... 2002side-chain at residue 210 is dispensable, as shown by the factthat H21 0A and H210S are as active as native API withVLK-MCA as a substrate (Table 1). This means thatTrp169 does not play a role as ... by the Trp169–His210 stacking, suggesting thatAPI has a catalytic quadruple apparatus, composed of Ser194, His57, Asp113 and His210, rather than a catalytictriad.ACKNOWLEDGEMENTWe are grateful ... In the struc-tural deviation, the solvent ASA of the side-chain of His210increased with the decrease in size of the side-chain atresidue 169 (Table 1 and Fig. 3A) . However, the ASAs of Asp113...