Prediction of the mechanism of action of omuralide (clasto-lactacystin β-lactone) on human cathepsin A based on a structural model of the yeast proteasome β5/PRE2-subunit/omuralide complex

Cathepsin A (CathA) is a lysosomal serine carboxypeptidase that exhibits homology and structural similarity to the yeast and wheat serine carboxypeptidases (CPY and CPW) belonging to the α/β-hydrolase fold family. Human CathA (hCathA) and CPW have been demonstrated to be inhibited by a proteasome (threonine protease) inhibitor, lactacystin, and its active derivative, omuralide (clasto-lactacystin β-lactone), as well as chymostatin. A hCathA/omuralide complex model constructed on the basis of the X-ray crystal structures of the CPW/chymostatin complex and the yeast proteasome β-subunit (β5/PRE2)/omuralide one predicted that the conformation of omuralide in the active-site cleft of proteasome β5/PRE2 should be very similar to that of chymostatin at the S1 catalytic subsites in the hCathA- and CPW-complexes. The relative positions of the glycine residues, i.e., Gly57 in hCathA, Gly53 in CPW, and Gly47 in β5/PRE2, present in the oxyanion hole of each enzyme were also highly conserved. These results suggest that omuralide might inhibit hCathA and CPW at the S1 subsite in their active-site clefts through direct binding to the active serine residue.