The functional involvement of Lys-38 in the heavy subunit of rat kidney γ-glutamylcysteine synthetase: Chemical modification and mutagenesis studies

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Abstract
Rat kidneyγ-glutamylcysteine synthetase (γGCS) was inactivated by reaction with trinitrobenzene sulfonate (TNBS), and the reaction followed pseudo-first-order kinetics. Inactivation kinetics revealed that only one of the amino acid residues modified by TNBS was essential for-γGCS activity. The addition of 10 mM Mg2+ to the TNBS inactivation reaction resulted in a 16-fold increase in the rate of inactivation. Chromatographic analysis on the tryptic hydrolyzates of trinitrophenylated (TNP) derivatives showed that Lys-38 in theγGCS heavy subunit was significantly modified in the presence of Mg2+. In contrast to small changes in the catalytic properties observed by mutation of Lys-38 to Arg, the mutants K38N and K38E had a marked decrease in enzymatic activity and about twofold increase inKm for glutamate. These results suggest that the positively charged Lys-38 may sbe involved in the binding of glutamate toγGCS.