CRYSTAL-STRUCTURE OF NADH-CYTOCHROME B5 REDUCTASE FROM PIG-LIVER AT 2.4 ANGSTROM RESOLUTION

The three-dimensional structure of NADH-cytochrome bs reductase from p ig liver microsomes has been determined at 2.4 Angstrom resolution by X-ray crystallography. The molecular structure reveals two domains, th e FAD binding domain and the NADH domain. A large cleft Lies between t hese two domains and contains the binding site for the FAD prosthetic group. The backbone structure of the FAD binding domain has a great si milarity to that of ferredoxin-NADP(+) reductase [Karplus, P. A., Dani els, M. J., & Herriott, J. R. (1991) Science 251, 60-65], in spite of the relatively low sequence homology (about 15%) between the two enzym es. On the other hand, the structure of the NADH domain has several st ructural differences from that of the NADP(+) domain of ferredoxin-NAD P(+) reductase. The size of the cleft between the two domains is large r in NADH-cytochrome b(5) reductase than in ferredoxin-NADP(+) reducta se, which may be responsible for the observed difference in the nucleo tide accessibility in the two enzymes.