Nonheme Metalloenzymes

N-substituted iron porphyrins form upon treatment of heme enzymes with many xenobiotics. The formation of these modified hemes is directly related to the mechanism of their enzymatic reactivity. N-alkyl porphyrins may be formed from organometallic iron porphyrin complexes, PFe-R (a-alkyl, o-aryl) or PFe = CR2 (carbene). They are also formed via a branching in the reaction path used in the epoxidation of alkenes. Biomimetic N-alkyl porphyrins are competent catalysts for the epoxidation of olefins, and it has been shown that iron N-alkylporphyrins can form highly oxidized species such as an iron(IV) ferryl, (N-R P)Fe v=0, and porphyrin ir-radicals at the iron(III) or iron(IV) level of metal oxidation. The N-alkylation reaction has been used as a low resolution probe of heme protein active site structure. Modified porphyrins may be used as synthetic catalysts and as models for nonheme and noniron metalloenzymes. 

Nitrite Reductase. Bacterial denitrification in anaerobic microorganisms involves a four-step process, with the overall reduction of nitrate (N03 ) to dinitrogen (N2). Each reaction in the reduction process is catalyzed by one or more different metalloenzymes, which have various transition metals found in diverse ligand environments nitrate reductase (NOR, molybdenum), nitrite reductase (NiR, iron or copper), nitric oxide reductase (NOR, heme and nonheme iron), and nitrous oxide reductase (N2OR, copper) (9). 

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