= Solution
At <pH> 7, a <glutamate residue> is normally carboxylate. \b[Its most natural role is general-base activation of the attacking <water>.] Draw the carboxylate <oxygen> <lone pair> toward a <water> H, the <water> O–H pair toward its <oxygen>, and that <oxygen>'s <lone pair> toward C3 while the conjugated pi system moves toward the carbonyl <oxygen>:
$$
\mathrm{Glu{-}CO_2^-\cdots H{-}OH\cdots C3}
\longrightarrow
\mathrm{Glu{-}CO_2H}+\text{bound beta-hydroxy enolate}.
$$
This can be concerted, avoiding a freely diffusing hydroxide intermediate. The protonated glutamate subsequently returns its <proton> to the <enolate> system, directly or through solvent, regenerating carboxylate. A favorable arrangement is needed; ordinary carboxylate does not quantitatively deprotonate bulk <water> in a prior equilibrium.
The side-chain amide of <glutamine> is neutral. \b[Its N–H can donate a <hydrogen bond> to the carbonyl <oxygen> and stabilize developing <enolate> charge], while its amide <oxygen> can accept a <hydrogen bond> to orient attacking <water>. Draw $\mathrm{Gln{-}CONH_2\cdots O{=}C1{-}C2{=}C3}$, with the dotted interaction from amide N–H to substrate <oxygen>. This assists polarization and <transition state stabilization>; the amide <nitrogen> is not an ordinary amine base because its <lone pair> is delocalized into its own carbonyl.
The guanidinium group of <arginine> is positively charged. \b[It can stabilize developing negative charge by hydrogen bonding and electrostatic attraction], drawn $\mathrm{Arg{-}guanidinium^+\cdots O^{\delta-}{-}C1}$. Its delocalized cation is not the preferred neutral general base at <pH> 7. Productive <catalysis> requires stronger stabilization of the reacting arrangement than of the unreactive ground-state complex.
<Phenylalanine> and <leucine> have hydrocarbon side chains and provide no conventional acid/base or hydrogen-bond donor/acceptor group here. They can position the <enone>, exclude unproductive solvent and select the attacked face through <hydrophobic effect> and <steric hindrance>. Those binding effects can assist <catalysis> but should not be drawn as an imaginary side-chain <proton> relay. The original schematic depicts the Glu <water> relay and the Gln/Arg oxyanion-stabilizing contacts.
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