Hemoglobins oxidation-reduction

The abihty of iron to exist in two stable oxidation states, ie, the ferrous, Fe ", and ferric, Fe ", states in aqueous solutions, is important to the role of iron as a biocatalyst (79) (see Iron compounds). Although the cytochromes of the electron-transport chain contain porphyrins like hemoglobin and myoglobin, the iron ions therein are involved in oxidation—reduction reactions (78). Catalase is a tetramer containing four atoms of iron peroxidase is a monomer having one atom of iron. The iron in these enzymes also undergoes oxidation and reduction (80). 

Nitrosoarenes are readily formed by the oxidation of primary N-hydroxy arylamines and several mechanisms appear to be involved. These include 1) the metal-catalyzed oxidation/reduction to nitrosoarenes, azoxyarenes and arylamines (144) 2) the 02-dependent, metal-catalyzed oxidation to nitrosoarenes (145) 3) the 02-dependent, hemoglobin-mediated co-oxidation to nitrosoarenes and methe-moglobin (146) and 4) the 0 2-dependent conversion of N-hydroxy arylamines to nitrosoarenes, nitrosophenols and nitroarenes (147,148). Each of these processes can involve intermediate nitroxide radicals, superoxide anion radicals, hydrogen peroxide and hydroxyl radicals, all of which have been observed in model systems (149,151). Although these radicals are electrophilic and have been suggested to result in DNA damage (151,152), a causal relationship has not yet been established. Nitrosoarenes, on the other hand, are readily formed in in vitro metabolic incubations (2,153) and have been shown to react covalently with lipids (154), proteins (28,155) and GSH (17,156-159). Nitrosoarenes are also readily reduced to N-hydroxy arylamines by ascorbic acid (17,160) and by reduced pyridine nucleotides (9,161). 

A number of copper -containing protein compounds are enzymes with an oxidase function (ascorbic acid oxidase, urease, etc 1 and these play an important role in Ihe biological oxidation-reduction system. There is a definite relationship of copper with iron in connection with utilization of iron in hemoglobin function. 

This composite satellite image displays areas on the surface of the Earth where chlorophyll-bearing plants are located. Chlorophyll, which is one of nature s most important biomolecules, is a member of a class of compounds called porphyrins. This Glass also includes hemoglobin and cytochrome c, which is discussed in Feature 19-1. Many analytical techniques have been used to measure the chemical and physical properties of chlorophyll to explore Its role in photosynthesis. The redox titration of chlorophyll with other standard redox couples reveals the oxidation/ reduction properties of the molecule that help explain the photophysics of the complex process that green plants use to oxidize water to molecular oxygen. 

Oxidation-reduction potentials are expressed in volts relative to the standard hydrogen electrode at 0 volts, and represent the tendency of a compound to lose electrons (oxidation) or gain electrons (reduction). Various enzyme systems in respiration reactions (e.g., the cytochromes) utilizing ascorbic acid, hemoglobin, as well as reactions in the Krebs cycle (succinic and fumaric acids) involve electron transfers. Drugs can affect all these systems, and the consequences must be taken into account. 

In addition to structural changes of this kind occurring, when the hemoproteins take part in oxidation-reduction reactions there is the further possibility of irreversible oxidative attack on the porphyrin ring itself. Lemberg, Legge, and Lockwood (27) have studied this process in the case of hemoglobin and have named the oxidation product chole-globin the precise nature of this compound has not yet been established. 

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