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The promiscuous active site specificity /binding preferences of the fungal aspartic proteinase, rhizopuspepsin



Rhizopuspepsin, a secreted proteinase from the fungus Rhizopus chinensis, has been used as a model system for studying the active site interactions of the aspartic proteinases. Substrates containing systematic substitutions at P5-P1 and P2-P1 were analyzed in an effort to identify molecular interactions that could be exploited in structure-based drug design. The recombinant enzyme showed a high degree of kinetic similarity to naturally occurring isozymes. Rhizopuspepsin was able to cleave substrates with side chains varying widely in size, shape, charge and hydrophobicity. Specificity constant (k,,/Km) values (0.06 to 3.84 s·1µM·1) were consistently two to five-fold higher than for the same peptides studied with the mammalian enzymes, porcine pepsin, cathepsin D and cathepsin E. The structural basis for this broad specificity, particularly in the S, subsite, was rationalized to be a result of glycine residues present at positions 290 and 292.
