Nitric Oxide Complexes of Ferrohemes in Proteins

Heme—NO complexes are the best known and most thoroughly studied of the biological metal-nitrosyl complexes. So many heme enzymes have been 

20 gauss in an X-band EPR experiment the other principal values of the A tensor are about half this large. Because of the bent Fe-N-O configuration, the g and A tensors are not colinear. The A tensor orientation is dominated by the direction of the N—O bond, while the largest value ofg lies approximately along the orientation in the heme plane perpendicular to the Fe-N-O plane. 

Yonetoni and co-workers (1972) have shown that hemoglobin and myoglobin form nitrosyl complexes with different bond angles at 77 K and at room temperature. The high-temperature species has less g tensor anisotropy (g = 2.03, gy = 1.98-1.99) and poorly resolved hyperfine splitting. Addition of glycerol at high concentrations prevented the transition between these forms. 

Addition of NO to oxidized cytochrome oxidase produces a state in which NO binds to the copper center rather than to the heme (Brudvig et al., 1980. The Cu(Il)-NO complex is diamagnetic EPR signals can be observed at g = 6 which probably result from the ferric heme a, now uncoupled from Cu(ll). It is also possible to assign these signals to some S = f coupled state involving both iron and cooper, but this is much less likely. 

In the presence of NO and azide, cytochrome oxidase forms a complex with integral spin EPR spectra that have been assigned to a triplet state formed by coupling of S = 2 heme and copper centers (Brudvig et al., 1980). This explanation is possible, but other net integral spin possibilities could also explain the 

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