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Glutamine has a neutral side-chain amide. Its N–H groups can donate hydrogen bonds and its carbonyl oxygen can accept them; its amide nitrogen is not an ordinary amine base.

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.^[ref]

At pH 7, a glutamate residue is normally carboxylate. 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: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. 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 $\mathrm{Gln}-\mathrm{CON}{\mathrm{H}}_2\cdots \mathrm{O}=C1-C2=C3$, 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. It can stabilize developing negative charge by hydrogen bonding and electrostatic attraction, drawn $\mathrm{Arg}-\mathrm{guanidiniu}{\mathrm{m}}^{+}\cdots {\mathrm{O}}^{\delta -}-C1$. 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.

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, 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 $p{K}_a$, 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 $p{K}_a\simeq 4.2$ only as an illustration, the deprotonated fraction is $[1+10^{p{K}_a-\mathrm{pH}}{]}^{-1}$: about 0.059 at pH 3 and 0.998 at pH 7. An enzyme environment can shift that $p{K}_a$ 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 $p{K}_a$ 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.

Y91F implicates the Tyr hydroxyl, while E57Q implicates the Glu carboxylic-acid chemistry. Replacing tyrosine by phenylalanine retains an aromatic side chain but removes its hydroxyl, supporting a role for Tyr91 as a hydrogen bond donor that polarizes the steroid C3 carbonyl and stabilizes an enolate-like intermediate or transition state. Its hydroxyl could participate in a proton relay, but inactivity alone does not establish that it supplies the final proton.

Replacing a glutamate residue by glutamine approximately preserves side-chain size while replacing a titratable carboxylic acid by an amide. Loss of activity, combined with the acidic activity range, supports protonated Glu57 as a carbonyl-activating acid and possible proton donor. The two residues can cooperate to stabilize oxygen charge during reduction. The inhibitor-binding structure supports local contacts; it is not a direct picture of a testosterone transition state, and an N–H contact of the aza inhibitor need not occur in testosterone.

To distinguish these chemical roles from indirect effects, compare mutant and wild-type abundance, membrane insertion and folding, cofactor binding, and steroid binding. Measure initial rates with matched enzyme amounts and include no-enzyme and inhibitor-treated wild-type controls. If binding survives but turnover fails, the evidence for a catalytic defect is stronger. The published study combines structure, mutagenesis and modeled testosterone placement, so the proposed residue roles should be distinguished from direct observation of proton trajectories.^[ref]