Proton-nuclear-magnetic-resonance/pH-titration studies of the histidines of pancreatic phospholipase A2.

The study by means of 1H nuclear magnetic resonance (NMR) of the histidines of phospholipase A2 isolated from porcine, bovine and equine pancreas is reported. Assignment of the histidine resonances was achieved by comparison of different enzymes and the use of paramagnetic probes. pH titration curves for various histidyl resonances were obtained and compared in the presence and absence of calcium. Calcium is shown to lower the pKa of the active site histidine. The NMR results are compared with the known X-ray three-dimensional structure for the bovine enzyme.