= Solution
A chemically coherent mechanism combines zinc-activated <water> addition with a <proton relay> involving His238 and Glu217. In a simple route consistent with the displayed contacts, neutral His238 accepts a proton from zinc-bound <water> while protonated Glu217 donates to <substrate> N1. These transfers can be coupled to the bond-forming event rather than requiring accumulation of a free, doubly activated <substrate>. Zinc-bound <hydroxide> attacks C6; the C6=N1 pi <electrons> move onto N1, yielding the tetrahedral C6 intermediate bearing OH and NH$_2$ while N1 bears H.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2024/paper-2-enzyme.png]
{title=One proton-relay mechanism consistent with water-derived oxygen and the catalytic residues}
Next, the exocyclic amino group receives a proton, through His238, Glu217 and/or intervening <water> as permitted by the changing geometry. The <oxygen> is deprotonated as it forms the C6 carbonyl, and the C6–N6 bond cleaves to release <ammonia>. These transfers can be coupled in the collapse step, avoiding expulsion of $\mathrm{NH_2^-}$. Zinc stabilizes the reacting <oxygen> and the organized intermediate; the ring regains conjugation in the <inosine> product. Product dissociation, fresh <water> binding and proton exchange with solvent reset the catalytic groups.
\b[The core result is zinc-assisted addition–elimination, with acid/base <proton transfers> and <water> supplying C6 <oxygen>.] The contributions are <metal ion catalysis>, <general base catalysis>, <general acid catalysis>, <electrostatic catalysis> and <proximity and orientation catalysis>. The proposed route does not require <covalent catalysis> by a protein side chain: a <substrate>–<water> C–O bond is not an <enzyme>–<substrate> covalent linkage. Asp296 can retain a structural hydrogen-bonding role without acting as an indispensable acid/base.
The data establish the importance of His238 and Glu217 but do not uniquely assign the entire <proton relay>. In particular, an alternative has Glu217 first deprotonate <water> and then deliver that proton to N1, with His238 assisting <hydroxide> stabilization and later amino-group <protonation>. The primary observation that His238 substitution need not abolish analogue hydration cautions against calling its initial-base role proven.<Site-directed mutagenesis of histidine 238 in mouse adenosine deaminase: substitution of histidine 238 does not impede hydroxylate formation by V. Sideraki and others>[1996, pp. 15019–15028, retained hydroxylate formation and proposed electrostatic contribution]{ref} pH-rate profiles, solvent isotope effects and time-resolved intermediate experiments would discriminate these refinements. The diagram is a proposed catalytic route, not a structure directly observed by the crystal experiment.
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