<p>3-hydroxyacyl-CoA dehydrogenase (<db_xref db="EC" dbkey="1.1.1.35"/>) (HCDH) [<cite idref="PUB00004646"/>] is an enzyme involved in fatty acid metabolism, it catalyzes the reduction of 3-hydroxyacyl-CoA to 3-oxoacyl-CoA. Most eukaryotic cells have 2 fatty-acid beta-oxidation systems, one located in mitochondria and the other in peroxisomes. In peroxisomes 3-hydroxyacyl-CoA dehydrogenase forms, with enoyl-CoA hydratase (ECH) and 3,2-trans-enoyl-CoA isomerase (ECI) a multifunctional enzyme where the N-terminal domain bears the hydratase/isomerase activities and the C-terminal domain the dehydrogenase activity. There are two mitochondrial enzymes: one which is monofunctional and the other which is, like its peroxisomal counterpart, multifunctional.</p><p>In <taxon tax_id="562">Escherichia coli</taxon> (gene fadB) and <taxon tax_id="296">Pseudomonas fragi</taxon> (gene faoA) HCDH is part of a multifunctional enzyme which also contains an ECH/ECI domain as well as a 3-hydroxybutyryl-CoA epimerase domain [<cite idref="PUB00004380"/>].</p><p>There are two major regions of similarity in the sequences of proteins of the HCDH family, the first one located in the N-terminal, corresponds to the NAD-binding site, the second one is located in the centre of the sequence. This represents the C-terminal domain which is also found in lambda crystallin. Some proteins include two copies of this domain.</p>