'Ph-Jump' Crystallographic Analyses of Gamma-Lactam-Porcine Pancreatic Elastase ComplexesWright, P.A., Wilmouth, R.C., Clifton, I.J., Schofield, C.J.
(2000) Biochem J 351: 335
- PubMed: 11023818
- Primary Citation of Related Structures:
1E34, 1E35, 1E36, 1E37, 1E38
- PubMed Abstract:
beta-Lactams inhibit a range of enzymes via acylation of nucleophilic serine residues. Certain gamma-lactam analogues of monocyclic beta-lactams have also been shown to be reversible inhibitors of porcine pancreatic elastase (PPE), forming acyl-enzyme complexes that are stable with respect to hydrolysis. Crystallographic analysis at pH 5 of an acyl-enzyme complex formed with PPE and one of these inhibitors revealed the ester carbonyl located in the oxyanion hole in a similar conformation to that observed in the structure of a complex formed between a heptapeptide (beta-casomorphin-7) and PPE. Only weak electron density was observed for the His-57 side chain in its 'native' conformation. Instead, the His-57 side chain predominantly adopted a conformation rotated approx. 90 degrees from its normal position. PPE-gamma-lactam crystals were subjected to 'pH-jumps' by placing the crystals in a buffer of increased pH prior to freezing for data collection. The results indicate that the conformation of the gamma-lactam-derived acyl-enzyme species in the PPE active site is dependent on pH, a result having implications for the analysis of other serine protease-inhibitor structures at non-catalytic pH values. The results help to define the stereoelectronic relationship between the ester of the acyl-enzyme complex, the side chain of His-57 and the incoming nucleophile during the reversible (de)acylation steps, implying it is closely analogous to the hydrolytic deacylation step during catalytic peptide hydrolysis.
The Oxford Centre for Molecular Sciences and The Dyson Perrins Laboratory, South Parks Road, Oxford OX1 3QY, UK.