Nitric Oxide Dioxygenase - Structure and Molecular Function

Structure and Molecular Function

The flavohemoglobin protein contains two domains: an oxidoreductase FAD-binding domain, and a b-type heme-containing "globin" domain and optionally an oxidoreductase NAD-binding domain. The reductase domain supplies an electron to the heme iron to achieve a high rate of catalytic NO dioxygenation. In addition to numerous flavohemoglobins, many distantly related members of the hemoglobin superfamily including the muscle myoglobin, the non-symbiotic plant hemoglobin and symbiotic plant leghemoglobin, the neuronal neuroglobin, and the mammalian cytoplasmic cytoglobin appear to function as nitric oxide dioxygenases (NODs), although the cellular electron donor(s) for many globins have yet to be defined. Electron donors may include ascorbate, cytochrome b₅ or ferredoxin reductase. The catalytic NO dioxygenation can be written in its simplest form:

NO + O₂ + e- NO₃-

Catalysis is very efficient. The reported bimolecular NO dioxygenation rate constants range from 2 x 107 M−1s−1 for cytoglobin to 3 x 109 M−1s−1 for flavohemoglobin, and turnover rates range from 1 to 700 s−1. Structure, O₂ binding, and reduction of globins appear optimized for a NO dioxygenase function.