Structural mobility and transformations in globular proteins

Functional significance of the conformational dynamics of the N-terminal segment of secreted phospholipase A2 at the interface

The kinetic and fluorescence properties of several pig pancreatic phospholipase A2 (PLA2) with substitutions and deletions in the N-terminal region and of tyrosines 52 and 73 are characterized. The substitutions Ala-1-D-Ala or -Gly, Trp-3-Phe, Gln-4-Nle, Arg-6-Glu, Tyr-52-Phe, and Tyr-73-Phe had at the most only a modest effect on the interfacial catalytic activity on the anionic interface to which they bind with high affinity. The observed rate of hydrolysis in the scooting mode by deletion mutants lacking one or more successive residues from the N-terminal region was lower by 50-95%. Detailed kinetic analysis of the deletion mutant lacking Ala-1 (des-1-AMPA) showed that the 50% decrease in the rate is due to a 5-fold increase in the interfacial Michaelis-Menten parameter, KM*, without a significant change in kcat. These results and direct measurements show that the primary effect of Ala-1 deletion is to lower the affinity for the active site directed ligands. Although the affinity of these mutants for anionic interface remains the same as for the wild type, the affinity for zwitterionic neutral diluents is considerably lower. Significant differences in the fluorescence quantum yields and the heterogeneity in the frequency-domain fluorescence intensity decays of these enzymes suggest that both in solution and at the interface the N-terminal region is an ensemble of conformations rather than a discrete state. Additional results suggest that the interfacial microenvironment of Trp-3 in des-1-AMPA is more polar and Trp-3 is more accessible to quenching by acrylamide.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorescence investigation on conformational state of rabbit muscle aldolase interacting with phosphatidylinositol liposomes

Evidence of conformational changes in rabbit muscle aldolase upon binding to phosphatidylinositol liposomes and the effect of the interaction on the thermal conformational transition are reported. Interaction with phosphatidylinositol liposomes significantly decreases the aldolase tryptophanyl fluorescence and shifts the maximum wavelength to higher values. The dynamic quenching constant for the aldolase fluorescence quenching by acrylamide in the presence of liposomes is much higher than that for unmodified enzyme; this signifies an increase in accessibility of some tryptophanyl residues to small polar molecules. Indirect interaction between single phospholipid molecules, small micelles or any soluble impurities able to penetrate into the protein molecule interior does not seem to be involved in the conformational rearrangement. Native and liposome-interaction-induced conformational states reveal different temperature dependences of the tryptophan residues exposure. The implications of the modification of the conformational state of the enzyme for its function in vivo are discussed.