The anatomy of a bifunctional enzyme: Structural basis for reduction of oxygen to water and synthesis of nitric oxide by cytochrome cd sub(1)

Cytochrome cd sub(1)-nitrite reductase is a bifunctional enzyme that catalyzes the one-electron reduction of nitrite to nitric oxide and the four-electron reduction of oxygen to water. The 1.55 angstroms crystal structure 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 sub(1) heme ligated by Tyr-25 and His-200. The d sub(1) heme is the mononuclear iron center where both oxygen and nitrite reduction take place. The two types of heme are located in separate domains whose arrangement suggests a mechanism requiring domain movement during catalysis.