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5-carboxymethyl-2-hydroxymuconate isomerase (CHMI) is a trimeric enzyme catalyzing the isomerization of the unsaturated ketone 5-(carboxymethyl)-2-hydroxymuconate to 5-(carboxymethyl)-2-oxo-3-hexene-1,6-dionate. This is one step in the homoprotocatechuate pathway, one of the microbial meta-fission pathways that degrade aromatic carbon sources to citric acid cycle intermediates. Despite the structural similarity of CHMI with 4-oxalocrotonate tautomerase (4-OT) and macrophage migration inhibitory factor (MIF), there is no significant sequence similarity among these protein families, and therefore, they are not combined in one hierarchy.
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