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Identification and purification of distinct isomerase and decarboxylase enzymes involved in the 4-hydroxyphenylacetate catabolic pathway of Escherichia coli.

The possible involvement of an isomerase in the 4-hydroxyphenylacetate meta-cleavage pathway has been studied. 5-Carboxymethyl-2-hydroxymuconate has been shown to undergo both spontaneous and enzyme-catalysed isomerisation to give 5-carboxymethyl-2-oxo-hex-3-ene-1,5-dioate, a compound with an absorbance maximum at 246 nm. The latter compound rather than the former is the substrate for a decarboxylase that produces 2-hydroxyhepta-2,4-diene,1,7-dioate. The isomerase and decarboxylase enzymes have been purified to over 90% homogeneity. Mg2+ is required for the decarboxylase reaction but not for the isomerase.