High-resolution refinement of orthorhombic bovine pancreatic phospholipase A₂

The X-ray structure of recombinant bovine pancreatic phospholipase A₂ (PLA2), which specifically catalyzes the cleavage of the sn-2 acylester bond of phospholipids, has been refined at 1.5 Å resolution. The crystal belongs to the space group P2₁2₁2₁ with unit-cell parameters a = 47.12, b = 64.59 and c = 38.14 Å similar to the native enzyme reported previously by Dijkstra et al. [J. Mol. Biol. (1981), 147, 97-123]. The refinement converged to an R value of 18.4% (R_free = 22.8%) for 16 374 reflections between 10.0 and 1.5 Å resolution. The surface-loop residues (60-70) are ordered in the present orthorhombic recombinant enzyme, but disordered in the trigonal recombinant enzyme. The active-site residues, His48, Asp99, and the catalytic water superimpose well with the trigonal form. Besides the catalytic water which is hydrogen bonded to His48, it is often seen that there is a second water attached to the same N atom of His48 and simultaneously hydrogen bonded to the O atom of Asp49. It is thought that the second water facilitates the tautomerism of His48 for enzyme catalysis. The catalytic water is also hydrogen bonded to the equatorial water coordinated to the calcium ion. In addition to the equatorial water, there is also an axial calcium water and the additional structural water. These five common water molecules are hydrogen bonded to the additional 16 water molecules in the present orthorhombic structure which may further enhance the structural integrity of the active site. Besides the protein and one calcium ion, a total of 134 water molecules were located in the present high-resolution refinement.