Question

2) Imagine an enzyme that has a histidine in the substrate binding pocket that regulates substrate binding based on its protonation state to control enzyme function. Substrates can only bind in the unprotonated state; in other words, substrate binding is blocked when the histidine is protonated and when the substrate is bound, the histidine cannot be protonated. This enzyme exhibits classical Michaelis-Menten behavior. a) Assume that the pka of the histidine in the substrate binding pocket the same as for free histidine. Is the enzyme more active at pH 5 or pH 8? Explain your logic with 1-2 sentences. [4 pts] b) Describe on the lines below how the following two mutations would likely influence substrate binding increase, decrease, or no change) at pH 6. Explain your logic below with 1-2 sentences. [8 pts] wild-type enzyme substrate binding pocket HN HN NH, c) You want to describe the Michaelis-Menten expression for initial rate of this enzyme (v.) as a function of [S], Vmax, and Km to reflect how pH influences vo. Please select the expression below that best describes how pH influences ve of this enzyme, fill in the box describing a and explain your answer with 1-2 sentences below. Hint: think about the acid dissociation constant and [H+] [6 pts] Vo = V = v = Vmax [S] akm + [S] a' max [S] a' a = + akm + [5] Km + [S]

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