Ferroheme-NO complexes

Ferroheme-NO complexes are the best known and most thoroughly studied NO adducts with biochemical relevance. Some of the vast body of existing work... 

Both siroheme enzymes form ferroheme-NO complexes in which the g value anisotropy appears somewhat smaller than in the corresponding complexes of most other enzymes. The EPR spectra of the complexes somewhat resemble the spectra of the high-temperature myoglobin-NO complexes. The hyperfine splitting from the NO nitrogen nucleus is evident at intermediate g values but is not well resolved. These enzymes are capable of reducing NO to ammonia if supplied with low potential reducing equivalents. Other heme proteins also catalyze oxidation reduction reactions with NO. 

Studies on nitrosylation reactions of metalloporphyrins are emerging and have been reviewed. 21 Reactions involving FeNO 7 and FeNO 6 species (considered as ferroheme and ferriheme, respectively) are crucial to enzymatic functions (activation of guanylyl cyclase, cytochrome oxidase, catalase inhibition). Reversible photodissociations of NO from nitrosyl metalloporphyrins have been studied by ns-pulsed laser techniques, providing values for kt and kA (Equation (12)). Dissociation processes are very slow, particularly for ferroheme complexes however, kA values in the 10-5—101 s 1 range have been measured for several five-coordinated FeNO 7 tetraarylpor-phyrins.103 The spread in the kA values is not well understood yet. 

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