AANP FNP Certification Practice Exam

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Passage 1: A phosphatase called PHDX removes phosphate groups from a peptide substrate. For wild-type PHDX, Km = 2.0 μM and Vmax = 120 nmol/min. An R45A variant has Km = 8.0 μM and Vmax = 118 nmol/min. A reversible inhibitor increases the apparent Km of wild-type PHDX to 10 μM without changing Vmax. A conserved histidine in the active site participates in proton transfer. The peptide substrate contains a negatively charged residue that contacts Arg45. Why would the R45A change tend to increase Km?

Alanine forms a stronger ionic interaction with the substrate The mutation increases the concentration of substrate in solution Alanine introduces a new disulfide bond at the active site Removing the positively charged arginine weakens electrostatic stabilization of substrate binding

Correct Answer:

Removing the positively charged arginine weakens electrostatic stabilization of substrate binding

Explanation:

Arginine is positively charged at physiological pH and can stabilize a negatively charged substrate group by electrostatic attraction. Alanine is nonpolar and cannot provide the same ionic interaction, so binding is weakened and Km can rise.

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