpyA better function for constant pressureH = E + PVIf P is constant, H = q H is the heat absorbed at constant PVolume is approx. constant for biochemical reactions (in solution)So H is approx. same as EThe Second LawSystems tend to proceed from ordered to disordered statesThe entropy change for (system + surroundings) is unchanged in reversible processes and positive for irreversible processesAll processes proceed toward equilibrium - i.e., minimum potential energyEntropyA measure of disorderAn ordered state is low entropyA disordered state is high entropydSreversible = dq/TThe Third LawThe entropy of any crystalline, perfectly ordered substance must approach zero as the temperature approaches 0 KAt T = 0 K, entropy is exactly zeroFor a constant pressure process:Cp = dH/dTFree EnergyHypothetical quantity - allows chemists to asses whether reactions will occurG = H - TSFor any process at constant P and T:G = H - TSIf G = 0, reaction is at equilibriumIf G < 0, reaction proceeds as writtenG versus Go’How can we calculate the free energy change for rxns not at standard state?Consider a reaction: A + B  C + DThen:G = Go’ + RT ln ([C][D]/[A][B])Energy TransferA 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, FADH2) High-energy phosphate compounds - free energy of hydrolysis larger than -25 kJ/mol) High-Energy BiomoleculesStudy 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!ATPAn 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 processesPhosphoric Acid AnhydridesWhy 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 factorsPhosphoric-Carboxylic AnhydridesThese mixed anhydrides - also called acyl phosphates - are very energy-richAcetyl-phosphate: G°´ = -43.3 kJ/mol1,3-BPG: G°´ = -49.6 kJ/molBond strain, electrostatics, and resonance are responsibleEnol PhosphatesPhosphoenolpyruvate (PEP) has the largest free energy of hydrolysis of any biomoleculeFormed by dehydration of 2-phospho-glycerateHydrolysis of PEP yields the enol form of pyruvate - and tautomerization to the keto form is very favorableIonization States of ATPATP has five dissociable protonspKa values range from 0-1 to 6.95Free energy of hydrolysis of ATP is relatively constant from pH 1 to 6, but rises steeply at high pHSince most biological reactions occur near pH 7, this variation is usually of little consequenceThe Effect of ConcentrationFree energy changes are concentration dependent We will use the value of -30.5 kJ/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 properlyIn typical cells, the free energy change for ATP hydrolysis is typically -50 kJ/mol