Sequence of a peptide susceptible to mixed-function oxidation. Probable cation binding site in glutamine synthetase

Mixed-function oxidation of glutamine synthetase from Escherichia coli causes loss of catalytic activity. The inactivation correlates with the loss of 1 of 16 histidine residues/subunit (Levine, R.L. (1983) J. Biol. Chem. 258, 11823-11827). A cyanogen bromide peptide containing the oxidizable histidine has been isolated. Within the protein, the sequence is Met-His-Cys-His-Met. This hydrophilic sequence likely forms one of the divalent metal-binding sites of glutamine synthetase. Binding of Fe2+ to this site permits generation of an activated oxygen species which reacts with a nearby histidine residue. This site-specific free radical mechanism accounts for the specificity of the mixed-function oxidation.