Past exam of the chemistry B course of the University of Cambridge / 2024 / Paper 2 / B10 / d / iii / Solution

Glu217 is important for catalysis, most naturally through proton transfer at the purine ring. Changing a glutamic acid residue to glutamine preserves much of the side-chain size and hydrogen-bonding geometry, while removing an ordinary carboxylic acid/base pair. Its placement near N1 makes acid donation there a plausible way to activate C6 for water addition and stabilize the resulting nonaromatic intermediate. The large turnover defect with little change in the Michaelis constant supports that catalytic interpretation, subject to the same $K_M$ and protein-integrity qualifications as above.

The protonated carboxyl drawn at pH 7 need not behave like free glutamate in water: local electrostatics and burial can shift its p$K_a$, and protonation may change during the cycle. The mutation alone cannot choose between an initially protonated Glu217 acid and a Glu217 residue that first accepts and then relays a proton. Primary mutational work supports catalytic roles for active-site histidine and glutamate groups while explicitly comparing purified enzymes and zinc content.^[ref]