Side-chain dynamics of two aromatic amino acids in pancreatic phospholipase A2 as studied by deuterium nuclear magnetic resonance

The flexibility of individual amino acid side chains of pancreatic phospholipase A2 in aqueous and micellar solutions was studied with deuterium nuclear magnetic resonance (2H NMR). Bovine pancreatic phospholipase A2 was selectively deuterated at the aromatic ring systems of Trp-3 and Phe-5 and porcine pancreatic phospholipase A2 at Trp-3 only. Solid-state 2H NMR spectra of the lyophilized enzymes exhibited quadrupole splittings on the order of 130 kHz, indicating almost complete immobilization of the aromatic ring systems. Exposure to a water-saturated atmosphere did not remove these steric constraints. However, side-chain mobility could be induced for the tryptophyl residue of the bovine enzyme by dissolving this enzyme in aqueous buffer or micellar solution whereas the phenyl ring always remained immobile and served as a probe for the protein's overall rotation. Typical correlation times for the tryptophyl and phenyl aromatic ring systems in aqueous solution were 7 ps and 13 ns (at 20 degrees C), respectively. The correlation time of the phenyl ring was longer than expected for the monomeric protein (approximately 6 ns), suggesting some aggregation of the protein at the high concentrations used for the NMR measurements. Addition of a micellar solution of oleoylphosphocholine had no influence on the motional freedom of the tryptophyl residue but approximately doubled the correlation time of the phenyl ring, indicating an increase of the effective volume of the tumbling particle due to lipid-protein interaction. A different behavior was observed for the Trp-3 residue of porcine phospholipase A2.(ABSTRACT TRUNCATED AT 250 WORDS)