Crotonyl-CoA reductase, a member of the medium chain dehydrogenase/reductase family, catalyzes the NADPH-dependent conversion of crotonyl-CoA to butyryl-CoA, a step in (2S)-methylmalonyl-CoA production for straight-chain fatty acid biosynthesis. Like enoyl reductase, another enzyme in fatty acid synthesis, crotonyl-CoA reductase is a member of the zinc-dependent alcohol dehydrogenase-like medium chain dehydrogenase/reductase family. The medium chain dehydrogenases/reductase (MDR)/zinc-dependent alcohol dehydrogenase-like family, which contains the zinc-dependent alcohol dehydrogenase (ADH-Zn) and related proteins, is a diverse group of proteins related to the first identified member, class I mammalian ADH. MDRs display a broad range of activities and are distinguished from the smaller short chain dehydrogenases (~ 250 amino acids vs. the ~ 350 amino acids of the MDR). The MDR proteins have 2 domains: a C-terminal NAD(P) binding-Rossmann fold domain of a beta-alpha form and an N-terminal catalytic domain with distant homology to GroES.