L-threonine dehydrogenase (TDH) catalyzes the zinc-dependent formation of 2-amino-3-ketobutyrate from L-threonine via NAD(H)- dependent oxidation. THD is a member of the zinc-requiring, medium chain NAD(H)-dependent alcohol dehydrogenase family (MDR). MDRs have a NAD(P)(H)-binding domain in a Rossmann fold of a beta-alpha form. NAD(P)(H)-dependent oxidoreductases are the major enzymes in the interconversion of alcohols and aldehydes, or ketones. 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. Sorbitol and aldose reductase are NAD(+) binding proteins of the polyol pathway, which interconverts glucose and fructose.