X-ray scattering studies of Aspergillus flavus urate oxidase: towards a better understanding of PEG effects on the crystallization of large proteins

The determination of the three-dimensional structures of biological macromolecules by X-ray diffraction generally requires large good-quality crystals, which are often difficult to obtain as crystal nucleation and growth depend upon a great number of physicochemical parameters. In the future, the emergence of structural genomic projects will require new and rapid methods to determine crystallization conditions. Until now, the prediction of crystallization conditions has essentially been based on the knowledge of interparticular interactions in solutions inferred from studies on small soluble proteins in the presence of salts. The present study, by small-angle X-ray scattering, of urate oxidase from Aspergillus flavus, a homotetrameric enzyme of 128 kDa, allowed the extension of the results to the crystallization of large proteins in the presence of polyethylene glycol (PEG). The protein crystallization, the nucleation rate and the different morphological crystal shapes obtained were correlated with the second virial coefficient (A₂), which was found to be in a restricted range at the low end of the `crystallization slot' proposed by George & Wilson [(1994). Acta Cryst. D50, 361-365].