This domain is found in 3-hydroxybutyryl-CoA dehydrogenase present in E. coli. 3-hydroxybutyryl-CoA dehydrogenase catalyzes the second step in the biosynthesis of n-butanol from acetyl-CoA, in which acetoacetyl-CoA is reduced to 3-hydroxybutyryl-CoA. This domain is a reduced Rossmann-fold domain and, unlike the first Rossmann-fold domain, it is missing the catalytic residues and an NAD(H) binding cleft [1] [2].

1: Crystal structure of (S)-3-hydroxybutyryl-CoA dehydrogenase from Clostridium butyricum and its mutations that enhance reaction kinetics. Kim EJ, Kim J, Ahn JW, Kim YJ, Chang JH, Kim KJ; J Microbiol Biotechnol. 2014;24:1636-1643. PMID:25112316

2: Sequestration of the active site by interdomain shifting. Crystallographic and spectroscopic evidence for distinct conformations of L-3-hydroxyacyl-CoA dehydrogenase. Barycki JJ, O'Brien LK, Strauss AW, Banaszak LJ; J Biol Chem. 2000;275:27186-27196. PMID:10840044