Structure of tetragonal crystals of human erythrocyte catalase

The structure of catalase from human erythrocytes (HEC) was determined in tetragonal crystals of space group I4₁ by molecular-replacement methods, using the orthorhombic crystal structure as a search model. It was then refined in a unit cell of dimensions a = b = 203.6 and c = 144.6 Å, yielding R and R_free of 0.196 and 0.244, respectively, for all data at 2.4 Å 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 catalase gene. The overall structures were otherwise identical in both crystal forms. The NADPH-binding sites were empty in all four subunits and bound water molecules were observed at the active sites. The structure of the C-terminal segment, which corresponds to the last exon, remained undetermined. The tetragonal crystals showed a pseudo-4₁22 symmetry in molecular packing. Two similar types of lattice contact interfaces between the HEC tetramers were observed; they were related by the pseudo-dyad axes.