Purification, some properties and nucleotide sequence of 5‐carboxymethyl‐2‐hydroxymuconate isomerase of Escherichia coli C

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Abstract
As part of an investigation into the evolution of catabolic pathway enzymes a cloned gene encoding the Escherichia coli C 5-carboxymethyl-2-hydroxymuconate (CHM) isomerase, an enzyme of the homoprotocatechuate catabolic pathway, was used to produce large amounts of the protein. The isomerase was purified to homogeneity and some of its properties determined. The reaction occurred optimally at pH 7.6 and the specificity constant was 5.8 × 105 M−1·s−1 with CHM and 6.0 × 102 M−1·s−1 with 2-hydroxyhepta-2,4-diene-1,7-dioate, the substrate of a second isomerase in the pathway. The pure protein showed one type of subunit of M r 14 000 whilst the molecular mass of the native enzyme was 30 000, suggesting that it was a dimer of identical subunits. The first 19 N-terminal amino acids were sequenced and the data used to confirm that the open reading frame of 378 bp, identified from the nucleotide sequence, encoded the CHM isomerase.