PHOSPHOLIPASE A(2) ENGINEERING .12. STRUCTURAL AND FUNCTIONAL ROLES OF THE HIGHLY CONSERVED ACTIVE-SITE RESIDUE ASPARTATE-49

Site-directed mutagenesis and high-resolution two-dimensional (2D) proton nuclear magnetic resonance (NMR) were used to probe the structural and functional roles of a highly conserved residue, Asp-49, in the interfacial catalysis by bovine pancreatic phospholipase A(2) (PLA2, overexpressed in Escherichia coli). According to crystal structures, the side chain carboxylate of Asp-49, along with the carbonyl oxygens of Tyr-28, Gly-30, and Gly-32, and two water molecules, provides the necessary Ligands for Ca2+ which is essential for the enzymatic activity. The Asp-49 of PLA2 was changed to Asn, Glu, Gln, Lys, and Ala; the resulting mutants are named D49N, D49E, D49Q, D49K, and D49A, respectively. The conformational stabilities of all five mutants are similar to that of WT as judged by guanidine hydrochloride-induced denaturation. The structural analyses by NMR indicated no global perturbations upon substitutions, although localized conformational perturbations can be observed for less conserved replacements. Direct Ca2+ binding studies showed no specific binding for D49A, D49N, D49Q, and D49K; however, D49E retains a 12-fold weaker calcium binding affinity (K-d,K-Ca = 23 mM). The specific activities of all five mutant enzymes decrease significantly, ranging from 5.4 x 10(2)- to 5.8 x 10(5)-fold in comparison with that of the wild-type enzyme. The observed activities of mutants require the presence of Ca2+. This demonstrates the functional importance of Asp-49 in the catalytic mechanism of PLA2, presumably by helping to bind and properly orient Ca2+ Fluorescence binding studies showed that the mutants fully retain the affinity for binding to the surfaces of zwitterionic micelles and anionic vesicles (i.e., the E to E* step), even though the spectroscopic property has been perturbed. On the other hand, D49A cannot bind active site-directed ligands at the interface (the E* to E*L step), apparently due to its inability to bind Ca2+. The catalytically inactive D49A was also used to study the binding of PLA2 to the interface of substrates, and confirmed the previous results obtained from ether analogs.