Conformational changes in phospholipase A2 upon binding to micellar interfaces in the absence and presence of competitive inhibitors. A 1H and 15N NMR study.

An NMR study has been made of porcine pancreatic phospholipase A2 (PLA) in three environments: free in solution, in a binary complex with dodecylphosphocholine micelles, and in a ternary complex with a micelle and the substrate-like inhibitor (R)-1-octyl-2-(N-dodecanoylamino)-2-deoxyglycero-3-phosph oglycol. 1H and 15N chemical shifts, amide exchange rates, and NOE intensities are compared for the enzyme in different environments. From these data, structural differences are found for the N-terminal part, the end of the surface loop at residues Tyr69 and Thr70, and the active site residue His48, and also for the Ca-binding loop (residues 28-32). Specifically, when binding to a micelle, the side chains of residues Ala1, Trp3, and Tyr69, as well as all protons of Thr70, are found to be closer together. After subsequent introduction of the competitive inhibitor, further changes are found for these residues. The N-terminus is flexible in PLA free in solution, in contrast with the crystal structures where it adopts an alpha-helical conformation. According to the NMR data, this helix is rigidly formed only in the ternary complex. Furthermore, in the ternary complex, the N-terminal amino group and the exchangeable hydrogen at N3 of the ring of His48 are observed. We propose that PLA is activated in two steps. An initial conformational change occurs upon binding to a micellar interface. The catalytically active conformation of the enzyme, which has an extensive network of hydrogen bonds, is formed only when binding a substrate or competitive inhibitor at a lipid-water interface.