The crystal structure of the secreted aspartic proteinase from Candida
tropicalis yeast (SAPT) has been determined to 1.8 A resolution. The classic
aspartic proteinase bilobal structure and domain topology is conserved in SAPT,
with the substrate binding cleft situated between the two domains. Structural
comparisons made with pepsin indicate that insertions and deletions in the
primary sequence modify the SAPT structure to create a more spacious substrate
binding cleft with altered specificity. An unexpected tetrapeptide has been
found to occupy binding sites S1'-S3', and this suggests the order of release of
peptide products in the catalytic mechanism of these enzymes. Structural
features are considered with regard to previous substrate specificity data.
