ch06.2 - Which of the following is true about the role of...

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Unformatted text preview: Which of the following is true about the role of enzymes in catalyzing chemical reactions? A. Enzymes alter the equilibrium of a reaction. B. Enzymes are needed to catalyze only thermodynamically unfavorable reactions. C. Enzymes enhance the rate of reactions. D. Enzymes eliminate the activation energy of a reaction. Catalyst lowers activation energy of the transition state V&V14-6 Activation energy of catalyzed reaction Transition state Activation energy of uncatalyzed reaction Where is the transition state? 1.Enzyme (E) first combines reversibly with substrate (S) to form enzyme-substrate complex (ES) in fast reaction: E + S ES 2.ES breaks down in a second reversible reaction, which is typically slower , to yield product, P and free enzyme: ES P + E 3.2nd reaction is rate limiting (slower) overall rate of reaction proportional to concentration of ES 1.Enzyme (E) first combines reversibly with substrate (S) to form enzyme-substrate complex (ES) in fast reaction: E + S ES 2.ES breaks down in a second reversible reaction, which is typically slower , to yield product, P and free enzyme: ES P + E 3.2nd reaction is rate limiting (slower) overall rate of reaction proportional to concentration of ES 4. At any given instant, enzyme exists in two forms: free (E) or combined (ES). Rate will be maximal when virtually all enzyme is in ES form . This condition will exist at very high [S], due to law of mass action: E + S ES (i.e. equilibrium will be pushed to right) 5. At high [S], essentially all free E will be in ES form. As ES breaks down into E + P in second reaction, E will immediately bind another S. Under these conditions, a steady state is achieved in which enzyme is always saturated with substrate (i.e. [ES] is constant) and reaction rate is maximal = V max . Michaelis-Menten equation relates initial velocity, v , maximal velocity, V max , and substrate concentration [S] to one constant, K m = Michaelis constant: v = V max [ S ] [ S ] + K m 6-11 K m = k 2 + k " 1 k 1 Meaning of K m Box 6-1 1 v o = K m V max 1 [ S ] + 1 V max Double-reciprocal plot (Lineweaver-Burk) Where are the highest [S] values? v = V max [ S ] " K m + [ S ] " = 1 + [ I ] K i where 6-15 K i = [ E ][ I ] [ EI ] v = V max [ S ] K m + " '[ S ] " ' = 1 + [ I ] K ' i K ' i = [ ES ][ I ] [ ESI ] 6-15 v = V max [ S ] " K m + " '[ S ] 6-15 Competitive Does K m change? Does V max change? Apparent K m increases No v = V max [ S ] " K m + [ S ] " = 1 + [ I ] K i Uncompetitive Does K m change?...
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ch06.2 - Which of the following is true about the role of...

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