Involvement of a residue at position 75 in the catalytic mechanism of a fungal aspartic proteinase, Rhizomucor pusilus pepsin. Replacement of tyrosine 75 on the flap by asparagine enhances catalytic efficiency

Residue 75 on the flap, a beta hairpin loop that partially coversthe active site cleft, is tyrosine in most members of the asparticproteinase family. Site-directed mutagenesis was carried outto investigate the functional role of this residue in Rhizomucorpusilus pepsin, an aspartic proteinase with high milk-clottingactivity produced by the fungus Rhizomucor pusillus. A set ofmutated enzymes with replacement of the amino acid at position75 by 17 other amino acid residues except for His and Gly wasconstructed and their enzymatic properties were examined. Strongactivity, higher than that of the wild-type enzyme, was foundin the mutant with asparagine (Tyr75Asn), while weak but distinctactivity was observed in Tyr75Phe. All the other mutants showedmarkedly decreased or negligible activity, less than 1/1000of that of the wild-type enzyme. Kinetic analysis of Tyr75Asnusing a chromogenic synthetic oligopeptide as a substrate revealeda marked increase in k_cat with slight change in K_m, resultingin a 5.6-fold increase in k_cat/k_m. When differential absorptionspectra upon addition of pepstatin, a specific inhibitor foraspartic proteinase, were compared between the wild-type andmutant enzymes, the wild-type enzyme and Tyr75Asn, showing strongactivity, had spectra with absorption maxima at 280, 287 and293 nm, whereas the others, showing decreased or negligibleactivity, had spectra with only two maxima at 282 and 288 nm.This suggests a different mode of the inhibitor binding in thelatter mutants. These observations suggest a crucial role ofthe residue at position 75 in enhancing the catalytic efficiencythrough affecting the mode of substrate-binding in the asparticproteinases.