= Solution
A <glutamic acid residue> supplies a carboxyl group whose <protonation> state can change with the <active site> environment. The following five schematic contributions are possible in a hypothetical <enzyme>; they are alternatives and overlapping physical effects, rather than five independently established reactions of this <enzyme>. In the diagrams $E$ is the protein tether, and $C6$ is the reacting <purine> <carbon>.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2024/paper-2-glutamate.png]
{title=Five possible catalytic contributions of a glutamate side chain to hydrolytic deamination}
\b[<general acid catalysis>:] $\mathrm{E{-}CO_2H\cdots N1\to E{-}CO_2^-+H{-}N1^+}$. Donation of the carboxyl proton activates the amidine-like C6 center for <water> attack. The same acid could instead assist the amino leaving group: $\mathrm{E{-}CO_2H+{-}NH_2\to E{-}CO_2^-+{-}NH_3^+}$. These are two locations for <general acid catalysis>, rather than two different categories.
\b[<general base catalysis>:] $\mathrm{E{-}CO_2^-\cdots H{-}OH\to E{-}CO_2H+OH^-}$, coupled to <oxygen> attack on C6. Accepting a proton makes <water> a more effective <nucleophile>. This does not require an appreciable pool of free <hydroxide> in bulk solution.
\b[<covalent catalysis>:] a glutamate <oxygen> can in principle attack activated C6 to form $\mathrm{C6{-}O{-}C(O){-}E}$. After addition/elimination replaces the amino group, <water> must hydrolyze that transient <enzyme>–<substrate> bond to release product and regenerate $\mathrm{E{-}CO_2H/CO_2^-}$. Such a pathway is a chemically distinct possibility, not established by the given structure.
\b[<electrostatic catalysis>:] the <carboxylate> can stabilize a developing positive charge, for example in a protonated <purine> or amino leaving group; a protonated carboxyl can also donate a <hydrogen bond> to an <oxygen> developing negative charge. This contributes only if the reacting configuration is stabilized relative to the bound starting state. A <carboxylate> placed next to an <oxyanion> without compensating geometry would instead be unfavorable.
\b[<proximity and orientation catalysis>:] donor and acceptor <hydrogen bonds> can hold the <purine> and attacking <water> in a productive arrangement, schematically $\mathrm{E{-}CO_2H\cdots N1{-}C6\cdots OH_2}$. Binding reduces unproductive orientations and can distort the <substrate> toward a reactive geometry. This last contribution is sometimes described through <substrate> binding or strain; a binding interaction is not automatically catalytic unless it improves access to the <transition state>.
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