May play a role in supplying butyryl-CoA for straight-chain fatty acid biosynthesis. Catalyzes the conversion of crotonyl-CoA to butyryl-CoA. It shows a high substrate specificity for crotonyl-CoA, a short-chain-length (C4), but no measurable activity is observed with shorter (C3) or longer-chain-length enoyl-CoA thioesters. butanoyl-CoA + NADP(+) = (2E)-butenoyl-CoA + H(+) + NADPH Inhibited by divalent cations (30-100%), beta-chloromercuribenzoate (85%), iodoacetamide (40%) and N-ethylmaleamide (80%). The presence of CoA thioesters containing 12-20 carbon atoms results in inhibition of enzyme activity. The greatest degree of inhibition is observed in the presence of palmitoyl-CoA and myristoyl-CoA. The branched-chain fatty acids, isopalmitoyl-CoA and isomyristoyl-CoA are less effective inhibitors of the crotonyl-CoA reductase. Concentrations of NADPH above 200 uM lead to inhibition of enzyme activity. Optimum pH is 6.5. Optimum temperature is 40 degrees Celsius. Homodimer. Belongs to the zinc-containing alcohol dehydrogenase family. Crotonyl-CoA carboxylase/reductase subfamily.