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METABOLIC BIOCHEMISTRY Summer 2005 Immo E. Scheffler MIDTERM EXAM All answers are to be written into the Blue Book. Leave the first inside page blank for scoring and write a column of numbers from 1 to 15. There are 15 questions. Make sure that each answer is clearly identified with the question number at the top or left side of the page. For the short answer questions you need to write down only the number for the correct answer, for example: 1a) A (or B, or C, or D, or E) There will be no credit for a wrong guess. Useful Information: Avogadro's number: 6.02 x 10 23 molecules / mole 1 Faraday = 96,494 Coulomb / mole = 96,494 Joules / Volt / mole Gas constant (R) = 8.31 Joules K -1 mol -1 = 1.987 cal K -1 mol -1 = 0.082 liter atm K -1 mol -1 1 calorie = 4.184 Joules QUESTION 1 (10 min) (one point for each correct answer; 2 points for (i)) a) One of the enzymes involved in glycolysis, aldolase, requires Zn 2+ for catalysis. Under conditions of zinc deficiency, when the enzyme may lack zinc, it would be referred to as the: A) apoenzyme. B) coenzyme. C) holoenzyme. D) prosthetic group. E) substrate. b) Enzymes are potent catalysts because they: A) are consumed in the reactions they catalyze. B) are very specific and can prevent the conversion of products back to substrates. C) drive reactions to completion while other catalysts drive reactions to equilibrium. D) increase the equilibrium constants for the reactions they catalyze. E) lower the activation energy for the reactions they catalyze . c) The benefit of measuring the initial rate of a reaction V 0 is that at the beginning of a reaction: A) [ES] can be measured accurately. B) changes in [S] are negligible, so [S] can be treated as a constant . C) changes in K m are negligible, so K m can be treated as a constant. D) V 0 = V max . E) varying [S] has no effect on V 0 .
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d) Which of the following statements about a plot of V 0 vs. [S] for an enzyme that follows Michaelis-Menten kinetics is false ? A) As [S] increases, the initial velocity of reaction V 0 also increases. B) At very high [S], the velocity curve becomes a horizontal line that intersects the y-axis at K m . C) K m is the [S] at which V 0 = 1/2 V max . D) The shape of the curve is a hyperbola. E) The y-axis is a rate term with units of μ m/min. e) Michaelis and Menten assumed that the overall reaction for an enzyme-catalyzed reaction could be written as k 1 k 2 E + S ES E + P k -1 Using this reaction, the rate of breakdown of the enzyme-substrate complex can be described by the expression: A) k 1 ([E t ] - [ES]). B) k 1 ([E t ] - [ES])[S]. C) k 2 [ES]. D) k -1 [ES] + k 2 [ES]. E) k -1 [ES]. f) The steady state assumption, as applied to enzyme kinetics, implies: A) K m = K s . B) the enzyme is regulated. C) the ES complex is formed and broken down at equivalent rates . D) the K m is equivalent to the cellular substrate concentration. E) the maximum velocity occurs when the enzyme is saturated.
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