Functional arginine residues involved in coenzyme binding by dihydrofolate reductase

Reaction of dihydrofolate reductase from amethopterin-resistant Lactobacillus casei with phenylglyoxal results in a complete loss of enzyme activity. This inactivation is concomitant with the modification of five of a total of eight arginine residues per mole of enzyme. In the presence of the reduced coenzyme, NADPH, two of the five reactive arginines are protected from chemical modification with complete retention of enzyme activity. The results suggest the involvement of essential arginine residues at or near the coenzyme binding site and thus at or near the active center of the enzyme.