{{Infobox protein family | Symbol = HD | Name = HD domain | image = PDB 2hek EBI.jpg | width = | caption = crystal structure of o67745, a hypothetical protein from aquifex aeolicus at 2.0 a resolution. | Pfam = PF01966 | Pfam_clan = CL0237 | InterPro = IPR006674 | SMART = | PROSITE = PDOC00924 | MEROPS = | SCOP = 1f62 | TCDB = | OPM family = | OPM protein = | CAZy = | CDD = cd00077 }} In molecular biology, the '''HD domain''' is a conserved protein domain, named after the conserved histidine (H) and/or aspartate (D) amino acid residues. It is found in a superfamily of enzymes with a predicted or known phosphohydrolase activity. These enzymes appear to be involved in nucleic acid metabolism, signal transduction and possibly other functions in bacteria, archaea and eukaryotes. The fact that all the highly conserved residues in the HD superfamily are histidines or aspartates suggests that coordination of divalent cations is essential for the activity of these proteins.<ref name="pmid9868367">{{cite journal | vauthors = Aravind L, Koonin EV | title = The HD domain defines a new superfamily of metal-dependent phosphohydrolases | journal = Trends in Biochemical Sciences | volume = 23 | issue = 12 | pages = 469–72 | date = December 1998 | pmid = 9868367 | doi = 10.1016/s0968-0004(98)01293-6 }}</ref>
== References == {{reflist}} {{InterPro content|IPR006674}}
Category:Protein families