Equivalence of the projected structure of thin catalase crystals preserved for electron microscopy by negative stain, glucose or embedding in the presence of tannic acid

Abstract

Thin crystals of beef liver catalase have been examined by electron microscopy following various preservation procedures. In the first part of this investigation, micrographs of three principal projections were obtained from thin sections of micro-crystals embedded in the presence of tannic acid. Computer reconstructions confirmed the space group assignment of P2 12 12 1 and permitted the packing arrangement of the catalase tetramers to be deduced to a resolution of about 20 Å. These results corroborate the packing model for this crystal form proposed by Unwin (1975) on the basis of molecular modeling of one projection. In the second part of this investigation, the projected structures of the thin crystals in various preserving media were compared. The negative contrasting of crystals embedded in the presence of tannic acid was confirmed by direct comparison with nonembedded, negatively stained thin platelet crystals. In addition, good agreement at 20 Å resolution was observed between the structure of negatively stained crystals and the structure of crystal platelets preserved in glucose and examined by lowdose methods, while moderate agreement was established with the published data of Taylor (1978) for crystals embedded in thin ice films. Tannic acid alone was also found to serve as a suitable medium for preserving catalase crystals to a resolution of 3.7 Å as judged by electron diffraction. Overall, we demonstrate that projections obtained from thin sections of catalase crystals embedded in the presence of tannic acid can provide a reliable, negatively contrasted representation of the protein structure to 20 Å resolution. Examination of sectioned crystals could thus provide a useful adjunct to X-ray crystallographic studies of protein crystals and three-dimensional reconstruction of crystal thin sections should ultimately be possible.