Fusarium oxysporum trypsin at atomic resolution at 100 and 283 K: a study of ligand binding

The X-ray structure of F. oxysporum trypsin has been determined at atomic r esolution, revealing electron density in the binding site which was interpr eted as a peptide bound in the sites S1, S2 and S3. The structure, which wa s initially determined at 1.07 Angstrom resolution and 283 K, has an Arg in the S1 specificity pocket. The study was extended to 0.81 Angstrom resolut ion at 100 K using crystals soaked in Arg, Lys and Gln to study in greater detail the binding at the S1 site. The electron density in the binding site was compared between the different structures and analysed in terms of par tially occupied and overlapping components of peptide, solvent water and po ssibly other chemical moieties. Arg-soaked crystals reveal a density more d etailed but similar to the original structure, with the Arg side chain visi ble in the S1 pocket and residual peptide density in the S2 and S3 sites. T he density in the active site is complex and not fully interpreted. Lys at high concentrations displaces Arg in the S1 pocket, while some main-chain d ensity remains in sites S2 and S3. Gln has been shown not to bind. The free peptide in the S1-S3 sites binds in a similar way to the binding loop of B PTI or the inhibitory domain of the Alzheimer's beta -protein precursor, wi th some differences in the S1 site.