Multiple Choice
An enzyme has two key catalytic residues, Glu 35 (pKa= 5.9) and Asp 52 (pKa= 4.5). Which of the following is likely true about the mechanism for this enzyme if the optimum pH = 5.2?
6. Enzymes and Enzyme Kinetics
Covalent Catalysis
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Suppose that the covalent catalytic mechanism of an enzyme depends on a single active site amino acid (Cys), whose pKa = 8.3. A mutation in a nearby amino acid residue of the enzyme only slightly alters the microenvironment so that the pKa of the Cys residue increases to 10.3. Would this mutation cause the enzyme-catalyzed reaction rate to increase or decrease? Explain.
A
Reaction rate increases.
B
Reaction rate decreases.
Verified step by step guidance1
Identify the role of the cysteine residue in the enzyme's catalytic mechanism. Cysteine often acts as a nucleophile, meaning it donates an electron pair to form a chemical bond in reaction mechanisms.
Understand the significance of pKa in this context. The pKa value indicates the pH at which the cysteine residue is half ionized. A lower pKa means the thiol group (S-H) is more likely to be deprotonated and thus more nucleophilic at physiological pH.
Analyze the effect of the mutation on the pKa value. The mutation increases the pKa from 8.3 to 10.3, meaning the thiol group is less likely to be deprotonated at physiological pH, reducing its nucleophilicity.
Consider the impact on the reaction rate. Since the nucleophilicity of the cysteine residue is reduced, the enzyme's ability to catalyze the reaction decreases, leading to a slower reaction rate.
Conclude that the mutation causes the enzyme-catalyzed reaction rate to decrease due to the reduced nucleophilicity of the cysteine residue at physiological pH.
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