THE CRYSTAL STRUCTURE OF LIPASE FROM MUCOR MIEHEI

Abstract

The crystal structure of lipase from the fungus Mucor miehei has been determined; it has revealed the enzyme's main chain structure as well as the details of the interactions madeby the individual sidechains. The enzymecontains a central 8 strand 6 sheet structure that extends acrossthe full depth of the molecule. Arranged across this, in some of the segmentslinking the strands, are several helices which pack against the sheet structure. There is an N terminal helix which appearsto sit at the centre of the convex surface created by the ß sheet. The serine (144) at the catalytic site has ben identified by chemical experiment. Inspection of the structure at this serine showedit to be part ofa triad: asp... his ... ser, equivalent at the active atomsto that seen in the serine proteases. Thereis no similarity in the lipase main chain structure to those of the trypsin related or the subtilisin related serine proteases - thus the appearanceofthe asp- his - ser triad is an example of an independentsolution of these side chainsfor a catalytic reaction. There is a small helix situated over the catalytic residues, effectively blocking them from the surrounding solvent. This lid explains the inactivity of the enzyme in aqueous conditions. The side chains on this helix are on one side polar and on the other nonpolar. This suggests that underthe influence of the interface at a micelle the lid could be destabilised by non-polar interactions andbe displaced, exposing the catalytic triad to the lipid at the interface.