This group contains proteins identified as sorbitol dehydrogenases and other sugar dehydrogenases of the medium-chain dehydrogenase/reductase family (MDR), which includes zinc-dependent alcohol dehydrogenase and related proteins. Sorbitol and aldose reductase are NAD(+) binding proteins of the polyol pathway, which interconverts glucose and fructose. Sorbitol dehydrogenase is tetrameric and has a single catalytic zinc per subunit. NAD(P)(H)-dependent oxidoreductases are the major enzymes in the interconversion of alcohols and aldehydes, or ketones. Related proteins include threonine dehydrogenase, formaldehyde dehydrogenase, and butanediol dehydrogenase. The medium chain alcohol dehydrogenase family (MDR) has a NAD(P)(H)-binding domain in a Rossmann fold of a beta-alpha form. The N-terminal region typically has an all-beta catalytic domain. These proteins typically form dimers (typically higher plants, mammals) or tetramers (yeast, bacteria), and have 2 tightly bound zinc atoms per subunit. Horse liver alcohol dehydrogenase is a dimeric enzyme and each subunit has two domains. The NAD binding domain is in a Rossmann fold and the catalytic domain contains a zinc ion to which substrates bind. There is a cleft between the domains that closes upon formation of the ternary complex.