= Solution
\b[<catalysis> requires a protonated group, consistent with a general-acid contribution.] Loss of its <proton> as <pH> rises would remove carbonyl activation or the <proton> needed to finish the conjugate reduction. A <glutamate residue> with a shifted active-site $pK_a$ is a plausible candidate, but a <pH> cutoff alone neither identifies the residue nor measures a precise $pK_a$. Irreversible unfolding or changes in substrate binding must also be excluded by reversible <pH> jumps, protein-integrity checks and initial-rate measurements.
For the mechanistic notation below, the reduced <cofactor (biochemistry)> is written <NADPH>, the established cofactor of human steroid <5α-reductase>. The original PDF explicitly labels it <NADH>; its abbreviated N-substituent does not display the phosphate distinction. <NADH> and <NADPH> have the same reactive reduced nicotinamide ring, so the local hydride-transfer arrows are unchanged, although their biochemical specificity is different.<Structure of human steroid 5α-reductase 2 with the anti-androgen drug finasteride by Qingpin Xiao and others>[2020, Fig. 3 and catalytic-mechanism discussion: NADPH-dependent reduction and protonated E57]{ref}
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