Cyanide reaction with cytochrome

The sixth coordination position of the heme iron in cytochrome c peroxidase, which is normally occupied by H2O, is available for reactions with extraneous ligands such as fluoride, cyanide, and azide, as well as substrates and substrate analogs. The acidic-alkaline transition of a hemo-protein, which is caused by the ionization of the bound water ligand, is usually accompanied by significant spectral changes. However, the visible absorption spectrum of cytochrome c peroxidase is not appreciably... 

Camerino PW and King TE (1966). Studies on cytochrome oxidase. 3. Reaction of cyanide with cytochrome oxidase insoluble and particulate forms. J Biol Chem, 241, 970-979. 

Despite the differences in catalytic and spectroscopic properties, cytochrome a and a3 are similar in many of their chemical properties. For instance, it is impossible to separate the two compounds by ultracentrifugation, electrophoresis, chromatography, ammonium sulfate fractionation, or serial addition of deoxycho-late, and, therefore, the proportion of cytochrome a or cytochrome a3 present in cytochrome a + a3 preparations is not well known. However, measurements of the absorption spectrum obtained after reaction with CO and cyanide has established that 50% of the Soret band and 28% of the 605 mp peak can be accounted for by the presence of cytochrome a in the cytochrome a + a3 preparation. 

The organization of xanthine oxidase appears to be quite complex. There is evidence that various substrates are not bound at the same site, and that the primary reaction of different substrates may occur with various ones of the cofactors. The oxidation of purines and aldehydes is inhibited by pteridyl aldehyde and by cyanide, but these reagents do not affect the oxidation of DPNH. It is possible that these inhibitors influence substrate binding sites and primary electron transport, respectively, and that the oxidation of DPNH involves a different binding site and avoids the cyanide-sensitive electron transport mechanism, which may well involve iron. Xanthine oxidase, and probably all flavoproteins, require —SH groups, but a definite function for these groups cannot be ascribed at this time. Similarly, various factors influence the reactions with oxidants differentially. Cyanide inhibits cytochrome reduction, but not the reactions with 0 or dyes. Reduction of either cytochrome c or nitrate depends upon the presence of molybdenum. These observations... 

Reactions of hemoglobin with cyanide and the magnetic properties of the cyanide complex. Equilibrium constant of the reaction. Reaction of cyanide with catalase and peroxidase. Magnetic properties of products. Reaction of hemoglobin with fluoride and its relation to the reaction with hydroxyl ion. Equilibrium constants for reactions of fluoride with cytochrome c, catalase, and peroxidase. Magnetic effects associated with these reactions. Velocity of reaction of ferrihemoglobin with hydro-sulfide. Magnetic properties of product. Reactions with azide. Reactions of hydroxyl ion with various heme proteins. Reconsideration of all the results in connection with possibility of interactions between hemes. 

Most cases of intoxication from industrial exposure have been mild, with rapid onset of eye irritation, headache, sneezing, and nausea weakness, light-headedness, and vomiting may also occur. Acute exposure to high concentrations may produce profound weakness, asphyxia, and death. Acrylonitrile is metabolized to cyanide by hepatic microsomal reactions. Deaths from acute poisoning result from inhibition of mitochondrial cytochrome oxidase activity by metabolically liberated cyanide. Inhalation of more moderate concentrations for a longer period of time leads to damage to the liver tissues in addition to central nervous system (CNS) effects. ... 

Cytochrome c peroxidase is reversibly inhibited by ligands that can combine with ferric heme—such as fluoride, azide, and cyanide—but it is not inhibited by carbon monoxide, which indicates that the formation of the ferrous form of the enzyme is not involved in its reaction cycle. The activities of cytochrome c oxidase and cytochrome c peroxidase in tissue may be readily differentiated by the use of carbon monoxide since only the former enzyme is selectively inhibited by carbon monoxide (.19). 

This is a remarkable reaction because the transition metal chemistry of N2O is sparse, especially with copper. Most N2O reductases are soluble, periplasmic homodimers however, there are examples of membrane-associated enzymes. " The best characterized N2O reductases are from Paracoccus denitrificans, Pseudomonas nautica, and Pseudomonas stutzeri, and most of the information presented here is derived from experiments on these enzymes. Where comparable data are available, N2O reductases from various organisms appear to be fairly similar, with the exception of the enzyme from Wolinella succinogenes, as noted above. The crystal stractmes of N2O reductase from P. nautica and more recently from P. denitrificans show two distinct copper clusters per subunit a bis-thiolate bridged dinuclear electron-transfer site (Cua), which is analogous to the Cua site in cytochrome c oxidase see Cyanide Complexes of the Transition Metals), and a novel four-copper cluster ligated by seven histidines, the catalytic copper site (Cuz), where N2O is thought to bind and be reduced. Cuz was proposed to be a copper-histidine cluster on the basis of the presence of nine strictly conserved histidine residues, and this was supported by a H NMR study that identified two non-CuA associated resonances that were assigned as copper-histidine N-H protons. ... 

VanBuuren, K. J. H., Nicholls, P., VanGelder, B. F. (1972). Biochemical and biophysical studies on cytochrome aa3 VI. Reaction of cyanide with oxidized and reduced enzyme. Biochimica... 

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