Structure of tetragonal crystals of human erythrocyte catalase

The structure of catalase from human erythrocytes (HEC) was determined in t etragonal crystals of space group I4(1) by molecular-replacement methods, u sing the orthorhombic crystal structure as a search model. It was then refi ned in a unit cell of dimensions a = b = 203.6 and c = 144.6 Angstrom, yiel ding R and R-free of 0.196 and 0.244, respectively, for all data at 2.4 Ang strom resolution. A major difference of the HEC structure in the tetragonal crystal compared with the orthorhombic structure was the omission of a 20- residue N-terminal segment corresponding to the first exon of the human cat alase gene. The overall structures were otherwise identical in both crystal forms. The NADPH-binding sites were empty in all four subunits and bound w ater molecules were observed at the active sites. The structure of the C-te rminal segment, which corresponds to the last exon, remained undetermined. The tetragonal crystals showed a pseudo-4(1)22 symmetry in molecular packin g. Two similar types of lattice contact interfaces between the HEC tetramer s were observed; they were related by the pseudo-dyad axes.