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Unformatted text preview: -1-LECTURE 6. 4 September 2009 (P. J. Hollenbeck)BIOL 231ENZYME KINETICS Reading for this lecture: chap. 3: 98-104, DVD 3.1, 3.2, 4.8 Problems: 18-20; exam I04, #4a, exam I 06, #5a<Last time you heard about energy changes in chemical reactions, and learned that thermodynamicsdoesnt tell us about their rate. You also heard about coupled reactions, in which a favorable reactioncan drive an unfavorable reaction, as long as the net reaction has G < 0. These two ideas cometogether in enzymes, which greatly accelerate biochemical reactions and, in the case of coupledreactions, provide the coupling. Enzymes are proteins (usually) that catalyze the reactions in the cell. These can involve typical chemical changes, force generation for movement, structural changes, etc.>I. Enzyme FunctionA(A) G and E of reactions(1) Remember that the standard free energy change of a reaction, G , is invarianteqand defines the equilibrium state. Using G we can calculate K , and vice versa.(2) G tells us what the spontaneous directionwill be for a reaction given the startingconditions (especially [R] and [P]). But it tellsus NOTHING about the RATE of the reaction. A reaction can be energetically favorable (thatis, G < 0) but require millions of years toreach equilibrium! (see OMP decarboxylaseand others in the table below)(3) Enzymes speed up reactions that areALREADY spontaneous, usually by a factorof 10 - 10 . The reaction rate is faster812because the enzyme lowers the activationAenergy (E ). It does this by stabilizing thetransition state, a short-lived, activatedcomplex of reactants in which bonds are beingformed and broken. Enzymes DO NOT alterthe G of reactions (see left & figs. 3-12,-13).(B) Enzyme structure and behavior(1) Enzymes are not consumed in reactions, and can typically catalyze the samereaction many times per second (see table below).(2) The active site is the part of the enzyme that binds reactants and performs thedirect catalysis of the reaction. It can be composed of R-groups from amino acidresidues that are widely distributed along the protein's primary structure but arebrought together in the active site by the folding of the enzyme (3Estructure). We willdiscuss this in more detail soon, when we study protein structure. discuss this in more detail soon, when we study protein structure....
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This note was uploaded on 12/18/2009 for the course BIOL 101 taught by Professor Wormer during the Fall '08 term at Purdue University-West Lafayette.
- Fall '08