| Abstract: | The alpha subunit of tryptophan synthase from Escherichia coli is inactivated by phenylglyoxal and by (p-hydroxyphenyl)glyoxal. The use of these chemical modification reagents to determine the role of arginyl residues in the alpha subunit of tryptophan synthase has been complicated by our finding that these reagents react with sulfhydryl groups of the alpha subunit, as well as with arginyl residues. Analyses of the data for incorporation of phenyl2-14C]glyoxal, for inactivation, and for sulfhydryl modification in the presence and absence of indole-3-glycerol phosphate indicate that two sulfhydryl groups and one arginine are essential for the activity. Our finding that the substrate protects the single essential arginyl residue but not the two sulfhydryl groups is consistent with the observed kinetics of partial protection by substrate or by a substrate analogue, indole-3-propanol phosphate. In contrast to phenylglyoxal, (p-hydroxyphenyl)glyoxal modifies two to three sulhydryl groups that are not protected by indole-3-glycerol phosphate and modifies none of the arginyl residues that are modified by phenylglyoxal. |
