Cytochromes

 The Cytochromes are electron-transferring proteins containing iron-porphyrin groups: they are found only in aerobic cells. Some are located in the inner mitochondria membrane, where they act sequentially to carry electrons originating from various dehydrogenase systems towards molecular oxygen. Other Cytochromes are found in the endoplasmic reticulum, where they play a role in specialized hydroxylation reactions. All Cytochromes undergo reversible Fe (II)-Fe (III) valence changes during their catalytic cycles. Their reduced forms cannot be oxidized by molecular oxygen, with the exception of the terminal cytochrome of mitochondrial respiration, namely, cytochrome a3 or cytochrome c oxidase, which also contains tightly, bound copper. In the mitochondria of higher animals, where the respiratory chain has been most thoroughly studied, at least five different cytochromes have been identified in the inner membrane: cytochrome b, c1, c, a and a3. At least, one of these, cytochrome b, occurs in two or more forms. In addition to cytochrome found only in the inner membrane of mitochondria, another type, cytochrome b5, occurs in the endoplasmic reticulum. Animal and plant cells also contain other heme enzymes, such as peroxidise and catalase.

            The porphyrin ring is present not only in various heme proteins but also in the chlorophylls of green plant cells. In their structure porphyrins can be considered as derivatives of a parent tetrapyrrole compound, porphin. The porphyrins are named and classified on the basis of their side-chain substituents, e.g. etioporphyrins, mesoporphyrins, uroporphyrins, coproporphyrins and protoporphyrins. Of these, protoporphyrins are by far the most abundant. Protoporphyrin contains four methyl groups, two vinyl groups, and two propionic acid groups. Fifteen different isomeric protoporphyrins differing in the sequence of substitution of the above groups in the eight available side chain positions can be written. Of these many possible forms, one, protoporphyrin IX, is the only form in nature. Is is found in haemoglobin, myoglobin, and most of the cytochromes.

             The cytochromes were first discovered and called histohematins in 1866 by C. MacMinn, but their significance in biological oxidation did not become clear until 1925, when they were rediscovered by D. Keilin in England. With a simple hand spectroscope he directly observed in intact insect muscle a number of absorption bands resembling those of reduced heme proteins. He showed that these bands appear and disappear in relationship to muscle activity. Keilin renamed these respiratory pigments cytochromes, postulated that they acted in a chain to carry electrons from nutrient molecules to oxygen, and grouped them into three major classes, a, b, and c, depending on the characteristic positions of their absorption bands in the reduced state. Each type of cytochrome in its reduced state has three distinctive absorption bands in the visible range, the, β and ɤ or Soret bands.

            With one exception the cytochromes are very tightly bound to the mitochondria and difficult to obtain in soluble and homogenous form. The exception is cytochrome c, which is readily extracted from mitochondria by strong salt solutions. The cytochrome c of many species has been obtained in crystalline form and their amino acids sequences determined. The iron poptoporphyrin group of cytochrome c is covalently linked to the protein via bridges between the porphyrin ring and two cysteine residues in the peptide chain. Cytochrome c is the only common heme protein in which the heme is bound to the protein by covalent linkage. In haemoglobin and myoglobin,   as in cytochrome b and a, the porphyrin ring is non covalently bound and can be removed by extraction of the acidified protein with pyridine or other solvents. In cytochrome c the fifth and sixth coordination positions of iron are believed to be occupied by the side chains of a histidine residue and a methionine residue, which prevent cytochrome c from reacting with oxygen or carbon monoxide at pH 7.0.

Source:- Biochemistry Second Edition, The Molecular Basis of Cell Structure and Function, Albert L. Lehninger, The Johns Hopkins University School of Medicine, Page number 489-493.