Structure of orthorhombic crystals of beef liver catalase

The growth mechanisms and physical properties of the orthorhombic crystal f orm of beef liver catalase were investigated using in situ atomic force mic roscopy (AFM). It was observed that the crystals grow in the (001) directio n by an unusual progression of sequential two-dimensional nuclei of half un it-cell layers corresponding to the 'bottoms' and 'tops' of unit cells. The se were easily discriminated by their alternating asymmetric shapes and the ir strong growth-rate anisotropy. This pattern has not previously been obse rved with other macromolecular crystals. Orthorhombic beef liver catalase c rystals exhibit an extremely high defect density and incorporate great numb ers of misoriented microcrystals, revealed intact by etching experiments, w hich may explain their marginal diffraction properties. To facilitate inter pretation of AFM results in terms of intermolecular interactions, the struc ture of the orthorhombic crystals, having an entire tetramer of the enzyme as the asymmetric unit, was solved by molecular replacement using a model d erived from a trigonal crystal form. It was subsequently refined by convent ional techniques. Although the packing of molecules in the two unit cells w as substantially different, with very few exceptions no significant differe nces in the molecular structures were observed. In addition, no statistical ly significant deviation from ideal 222 molecular symmetry appeared within the tetramer. The packing of molecules in the crystal revealed by X-ray ana lysis explained in a satisfying way the process of crystal growth revealed by AFM.