Control of the chemical step by leucine-31 of pancreatic phospholipase A2.

A well-defined region of pancreatic and other secreted phospholipase A2 (PLA2), which we call the i-face, makes a molecular contact with the interface to facilitate and control the events and processivity of the interfacial catalytic turnover cycles. The structural features of the i-face and its allosteric relationship to the active site remain to be identified. As a part of the calcium binding (26−34) loop, Leu-31 is located on the surface near the substrate binding slot of PLA2. Analysis of the primary rate and equilibrium parameters of the Leu-31 substitution mutants of the pig pancreatic PLA2 shows that the only significant effect of the substitution is to impair the chemical step at the zwitterionic interface in the presence of added NaCl, and only a modest effect is seen on k*cat at the anionic interface. Leu-31 substitutions have little effect on the binding of the enzyme to the interface; the affinity for certain substrate mimics is modestly influenced in W3F,L31W double mutant. The fluorescence e...