NMR studies of interactions between inhibitors and porcine pancreatic phospholipase A2

Two-dimensional NMR studies were performed on the complexes of porcine pancreatic phospholipase A2, bound to a micellar lipid-water interface of fully deuterated dodecylphosphocholine, with competitive inhibitors derived from the following general structure: [formula: see text] X and Y are alkyl chains with various 'reporter groups'. The interactions between the inhibitor and the enzyme were localized by comparison of 2-D nuclear Overhauser effect spectra using protonated and selectively deuterated inhibitors, and inhibitors with groups having easily identifiable chemical shifts. These experiments led us to the following conclusions for the phospholipase A2/inhibitor/micelle complex: i) the His48 C2 ring proton is in close proximity to both the amide proton and the methylene protons at the sn-1 position of the glycerol skeleton of the inhibitor, ii) the acyl chain of the inhibitor at the sn-2 position makes hydrophobic contacts near Phe5, Ile9, Phe22 and Phe106; iii) no interactions between the acyl chain at the sn-1 position and the protein could be identified. Comparison of our results on the enzyme/inhibitor/micelle ternary complex with the crystal structure of the enzyme-inhibitor complex shows that the mode of inhibitor binding is similar. However, in several cases we found indications that the hydrophobic chains of the inhibitors can have multiple conformations.