= Solution
The beta-carbon bond-forming <transition state> retains the same basic connectivity across this <pH> range, but the predominant proton-transfer route and enzyme <protonation> change. Near the acidic end, \b[protonated glutamate can act as a general acid], donating a <proton> or a strong <hydrogen bond> to carbonyl <oxygen> and making C3 more <electrophilic>; it is less available as the carboxylate general base that activates <water>. Above its local $pK_a$, \b[carboxylate-mediated <water> activation is favored]. Acid <catalysis> therefore shifts positive-charge stabilization toward a protonated carbonyl, whereas base-assisted attack reduces positive-charge buildup on incoming <water>. Neither a unique transition-state geometry nor a numerical <pH> optimum follows from <pH> alone.
Using a free-solution glutamate side-chain $pK_a\simeq4.2$ only as an illustration, the deprotonated fraction is $[1+10^{pK_a-\mathrm{pH}}]^{-1}$: about 0.059 at <pH> 3 and 0.998 at <pH> 7. An enzyme environment can shift that $pK_a$ substantially. The <glutamine> amide and the hydrocarbon side chains remain in their ordinary forms from <pH> 3 to 10; <arginine> guanidinium, with a typical free side-chain $pK_a$ around 12.5, remains predominantly positive even at <pH> 10. Thus the major side-chain switch among those listed is glutamate. At the alkaline end, bulk hydroxide can also contribute to nonenzymatic <conjugate addition>. A measured pH-rate profile must control substrate stability and enzyme folding before assigning every loss of activity to an active-site titration.
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