Structure of human erythrocyte NADH-cytochrome b₅ reductase

Erythrocyte NADH-cytochrome b₅ reductase reduces methaemoglobin to functional haemoglobin. In order to examine the function of the enzyme, the structure of NADH-cytochrome b₅ reductase from human erythrocytes has been determined and refined by X-ray crystallography. At 1.75 Å resolution, the root-mean-square deviations (r.m.s.d.) from standard bond lengths and angles are 0.006 Å and 1.03°, respectively. The molecular structure was compared with those of rat NADH-cytochrome b₅ reductase and corn nitrate reductase. The human reductase resembles the rat reductase in overall structure as well as in many side chains. Nevertheless, there is a large main-chain shift from the human reductase to the rat reductase or the corn reductase caused by a single-residue replacement from proline to threonine. A model of the complex between cytochrome b₅ and the human reductase has been built and compared with that of the haem-containing domain of the nitrate reductase molecule. The interaction between cytochrome b₅ and the human reductase differs from that of the nitrate reductase because of differences in the amino-acid sequences. The structures around 15 mutation sites of the human reductase have been examined for the influence of residue substitutions using the program ROTAMER. Five mutations in the FAD-binding domain seem to be related to cytochrome b₅.