= Solution
\b[Hydride is delivered to steroid C5 and a <proton> to C4; the C3 ketone is retained.] Use the conjugated fragment $\mathrm{O{=}C3{-}C4{=}C5}$. Carbonyl <hydrogen bonds> from Tyr91–OH and protonated Glu57 lower the barrier to <conjugate reduction>. Draw a curved arrow from the nicotinamide C4–H bond of <NADPH> to steroid C5, shift the C4=C5 pi pair toward C3–C4, and shift C3=O pi <electrons> onto <oxygen>. The nicotinamide ring rearomatizes to <NADP+>; the steroid becomes a bound <enolate>, or its protonated <enol> equivalent. <electron> shifts across the nicotinamide ring must accompany C–H bond cleavage rather than leaving an unexplained neutral <carbon> radical.
Next, reform C3=O and protonate steroid C4 from an appropriately positioned acid, directly or through an <enol>/proton-relay route. This regenerates the catalytic acid after solvent reprotonation. The enzyme fixes the hydride-accepting face so that the new C5–H has \b[the 5α configuration], giving <dihydrotestosterone>; delivery to the opposite face would give a different ring-junction configuration. This is not simple 1,2 reduction of the C3 carbonyl to an alcohol. The net local bookkeeping is one hydride plus one <proton> across C4=C5, with two <electrons> supplied by <NADPH>. The original reactive-fragment diagram shows the enolate-forming arrows and the separate protonation step.
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