Chemical modifications of Serratia marcescens anthranilate synthase component I

Serratia marcescens anthranilate synthase Component I (AS I) was purified from a plasmid-containing Escherichia coli strain. Residues essential for AS I function were studied by chemical modification reactions. Phenylglyoxal and 1,2-cyclohexanedione modified 2-5 arginine residues and inactivated AS I. The substrate chorismate reduced the rate of inactivation. Analysis of inactivation data indicated that 1 arginine residue is essential for activity. Histidine residues in AS I were modified by ethoxyformic anhydride and by photooxidation. Enzyme inactivation accompanied modification of histidine residues. Inactivation was prevented by substrate. Comparison of the number of carbethoxy groups incorporated between substrate-protected and unprotected AS I indicated that 1 histidine residue is required for activity. AS I was also inactivated by bromopyruvate. Substrate retarded inactivation by bromopyruvate. A differential labeling experiment indicated that the loss of AS I activity was correlated with alkylation of 1 cysteine residue. A tryptic peptide containing the essential cysteine residue was isolated. The peptide has the amino acid sequence of Ile-Cys-Gln-Ala-Gly-Ser-Arg.