Lipases belong to a class of esterases whose activity on triglycerides is greatly enhanced at lipid-water interfaces. This phenomenon, called interfacial activation, has a structural explanation: a hydrophobic lid, which at rest covers the catalytic site, is displaced on substrate or inhibitor binding and probably interacts with the lipid matrix. Fusarium solani pisi cutinase belongs to a group of homologous enzymes of relative molecular mass 22-25K (ref. 7) capable of degrading cutin, the insoluble lipid-polyester matrix covering the surface of plants, and hydrolysing triglycerides. Cutinases differ from classical lipases in that they do not exhibit interfacial activation; they are active on soluble as well as on emulsified triglycerides. Cutinases therefore establish a bridge between esterases and lipases. We report here the three-dimensional structure of a recombinant cutinase from F. solani pisi, expressed in Escherichia coli. Cutinase is an alpha-beta protein; the active site is composed of the triad Ser 120, His 188 and Asp 175. Unlike other lipases, the catalytic serine is not buried under surface loops, but is accessible to solvent. This could explain why cutinase does not display interfacial activation.

Protein engineering and x-ray crystallography have been used to study the role of a surface loop that is present in pancreatic phospholipases but is absent in snake venom phospholipases. Removal of residues 62 to 66 from porcine pancreatic phospholipase A 2 does not change the binding constant for micelles significantly, but it improves catalytic activity up to 16 times on micellar (zwitterionic) lecithin substrates. In contrast, the decrease in activity on negatively charged substrates is greater than fourfold. A crystallographic study of the mutant enzyme shows that the region of the deletion has a well-defined structure that differs from the structure of the wild-type enzyme. No structural changes in the active site of the enzyme were detected.

The cDNA coding for the porcine pancreatic prophospholipase A2 (proPLA) has been cloned and expressed in E. coli. Expression of proPLA could only be obtained in the form of intracellular aggregates after fusing the 15 kDa proPLA to a large (greater than or equal to 45 kDa) bacterial peptide. The fusion protein was readily purified from cell lysates, and specifically cleaved. Cleavage of the fusion protein was achieved with either hydroxylamine (at Asn/Gly sequences in the denatured protein), or trypsin (between the pro- and the mature PLA in the renatured fusion protein). The former method releases a proPLA-like enzyme, while the latter directly yields PLA. Renaturation of the fusion protein was made possible by the use of a recently reported new S-sulphonation method. The released (pro)PLA was purified (yields of 2-3 mg/ltr of culture medium), and showed identical properties compared to native (pro)PLA.

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