Chemical modification of critical catalytic residues of lysine, arginine, and tryptophan in human glucose phosphate isomerase.

Human glucose phosphate isomerase was subjected to a series of chemical modifications aimed at identifying residues essential for catalytic activity. A specific lysine was found to stoichiometrically react with pyridoxal 5'-phosphate forming a reversible Schiff base which could be reduced with NaBH4. The covalently modified enzyme was specifically cleaved with hydroxylamine at three labile Asn-Gly sequences yielding a series of peptides which were separated by sodium dodecyl sulfate-polyacrylamide electrophoresis. The modified lysine was located in the COOH-terminal peptide. A critical arginine residue/subunit was found to be stoichiometrically modified with either 2,3-butadione or cyclohexadione. At high concentrations of butadione, an irreversible nonspecific modification of essentially all arginines occurred. An essential tryptophan residue was found to be stoichiometrically modified with N-bromosuccinimide in a similar fashion. Each of the chemical modifications of these three residues followed pseudo-first order and rate saturation kinetics and the modifications were prevented by the presence of substrates or competitive inhibitors. Circular dichroic spectral studies and analytical gel filtration indicated that these modifications have no effect on the quarternary structure and little effect on the secondary and tertiary structures of the enzyme. However, the extensive modification of arginine with butadione caused a dissociation of the enzyme into monomers and significant changes in tertiary structure. These studies provide new insights into functional aspects of isomerization and also provide an effective method for evaluating structural consequences of chemical or genetic modification of the enzyme.