Ch8 - Chapter 8 Enzyme Catalysis Kinetics...

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Chapter 8 Enzyme Catalysis Kinetics
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Enzymes Catalyze a Variety of Reactions Carbonic anhydrase Assists transfer of CO2  through blood: million  molecules/sec Proteolytic  enzymes Specificity:T Thrombin (B) Papain  cleaves any  peptide bond Example of specificity: DNA polymerase I Specificity arises from 3D structure of protein Substrates = reactants
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Enzymes Enhance Rates of Reactions that Would not  Occur in the Cell Spontaneously Rate is enhanced by selective stabilization of the transition state
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Cofactors Apoenzyme +  cofactor =  holoenzyme Coenzymes =  organic molecule  cofactors Some vitamin  coenzymes are used  by several enzymes  with similar  mechanism Tightly bound  coenzymes are  called “prosthetic  groups”
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Free Energy and Activation Free  G is the difference in Free Energy  between the products and the reactants G < 0 , spontaneous (exergonic) reaction G = 0  the system is in equilibrium G > 0 , reaction requires energy input  (endergonic) G‡ is the activation free energy G is independent of path,  G‡is related to 
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Standard Free Energy and the Equilibrium Constant A + B C + D G =  G° + RTln[C][D]/[A][B]  = standard free energy change,  R  = gas constant,  T  =  temperature, [] are molar concentrations Standard conditions  = 1 molar concentration, atmospheric pressure G°’  = free energy change under biological conditions, pH =7 At Equilibrium,  G = 0, then G°’ = -RT ln[C][D]/[A][B] = -RT ln K’ eq 
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Values for Free Energy Changes G°’  = -RT ln K’ eq  = -2.303RT log10 K’eq Therefore: K’ eq = 10- G°’/ (2.303RT) R = 1.987 X 10-3 kcal mol-1 deg-1, & T = 298 K gives K’ eq = 10- G°’/1.36   Isomerization of DHAP to GAP K’ eq = 0.0475 at 298 K G°’  = -2.303 RTlog10 K’ eq = - 2.303 X 1.987 X 10-3 X 298 X  log10(0.0475)   = 1.80 kcal mol-1 (7.53 kJ mol-1) 3 X 10-6 M G is -0.69 kcal mol-1 Reaction is spontaneous!
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Enzymes Act by Modifying G‡, not  G A B 10-4s-1 10-6s-1 K = [B]/[A] = kF/kR = 10-4/10-6 = 100 Ratio of B to A is not affected by enzyme! G‡ = GS‡ - GS
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Ch8 - Chapter 8 Enzyme Catalysis Kinetics...

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