Proton NMR studies of bovine and porcine phospholipase A2: assignment of aromatic resonances and evidence for a conformational equilibrium in solution

Bovine and porcine pancreatic phospholipases A2, and porcine isophospholipase A2, have been investigated by one- and two-dimensional 1H NMR spectroscopy. Resonances have been assigned for 20-26 residues in each enzyme, including all the aromatic residues, by a strategy based on the semiquantitative comparison of proximity relationships deduced from NOE experiments with those seen in the crystal structure NOE experiments indicate that the loop comprising residues 59-70, which has a different conformation in the crystal structures of the bovine and porcine enzymes, has the same conformation in these two enzymes in solution. Selective changes in the line width of a limited number of resonances as a function of pH, temperature, and calcium concentration provide evidence for a local conformational equilibrium. This equilibrium involves a limited region of the protein structure around residues 25, 41, 106, and 111; it has been identified in the bovine enzyme and porcine isoenzyme but is not apparent in the porcine enzyme.