Structure of orthorhombic crystals of beef liver catalase.

The growth mechanisms and physical properties of the orthorhombic crystal form of beef liver catalase were investigated using in situ atomic force microscopy (AFM). It was observed that the crystals grow in the <001> direction by an unusual progression of sequential two-dimensional nuclei of half unit-cell layers corresponding to the 'bottoms' and 'tops' of unit cells. These were easily discriminated by their alternating asymmetric shapes and their strong growth-rate anisotropy. This pattern has not previously been observed with other macromolecular crystals. Orthorhombic beef liver catalase crystals exhibit an extremely high defect density and incorporate great numbers of misoriented microcrystals, revealed intact by etching experiments, which may explain their marginal diffraction properties. To facilitate interpretation of AFM results in terms of intermolecular interactions, the structure of the orthorhombic crystals, having an entire tetramer of the enzyme as the asymmetric unit, was solved by molecular replacement using a model derived from a trigonal crystal form. It was subsequently refined by conventional techniques. Although the packing of molecules in the two unit cells was substantially different, with very few exceptions no significant differences in the molecular structures were observed. In addition, no statistically significant deviation from ideal 222 molecular symmetry appeared within the tetramer. The packing of molecules in the crystal revealed by X-ray analysis explained in a satisfying way the process of crystal growth revealed by AFM.