Labeling of a specific arginine residue at the active site of glutamine synthetase ( [formula omitted

Abstract

Chemical modification of a specific arginine residue of Escherichia coli glutamine synthetase has been accomplished by the use of the arginine-specific reagents p-hydroxyphenylglyoxal, phenylglyoxal, and methylglyoxal. Modification of one arginine residue results in complete inactivation of the enzyme and the modified enzyme seems to be extremely stable since no reactivation is observed upon addition of free arginine or dialysis. Saturating levels of ATP but not L-glutamate, L-methionine sulfoximine, or inorganic phosphate provide substantial protection against inactivation of the enzyme suggesting the modified amino acid is at or near the ATP substrate binding site. However, an ATP affinity analog is not prevented from binding upon modification of the arginine residue indicating that the reduction in catalytic activity is not solely due to alteration in substrate binding but may also reflect a catalytic role for the arginine residue.