Cytochrome bd from Azotobacter vinelandii:: Evidence for high-affinity oxygen binding

Cytochrome bd from Azotobacter vinelandii is a respiratory quinol oxidase that is highly efficient in reducing intracellular oxygen concentration, thus enabling nitrogen fixation under ambient aerobic conditions. Equilibrium measurements Of 02 binding to ferrous heme d in the one-electron-reduced form of the A. vinelandii enzyme give K-d(O2)= 0.5 mu M, close to the value for the Escherichia coli cytochrome bd (ca. 0.3 mu M); thus, both enzymes have similar, high affinity for oxygen. The reaction of the A. vinelandii cytochrome bd in the one-electron-reduced and fully reduced states with 02 is extremely fast approaching the diffusion-controlled limit in water. In the fully reduced state, the rate Of 02 binding depends linearly on the oxygen concentration consistently with a simple, single-step process. In contrast, in the one-electron-reduced state the rate of oxygen binding is hyperbolic, implying a more complex binding pattern. Two possible explanations for the saturation kinetics are considered: (A) There is a spectroscopically silent prebinding of oxygen to an unidentified low-affinity saturatable site followed by the oxygen transfer to heme d. (B) Oxygen binding to heme d requires an activated state of the enzyme in which an oxygen channel connecting heme d to the bulk is open. This channel is permanently open in the fully reduced enzyme (hence no saturation behavior) but flickers between the open and closed states in the one-electron-reduced enzyme.