Single crystals of bacterial catalase from Micrococcus luteus have been examined by EPR at 77 K. X-ray perfect crystals gave a set of four prominent resonances in all three orthogonal planes which yielded eight heme direction cosine matrices to an accuracy of +/- 2 degrees as expected for the P4(2)2(1)2 space group and unit cell parameters previously determined. These matrices are related by D4 symmetry operation of the space group. There were additional weaker resonances only resolved in two or even one plane. A method of g-contour mapping was devised to solve for the orientations of hemes that give rise to these weaker resonances. Three additional sets of heme orientations, also following D4 symmetry, were determined. All of the above sites have the same principal g values, 2.0, 5.4, and 6.6. The EPR crystallographic results imply that several conformational substates may be trapped at 77 K.
Catalase Crystallography