THE SYNTHESIS OF NITRIC OXIDE IN CYTOCHROME cd₁ NITRITE REDUCTASE. Vilmos Fülöp, Pamela Williams & János Hajdu. Laboratory of Molecular Biophysics & Oxford Cenre for Molecular Sciences, University of Oxford, U.K.

Cytochrome cd₁ nitrite reductase is a bifunctional enzyme that catalyses the one-electron reduction of nitrite to nitric oxide and the four-electron reduction of oxygen to water. We recently reported the 1.55 Å resolution structure of the dimeric enzyme isolated from Thiosphaera pantotropha. Each subunit contains a covalent c haem and a unique non-covalent d₁ haem. The d₁ haem is the mononuclear centre where both oxygen and nitrite reduction takes place. The two types of haems are located in separate domains whose arrangement suggests a mechanism requiring domain movement during catalysis. A mechanism of NO release linked to domain movement may have wider implications for haem catalysis and signalling, for example, in the NO-dependent haem-containing guanyl cyclase. Preliminary studies show that cytochrome cd₁ is active in the crystal. Crystals in the act of catalysis diffract to high resolution. The study of the structure of the reduced form indicates that there are substantial regions of conformational change. Implications for time-resolved studies will be presented.

Reference

Fülöp, V., Moir, J. W. B., Ferguson, S. J. & Hajdu, J. (1995), Cell 81,

369-377.