ID   4.2.1.181
DE   3-carboxymethyl-3-hydroxy-acyl-[ACP] dehydratase.
AN   3-carboxymethyl-3-hydroxy-acyl-[acyl-carrier protein] dehydratase.
CA   a 3-carboxymethyl-3-hydroxyacyl-[ACP] = a 4-carboxy-3-alkylbut-(2E)-
CA   enoyl-[ACP] + H2O.
CC   -!- This family of enzymes participates in a process that introduces a
CC       methyl branch into nascent polyketide products.
CC   -!- The process begins with EC 4.1.1.124 which converts the common
CC       extender unit malonyl-[ACP] to acetyl-[ACP]. The enzyme is a mutated
CC       form of a ketosynthase enzyme, in which a Cys residue in the active
CC       site is modified to a Ser residue, leaving the decarboxylase function
CC       intact, but nulifying the ability of the enzyme to form a carbon-
CC       carbon bond.
CC   -!- Next, EC 2.3.3.22 utilizes the acetyl group to introduce the branch
CC       at the beta position of 3-oxoacyl intermediates attached to a
CC       polyketide synthase, forming a 3-hydroxy-3-carboxymethyl
CC       intermediate.
CC   -!- This is followed by dehydration catalyzed by EC 4.2.1.181 (often
CC       referred to as an ECH1 domain) leaving a 3-carboxymethyl group and
CC       forming a double bond between the alpha and beta carbons.
CC   -!- The process concludes with decarboxylation catalyzed by EC 4.1.1.125
CC       (often referred to as an ECH2 domain) leaving a methyl branch at the
CC       beta carbon.
CC   -!- The enzymes are usually encoded by a cluster of genes referred to as
CC       an "HMGS cassette", based on the similarity of the key enzyme to
CC       EC 2.3.3.10. cf. EC 4.2.1.18.
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