Enoyl-CoA hydratase (EC 18.104.22.168) (ECH)  and D3,D2-enoyl-CoA isomerase
(EC 22.214.171.124) (ECI)  are two enzymes involved in fatty acid metabolism. ECH
catalyzes the hydratation of 2-trans-enoyl-CoA into 3-hydroxyacyl-CoA and ECI
shifts the 3- double bond of the intermediates of unsaturated fatty acid
oxidation to the 2-trans position.
Most eukaryotic cells have two fatty-acid β-oxidation systems, one located
in mitochondria and the other in peroxisomes. In mitochondria, ECH and ECI are
separate yet structurally related monofunctional enzymes. Peroxisomes contain
a trifunctional enzyme  consisting of an N-terminal domain that bears both
ECH and ECI activity, and a C-terminal domain responsible for 3-hydroxyacyl-CoA dehydrogenase (HCDH) activity.
In Escherichia coli (gene fadB) and Pseudomonas fragi (gene faoA), ECH and ECI
are also part of a multifunctional enzyme which contains both a HCDH and a
3-hydroxybutyryl-CoA epimerase domain .
A number of other proteins have been found to be evolutionary related to the
ECH/ECI enzymes or domains:
3-hydroxbutyryl-coa dehydratase (EC 126.96.36.199) (crotonase), a bacterial
enzyme involved in the butyrate/butanol-producing pathway.
Naphthoate synthase (EC 188.8.131.52) (DHNA synthase) (gene menB) , a
bacterial enzyme involved in the biosynthesis of menaquinone (vitamin K2).
DHNA synthase converts O-succinyl-benzoyl-CoA (OSB-CoA) to 1,4-dihydroxy-2-
naphthoic acid (DHNA).
4-chlorobenzoate dehalogenase (EC 184.108.40.206) , a Pseudomonas enzyme which
catalyzes the conversion of 4-chlorobenzoate-CoA to 4-hydroxybenzoate-CoA.
A Rhodobacter capsulatus protein of unknown function (ORF257) .
Bacillus subtilis putative polyketide biosynthesis proteins pksH and pksI.
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