BCHM 561 exam 3

BCHM 561 exam 3 - BCHM 561 EXAM III Fall 2009 NAME(please...

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- 1 - NAME ( please print clearly ) Please write legibly in the space provided. If additional space is needed to write your answers, use the back of the pages provided making sure you include the question number. The total points assigned to each question are enclosed in brackets. There are a total of 200 points. There may be more than one correct selection for multiple choice questions. Circle all correct choices. ENZYME MECHANISMS AND REGULATION (Chapters 9 and 10) 1. [4] Briefly, describe the events that occur at the active site of an enzyme that uses covalent catalysis to form and stabilize the transition state. 2. [6] (A) How do Mg 2+ ions enhance catalysis by adenylate kinase? (B) Do Mg 2+ ions play a similar role in the reaction catalyzed by protein kinases? 3. [6] (A) Briefly, describe the key characteristics of an allosteric enzyme. (B) Explain how measuring the rate of an enzyme at varying substrate concentrations might reveal that an enzyme is allosterically regulated? BCHM 561 EXAM III Fall 2009 Enzymes that act via covalent catalysis have an active site containing an amino acid with a chemically reactive side chain, which is usually a good nucleophile. The reactive side chain forms a covalent bond with the substrate or an intermediate derived from the substrate. To regenerate free enzyme, this covalent link to the protein is transient and must be broken as the reaction progresses. See Stryer: p 269 Fig. 9.50 See Stryer: p 242 (1) Mg 2+ is coordinated to oxygen atoms on the β and γ phosphate groups of ATP, and thereby helps to position the γ phosphate for optimal transfer to the nucleotide monophosphate, the second substrate of the enzyme. (2) Through water molecules, the Mg 2+ ion bound to ATP provides additional points of contact between ATP and the enzyme to increase the binding energy for ATP. (1) An allosteric enzyme is a regulatory enzyme with catalytic activity that is modulated by the noncovalent binding of a molecule (usually a metabolite) at a site distinct from and often distal to the active site. The binding of this molecule can positively or negatively effect activity. (2) In most cases, allosteric enzymes are multimeric and exhibit cooperative substrate binding. Yes In most cases, allosteric enzymes do not exhibit a plot of V versus [S] that is hyperbolic and conforms to Michaelis-Menton kinetics, instead the curve from such plots will be sigmoidal (“S”-shaped), a hallmark of enzymes exhibiting cooperative substrate binding. See Stryer: p 277
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- 2 - 4. [6] The conversion of threonine to isoleucine is catalyzed by a biosynthetic pathway consisting of five distinct enzymes. The enzyme ( threonine dehydratase ) catalyzing the first step in this sequence of reactions is inhibited by isoleucine, but not by any of the four intermediates in the pathway. (A) What terms are used to describe this mode of regulation? (B) Why is this mode of regulation advantageous to the cell?
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BCHM 561 exam 3 - BCHM 561 EXAM III Fall 2009 NAME(please...

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