<p>The crotonase superfamily is comprised of mechanistically diverse proteins that share a conserved trimeric quaternary structure (sometimes a hexamer consisting of a dimer of trimers), the core of which consists of 4 turns of a (beta/beta/alpha)n superhelix. Some enzymes in the superfamily have been shown to display dehalogenase, hydratase, and isomerase activities, while others have been implicated in carbon-carbon bond formation and cleavage as well as the hydrolysis of thioesters [<cite idref="PUB00035962"/>]. However, these different enzymes share the need to stabilise an enolate anion intermediate derived from an acyl-CoA substrate. This is accomplished by two structurally conserved peptidic NH groups that provide hydrogen bonds to the carbonyl moieties of the acyl-CoA substrates and form an "oxyanion hole". The CoA thioester derivatives bind in a characteristic hooked shape and a conserved tunnel binds the pantetheine group of CoA, which links the 3'-phosphate ADP binding site to the site of reaction [<cite idref="PUB00035964"/>]. Enzymes in the crotonase superfamily include:</p><p> <ul><li>Enoyl-CoA hydratase (crotonase; <db_xref db="EC" dbkey="4.2.1.17"/>), which catalyses the hydratation of 2-trans-enoyl-CoA into 3-hydroxyacyl-CoA [<cite idref="PUB00027360"/>]. </li><li>3-2trans-enoyl-CoA isomerase (or dodecenoyl-CoA isomerise; <db_xref db="EC" dbkey="5.3.3.8"/>), which shifts the 3-double bond of the intermediates of unsaturated fatty acid oxidation to the 2-trans position [<cite idref="PUB00000748"/>].</li><li>3-hydroxbutyryl-CoA dehydratase (crotonase; <db_xref db="EC" dbkey="4.2.1.55"/>), a bacterial enzyme involved in the butyrate/butanol-producing pathway.</li><li>4-Chlorobenzoyl-CoA dehalogenase (<db_xref db="EC" dbkey="3.8.1.6"/>), a Pseudomonas enzyme which catalyses the conversion of 4-chlorobenzoate-CoA to 4-hydroxybenzoate-CoA [<cite idref="PUB00035965"/>].</li><li>Dienoyl-CoA isomerise, which catalyses the isomerisation of 3-trans,5-cis-dienoyl-CoA to 2-trans,4-trans-dienoyl-CoA [<cite idref="PUB00024165"/>].</li><li>Naphthoate synthase (MenB, or DHNA synthetase; <db_xref db="EC" dbkey="4.1.3.36"/>), a bacterial enzyme involved in the biosynthesis of menaquinone (vitamin K2) [<cite idref="PUB00033366"/>]. </li><li>Carnitine racemase (gene caiD), which catalyses the reversible conversion of crotonobetaine to L-carnitine in <taxon tax_id="562">Escherichia coli</taxon> [<cite idref="PUB00035966"/>]. </li><li>Methylmalonyl CoA decarboxylase (MMCD; <db_xref db="EC" dbkey="4.1.1.41"/>), which has a hexameric structure (dimer of trimers) [<cite idref="PUB00024526"/>].</li><li>Carboxymethylproline synthase (CarB), which is involved in carbapenem biosynthesis [<cite idref="PUB00035963"/>].</li><li>6-oxo camphor hydrolase, which catalyses the desymmetrisation of bicyclic beta-diketones to optically active keto acids [<cite idref="PUB00031259"/>].</li><li>The alpha subunit of fatty oxidation complex, a multi-enzyme complex that catalyses the last three reactions in the fatty acid beta-oxidation cycle [<cite idref="PUB00035967"/>].</li><li>AUH protein, a bifunctional RNA-binding homologue of enoyl-CoA hydratase [<cite idref="PUB00025845"/>].</li></ul> </p> Enoyl-CoA hydratase/isomerase, conserved site