Cytochrome component

Hill, R., and Bendall, F. 1960. Function of the two cytochrome components in chloroplasts A working hypothesis. Nature 186 136-137. 

Fig. 15. The Z-scheme for two photosystems (A) The original Z-scheme of Hill and Bendall (1960) (B) an expanded Z-scheme of Hill (1965) (C) a contemporary Z-scheme. (A) modified from Hill and Bendall (1960) Function of two cytochrome components in chloroplasts A working hypothesis. Nature 186 137 (B) from Hill (1965) The biochemist s green mansions the photosynthetic electron-transport chain in plants. In PN Campbell and GD Greville (eds) Essays in Biochemistry, 1 143. Acad Press.

Fig. 8. Difference spectra of the three spinach cytochromes isolated by imposing appropriate redox potentials In the sample and reference. See text for details. Figure source DS Bendall, HE Davenport and R Hill (1974) Cytochrome components of the higher plants. Methods in Enzymology 23 341.

Liu M-C, Costa C, Coutinho IB, Moura I, Xavier AV, LeGall J (1988) Cytochrome components of nitrate- and sulfate-respiring Desulfovibrio desulfuricans ATCC 27774. J Bacteriol 170 5545-5551... 

Yamanaka T, Ota A, Okunuki K (1961) A nitrite reducing system reconstituted with purified cytochrome components of Pseudomonas aeruginosa. Biochim Biophys Acta 53 294-308... 

Fig. 27. Electron flow in green plant photosynthesis. Vertical wavy arrows represent excitation of chlorophyll molecules by absorbed light. Reaction intermediates are as follows Z , unknown intermediate donating electrons to photocenter II Q , unknown intermediate accepting electrons from excited chlorophyll PQ, plasto-quinone, structurally similar to coenzyme Q or ubiquinone of Fig. 4 bss>, haa,, cytochrome components PC, plastocyanin, a copper-containing nonheme protein FR8, unknown ferredoxin reducing substance FD, ferredoxin FP, flavoprotein mediating reduction of NADP. ...

The ultimate test, but not one achieved easily, is always the isolation and purification of individual cytochrome components. Cytochrome c has been studied more extensively than b or a cytochromes just because it can be isolated easily, and Table XX is an indication of the success to date. When possible, reconstitution experiments between a purified, extracted component and membrane fragments depleted of that component can provide strong evidence for function. Unfortunately, bacterial systems have proved to be more sensitive to handling then mitochondria, and extraction of cytochrome usually denatures the chain irreversibly (368). 

Borregaard N, Heiple JM, Simons ER, Clark RA Subcellular localization of the b-cytochrome component of the human neutrophil microbicidal oxidase Translocation during activation. J Cell Biol 1983 97 52-61. 

The particulate preparations which are loosely referred to as preparations of cytochrome oxidase contain in addition to the oxidase the entire electron transfer system other than the cytochromes and variable amoimts of the various cytochrome components. Such preparations will readily oxidize reduced cytochrome c by molecular oxygen, though whether reduced cytochrome c is oxidized by a sequence as shown above or directly by cytochrome oxidase is indeterminate at present. 

The studies show that if the cell unsaturated fatty acid content is low enough, the assembly of the mitochondrial membrane is profoundly affected. We have concentrated mainly on the mitochondrially synthesized components, but the mitochondrial level of cytochrome c, which is synthesized in the cytoplasm, was also reduced under lipid-depleted conditions. Since petite mutant cells, which lack a mitochondrial protein-synthesizing system, contain normal levels of cytochrome c when derepressed (Marzuki and Linnane, unpublished observation), this effect cannot be due to a simple feedback mechanism which coordinates the synthesis of cytoplasmically synthesized cytochrome components with mitochondrially synthesized components. More likely, it is a reflection of the altered physical structure of the membrane under these conditions, which may not allow the integration of cytochrome c into the membrane. This system offers great potential for resolving some of the problems of the mechanism of mitochondrial membrane assembly. 

In addition, it was confirmed that major electron transfer from reduced cytochrome c to oxidized cytochrome a occurred via cytochrome c rather than directly, because reduced cytochrome Cx can reduce oxidized cytochrome c instantaneously. Furthermore, it was ascertained that reduced cytochrome c was rapidly oxidized by a purified preparation of oxidized cytochrome a and that the latter component was reduced under anaerobic conditions. Although it was established that the normal oxido-reduction potentials of the cytochrome components are in the following order a > c > Cj > b, Yakushiji and Okunuki proposed the following electron transferring system, to explain the discrepancy between Eqs. (1) and (2) ... 

The above solution is stored in an icebox for 2 days. During this time, most of cytochromes b and c become insoluble and precipitate. After centrifugation (8,000 X g, 20 minutes) the supernatant fluid is made up to 35% saturation with saturated ammonium sulfate solution buffered at pH 7.5. Cytochrome a and the rest of the cytochromes b and Cj are precipitated and only a little cytochrome Cj remains in the supernatant. The precipitate is dissolved in 50 ml. of the above 0.1 M phosphate buffer (containing 25% saturation of ammonium sulfate and 0.5% cholic acid) and by addition of more saturated ammonium sulfate solution the fraction between 25 and 35% saturation is obtained (Fraction S3). This fraction has no cytochrome b and little c. To remove cytochrome Cj from the fraction, the above fractionation is repeated two or three times. The fraction obtained after each fractionation should always be stored overnight before the next treatment. In this way, a clear, deep-reddish colored concentrated solution of cytochrome a is obtained which contains no other cytochrome components (Fraction S4). 

By the procedure described in the foregoing section, cytochrome a is purified and no other cytochrome components are observed spectro-photometrically, so that the sample is sufficiently pure to investigate the mechanism of the cytochrome oxidase reaction. However, there is sometimes a small absorption shoulder at about 430 m/A. To remove... 

---