Transmission electron microscopic studies on the quaternary structure of human erythrocyte catalase

Abstract

Two-dimensional crystals of the M r 256,000 human erythrocyte catalase with typically a × b = 8.44 × 8.43 ± 0.5 nm (α=81.6°) have been prepared by the negative staining-carbon film technique, in the presence of 0.05% (w/v) polyethylene glycol ( M r 10,000). In the absence of polyethylene glycol no crystal were detected. One predominant molecular projection arbritarily defined as the face-on-view, is present within the two-dimensional crystals, although the enzyme also has a tendency to form rows of molecules. Assuming a projection normal to the latter (side-on-view), the human erythrocyte catalase is approx 6 nm in height. Digital Fourier peak filtering has been performed on the two-dimensional crystals. Averaged unit cell projections revealed the four subunits of the molecule. Projections of single molecules provided further clues about the quarternary structure of human erythrocyte catalase. In conclusion, the three-dimensional structure of human erythrocyte catalase at low resolution ( ca 2 nm) resembles a flattened tetrahedron with its two principal axes being not orthogonal to each other and with its four edges corresponding to the distal ends of two pairs of subunits comprising two separate planes. Each subunit has a pseudo-spherical appearance with ellipsoidal domains towards the periphery. In a face-on projection the long axes of these domains are not parallel with the two principal axes of the tetrahedron, thus giving rise to a chirality. Owing to the proposed three-dimensional structure, two of the subunits predominate in the averaged face-on projection of the molecule.