Stereospecificity of the interaction of porcine pancreatic phospholipase A2 with micellar and monomeric inhibitors. A time-resolved fluorescence study of the tryptophan residue

Tryptophan-containing mutant of human (group IIa) secreted phospholipase A2 has a dramatically increased ability to hydrolyze phosphatidylcholine vesicles and cell membranes

Human nonpancreatic (group IIa) secreted phospholipase A2 (human sPLA2) is associated with a number of inflammatory disorders in which the extracellular concentrations of this enzyme can become highly elevated. It is probable that the enzyme normally acts as an acute-phase protein whose function is to facilitate the removal of infectious organisms or damaged host cells as part of the normal inflammatory response. The enzyme shows negligible activity with phosphatidylcholine (PC) vesicles and cell membranes, presumably reflecting the enzyme's lack of ability to bind productively to such condensed neutral interfaces. Mammalian pancreatic enzymes show modest activity with such interfaces and contain a unique tryptophan at position 3, which is part of the presumptive interfacial binding surface of these enzymes. Human sPLA2 does not contain tryptophan. The amphiphilic indole side chain of tryptophan is noted for its ability to penetrate the lipid interface of membranes, and tryptophan residues appear to be associated with the ability of lipases and phospholipases A2 to bind to and hydrolyze such interfaces. We have investigated in detail the properties of a V3W mutant of human sPLA2, which has a unique tryptophan on the interfacial binding surface of this enzyme. Although this enzyme shows a modest ( approximately 50%) reduction in activity when anionic substrates are used under standard assay conditions, the activity of the enzyme on phosphatidylcholine vesicles and cell membranes is dramatically increased compared with human sPLA2. This is particularly the case with small unilamellar vesicles of PC, where activity is enhanced over 250-fold compared to the almost zero activity expressed by human sPLA2. This enhanced activity is best explained by increased interfacial binding and activation of the V3W mutant and is not due to enhanced active-site binding and hydrolysis. The results highlight the important role that tryptophan residues can play in interfacial binding, particularly to condensed zwitterionic interfaces. The interfacial characteristics of the mutant human enzyme now resemble more closely the mammalian pancreatic enzymes that already have a tryptophan at position 3.

Adsorption-Induced Conformational Changes in the Serine Proteinase Savinase: A Tryptophan Fluorescence and Circular Dichroism Study

In this paper spectroscopic data of a proteolytic enzyme adsorbed on solid-liquid interfaces are discussed. The experiments consisted of time-resolved and steady-state fluorescence of tryptophan residues and of circular dichroism (CD) which give information on the tertiary and secondary state of the protein, respectively. The spectroscopic properties are measured for the inhibited form of subtilisin 309 in situ on a hydrophilic silica surface and on a hydrophobic Teflon surface. The results are compared with those obtained for the protein in solution. In the case of fluorescence it is reasoned that the average excited-state lifetime and short internal rotation correlation times are indicative parameters for structural changes in the protein. The internal rotation is superimposed on the rotation of the adsorbed protein which is immobile on the fluorescence time scale. Fluorescence and CD both prove that the protein alters its conformation when it adsorbs at low surface coverage on hydrophobic Teflon particles. In that case the tryptophan fluorescence lifetime is shortened which is accompanied by an increase in the alpha-helix content. At monolayer coverage the protein maintains its original structure, although minor changes in fluorophore dynamics occur. On hydrophilic silica particles the results from both techniques do not point in the same direction. The fluorescence was not affected, irrespective of the surface occupation, while the CD experiments show a decrease in alpha-helix content at low surface coverage. Copyright 1997 Academic Press.

Human non-pancreatic (group II) secreted phospholipase A2 expressed from a synthetic gene in Escherichia coli: characterisation of N-terminal mutants

A gene coding for human non-pancreatic (group II) secreted phospholipase A2 (hnpsPLA2) has been constructed by the single-step ligation of twelve synthetic oligonucleotides. The gene has been cloned into a modification of the bacterial expression vector pET 11 which allows protein over-expression as inclusion bodies and enables about 3 mg/litre of pure refolded fully active enzyme to be obtained. The protein was expressed as a 1-Ala mutant (N1A) to allow removal of the initiator methionine by the Escherichia coli amino-peptidase. This mutant had very similar properties to the wild-type enzyme. A double mutant, N1A, V3W was also constructed and expressed in high yield. This tryptophan-containing mutant showed similar properties to the wild-type and N1A mutant but had about 40% of the activity under the assay conditions used. This tryptophan was used as a reporter group for interfacial binding and its properties were compared to those of the corresponding tryptophan in PLA2 from procine pancreas. Expression of the wild-type gene sequence for hnpsPLA2 in E. coli gave the expected mutant protein still with the initiator methionine and with much reduced activity. Interfacial binding of all hnpsPLA2 mutants to anionic phospholipids was very similar when assessed by fluorescence methods. Comparisons of these mutants with the pancreatic enzyme revealed significant differences in terms of the effect of calcium on interfacial binding. The ability to express reasonably large amounts of the N1A mutant in E. coli will provide a basis for future site directed mutagenesis studies of this important human enzyme.