THE ANATOMY OF A BIFUNCTIONAL ENZYME - STRUCTURAL BASIS FOR REDUCTIONOF OXYGEN TO WATER AND SYNTHESIS OF NITRIC-OXIDE BY CYTOCHROME CD(1)

Cytochrome cd(1)-nitrite reductase is a bifunctional enzyme that catal yzes the one-electron reduction of nitrite to nitric oxide and the fou r-electron reduction of oxygen to water. The 1.55 Angstrom crystal str ucture of the dimeric enzyme from Thiosphaera pantotropha is reported here. The protein was sequenced from the X-ray structure. Each subunit contains a covalent c heme with two axial His ligands (His-17, His-69 ) and a unique noncovalent d(1) heme ligated by Tyr-25 and His-200. Th e d(1) heme is the mononuclear iron center where both oxygen and nitri te reduction take place. The two types of heme are located in separate domains whose arrangement suggests a mechanism requiring domain movem ent during catalysis.