Identification of a cysteine residue at the active site of Escherichia coli isocitrate lyase.

Escherichia coli isocitrate lyase was inactivated by iodacetate in a pseudo-first-order process. Complete inactivation was associated with the incorporation of only one carboxymethyl group per enzyme subunit. The substrate and products of the enzyme protected against inactivation, suggesting that the reactive group may be located at the active site. Isolation and sequencing of a carboxymethylated peptide showed that the modified residue was a cysteine, in the sequence Cys-Gly-His-Met-Gly-Gly-Lys. The reactivity of isocitrate lyase to iodoacetate declined with pH, following a titration curve for a group of pKa 7.1. The Km of the enzyme for isocritrate declined over the same pH range.