This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donor with NAD+ or NADP+ as acceptor.
In bacteria, HDH is a single chain polypeptide; in fungi it is the C-terminal domain of a multifunctional enzyme which catalyses three different steps of histidine biosynthesis; and in plants it is expressed as a nuclear encoded protein precursor which is exported to the chloroplast.
[4] However, according to newer studies with histidinol dehydrogenase from E. coli, the mechanism is catalyzed by four bases, B1-B4.
[4] The reaction proceeds via a tightly- or covalently-bound intermediate, and requires the presence of 2 NAD molecules.
[5] As of late 2007, 4 structures have been solved for this class of enzymes, with PDB accession codes 1K75, 1KAE, 1KAH, and 1KAR.