L-threonine dehydrogenase (TDH)-likeMDR/AHD-like proteins, including a protein annotated as a threonine dehydrogenase. L-threonine dehydrogenase (TDH) catalyzes the zinc-dependent formation of 2-amino-3-ketobutyrate from L-threonine via NAD(H)-dependent oxidation. The zinc-dependent alcohol dehydrogenases (ADHs) catalyze the NAD(P)(H)-dependent interconversion of alcohols to aldehydes or ketones. Zinc-dependent ADHs are medium chain dehydrogenase/reductase type proteins (MDRs) and have a NAD(P)(H)-binding domain in a Rossmann fold of an beta-alpha form. The N-terminal region typically has an all-beta catalytic domain. In addition to alcohol dehydrogenases, this group includes quinone reductase, sorbitol dehydrogenase, formaldehyde dehydrogenase, butanediol DH, ketose reductase, cinnamyl reductase, and numerous others. These proteins typically form dimers (typically higher plants, mammals) or tetramers (yeast, bacteria), and generally have 2 tightly bound zinc atoms per subunit. The active site zinc is coordinated by a histidine, two cysteines, and a water molecule. The second zinc seems to play a structural role, affects subunit interactions, and is typically coordinated by 4 cysteines.