5. In contrast to ibuprofen, aspirin is an irreversible inhibitor of PGH2 syntha

Question

5. In contrast to ibuprofen, aspirin is an irreversible inhibitor of PGH2 synthase, reacting with a Ser residue in the active site. You have incubated 5 uM enzyme with 1 mM aspirin and have found that 85% of the enzyme was inactivated in 30 min. (a) Calculate the pseudo first-order rate constant for the reaction under these con- (b) Calculate the second order rate constant for the inactivation of PGH2 by aspirin. (c) Suppose that you wanted to use 5 mM aspirin and inactivate 99% of the enzyme. (d) Suppose that you incubate 1 MM enzyme with 1 mM aspirin for 30 min. Would ditions For how long should you allow the reaction to proceed? you expect more or less than 85% of the enzyme to be inactivated? Explain your reasoning. (e) Suppose that you incubate 1 mM enzyme with 1 mM aspirin for 30 min. Would you expect more or less than 85% of the enzyme to be inactivated? Explain your reasoning.

Explanation / Answer

a) if we consider it as pseudofirst order then

ln [At] = -kt + ln [A0]

[At] = 0.85 [A0]

kt = ln[A0] / [At] = ln (1/0.85) = 0.163

K = rate constant = 0.163 X time = 0.163 / 30 = 0.00543 min-1

b) For second order reaction

1/[At] = kt +1/[A0]

[At] = 0.85 [A0]

1/0.85[A0] - 1/[A0] = kt

0.176 = kt

k = rate constant = 0.176 / 30 = 0.00587

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