= Solution
\b[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.<Mutational analysis of active site residues of human adenosine deaminase by D. Bhaumik and others>[1993, pp. 5464–5470, kinetic and zinc-content comparisons]{ref}
Back to article page
