Cytochrome -cytochrome

Let us pass to other cytochromes. Cytochrome f (or cytochrome C552) (FW = 15 000), the crystal structure of which is known,11 is an electron carrier present in the photosynthetic chain and also possesses a positive overall charge. It exhibits a reversible Fe(III)/Fe(II) reduction at a gold electrode modified with 4,4/-dithiopyridine,12 Figure 10. 

First the structures of cytochrome cytochrome c peroxidase [21] are both known at high resolution. Although the precise three dimensional structure of the protein-protein complex is unknown (and, we shall argue, unknowable), molecular modeling has produced detailed stereochemical models for the c ccp complex which are subject to experimental testing and subsequent improvement, as detailed below. 

The chain of carriers between the two photosystems includes the cytochrome b6f complex and a copper protein, plastocyanin. Like the mitochondrial and bacterial cytochrome be i complexes, the cytochrome b(J complex contains a cytochrome with two b-type hemes (cytochrome b6), an iron-sulfur protein, and a c-type cytochrome (cytochrome /). As electrons move through the complex from reduced plastoquinone to cytochrome/, plastoquinone probably executes a Q cycle similar to the cycle we presented for UQ in mitochondria and photosynthetic bacteria (see figs. 14.11 and 15.13). The cytochrome bbf complex provides electrons to plastocyanin, which transfers them to P700 in the reaction center of photosystem I. The electron carriers between P700 and NADP+ and between H20 and P680 are... 

Cytochrome oxidase (also called complex IV) contains two cytochromes (cytochrome a and a3). Cytochrome a is paired with a copper atom, CuA, and cytochrome a3 is paired with a different copper atom, CuB. During electron... 

Extensive work by Cheah (121, 122, 123, 128, 130), mainly with M. expansa, has shown that large cestodes possess a cytochrome chain which differs from the mammalian system in being branched and possessing multiple terminal oxidases (Fig. 5.11). One branch resembles the classical chain with cytochrome a3 as its terminal oxidase. The terminal oxidase of the alternative pathway, which branches at the level of rhodoquinone or vitamin K, is an o-type cytochrome. Cytochrome o is an autoxidisable b-type cytochrome which is commonly found in micro-organisms, parasitic protozoa and plants. The classical chain constitutes about 20% of the oxidase capacity in cestodes and cytochrome o is quantitatively the major oxidase. Cyanide-insensitive respiration - i.e. where oxygen uptake occurs in the presence of cyanide - is characteristic of most helminths (39). Cytochrome o binds cyanide much less strongly than cytochrome a3, and it seems reasonable, therefore, to equate cyanide-insensitive respiration with the non-classical pathway. 

The assimilatory enzyme from the mold Neurospora crassa has been intensively studied for over two decades, particularly by Nason and his collaborators. Thus, Nason and Evans (39) identified FAD as a prosthetic group in the enzyme Nicholas, Nason, and McElroy (40) showed that molybdenum was required for the synthesis of nitrate reductase Nicholas and Nason (41) suggested its presence in the enzyme Garrett and Nason (42) showed that a b-type cytochrome (cytochrome 6557) co-purifies with this nitrate reductase and Nason et al. (11) suggested, from in vitro complementation experiments with nitrate reductaseless mutants, that the enzyme consists of at least two components required for activity. These workers have suggested that the electron transfer pathway is ... 

Carbonic Anhydrase Cytochromes Cytochrome P-450 Aldehyde Oxidase Catalase, Peroxidase Hydrogenase Methylmalonyl Isomerase Kinase... 

Measurements of many kinds have been made between natural donor-acceptor pairs such as cytochrome c-cytochrome cytochrome c-cytochrome... 

The spectral and potentiometric characteristics of type b cytochromes in a succinate-cytochrome c reductase, prepared from pigeon breast muscle mitochondria using a mixture of ionic and nonionic detergents, have shown a close resemblance to those in intact mitochondria 8,34,37). The succinate-cytochrome c reductase may be fractionated into two complexes, succinate-ubiquinone reductase and ubiquinone-cytochrome c reductase, so-called complexes II and III, respectively. Complex III contains type b cytochromes, cytochrome c, and nonheme iron protein in a stoichiometry of 2 1 1 (38). One of the type b cytochromes shows an a peak at 562 nm (559.5 nm at 77°K) by the reduction with succinate and is identified as cytochrome b. The other, with the a peak at 566 nm (562.5 and 554 nm at 77°K), is reduced by succinate only in the presence of antimycin or by dithionite and is identified as cytochrome bi- These two cytochromes do not combine with CO. 

Among the respiratory electron carriers are three b-type cytochromes, cytochromes c and cl, and cytochromes a and a3. Cytochromes b, c, and cl all contain the same heme found in hemoglobin and myoglobin - iron complexed with protoporphyrin IX (Figure 7,4). In cytochromes c and cl, but not b, this heme is linked covalently to the protein component via thioether bonds formed between two of the vinyl side chains and two cysteine residues (Figure 15.6a). 

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