Rhodobacter sphaeroides haem protein: a novel cytochrome with nitric oxide dioxygenase activity

Rhodobacter sphaeroides produces a novel cytochrome, designated as SHP (sphaeroides haem protein), that is unusual in having asparagine as a redox-labile haem ligand. The gene encoding SHP is contained within an operon that also encodes a DHC (dihaem cytochrome c) and a membrane-associated cytochrome b. DHC and SHP have been shown to have high affinity for each other at low ionic strength (K_d=0.2 μM), and DHC is able to reduce SHP very rapidly. The reduced form of the protein, SHP²⁺ (reduced or ferrous SHP), has high affinity for both oxygen and nitric oxide (NO). It has been shown that the oxyferrous form, SHP²⁺–O₂ (oxygen-bound form of SHP), reacts rapidly with NO to produce nitrate, whereas SHP²⁺–NO (the NO-bound form of SHP) will react with superoxide with the same product formed. It is therefore possible that SHP functions physiologically as a nitric oxide dioxygenase, protecting the organism against NO poisoning, and we propose a possible mechanism for this process.