103 - final review

103 - final review - Chemistry review notes Lecture 17...

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Chemistry review notes Lecture 17 – kinetics, catalysis, and enzymes Living systems depend on the efficient, controlled execution of chemical reactions o Problem: biochemical reactions are very slow in water at ambient temperature, neutral pH, even when G is negative Enzymes: Achieve phenomenal rate enhancements (up to 10 17 fold) Display exquisite specificity (include stereospecificity) Most are true catalysts, not consumed during the reaction The removal of the end amino acid from the protein by reaction with a molecule of water o Water molecule + protein amino acid + new protein Equilibrium constant: Keq = [P]/[S] From equilibrium thermodynamics o G = -RTln Keq Enzyme kinetics o 1. Enzymes don’t alter the equilibrium constant o 2. From transition state theory, catalysis consists of (and indeed can be defined as)) the selective stabilization of the transition state relative to the ground state Lecture 18 Mechanism o E + S ES (fast) o ES E + P (slow) o E total = E free + ES K = [ES] / [E][S] When [S] is low, E total [ES] o Rate = k 2 K[E total ][S] When [S] is very large, [ES] E total , pseudo first order in [E] o Back rate is involved, but simplifies to V 0 = rate = k[E] total When [S] is high and vo = vmax, then v max = k cat [E total ] To be effective, an enzyme must be most complementary to the transition state, not the ground state Binding energy/complementarity explain two of the central features of enzymes: rate enhancement (catalysis) and specificity. Both arise from the same phenonmenon Electrostatic potential occurs around the enzyme Cu, Zn-superoxide dismutase K m behaves somewhat like the dissociation constant for a weak acid: the smaller the value the more tightly bound the substrate Diffusion-limited enzymes are said to be “catalytically perfect”; no changes to the active site or mechanism will improve the rate k cat /K M can’t be higher than the rate of diffusion (10 8 /Ms) Cell potential o Maximum useful work obtainable from a chemical reaction
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o Force behind the movement of electrons is defined in terms of volts o A positive cell potential corresponds to a spontaneous reaction - - Lecture 19 – electrochemistry As the cell discharges over time … Ion movement is required for conduction – circuit’s broken if there’s no bridge Eventually the cell voltage goes to zero. Once at equilibrium, there is no further driving force (G=0) until the system is altered and the equilibrium is disturbed o G = G ° + RT lnQ o At equilibrium Q=K and G=0 G ° = -RTlnK 1 V = potential = E = 1 joule/coulomb o This potential difference provides the driving force that pushes charge (e-) through an external circuit EMF – electromotive force (causing e- movement) Current = charge/time = amp = coulombs/second o Coulombs: quantity of charge that moves through a point in a circuit then 1 amp flows for 1 second
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This note was uploaded on 04/25/2008 for the course CHEM 103 taught by Professor O'halloran during the Spring '08 term at Northwestern.

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103 - final review - Chemistry review notes Lecture 17...

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