Cytochrome function

B. Chance and D. Parsons, Cytochrome function in relation to inner membrane structure of mitochondria. Science 142 1176, 1963, 1963 by the AAAS.)... 

The biochemistry of the aerobic carbon monoxide-oxidizing bacteria is of considerable interest in that the respiratory chain is not sensitive to inhibition by CO, due, it is suggested, to the presence of a CO-insensitive o-type cytochrome functioning as a terminal oxidase.1037... 

Ferricyanide or cytochromes function as electron acceptors for the enzyme. 

In addition to cytochrome aa3, cytochromes ba3, baa3, and aco (or cao3) are known, which have heme A together with other types of heme, and function as cytochrome c oxidases (see Yamanaka, 1992). Furthermore, cytochrome a,cl occurs, which has two molecules of each of hemes A and C, one molybdenum atom, and 5 [Fe4S4] clusters in the molecule. The cytochrome functions as nitrite oxidoreductase it catalyzes the oxidation of nitrite to nitrate (Tanaka et al., 1983 Fukuoka et al., 1987 Suzuki et al., 1997). 

Cytochrome having hemes O and B is known as cytochrome bo3 (or bo). This cytochrome functions as the terminal oxidase in many bacteria. It resembles cytochrome aa3 in structure heme O and Cub of the cytochrome compose the dimetallic center to reduce molecular oxygen (Mogi et al., 1998). However, this cytochrome does not utilize ferrocytochrome c as the electron donor. The electron donor for cytochrome bo3 is ubiquinol it is an ubiquinol oxidase. Moreover, it shows proton pumping activity. 

Starkeya novella cytochrome c-550 was first partially purified by Charles and Suzuki (1966b), and thereafter highly purified by Yamanaka et al. (1971, 1991b). The cytochrome functions as the electron acceptor for sulfite-cytochrome c oxidoreductase as mentioned above, and as the electron donor for cytochrome c oxidase (Yamanaka and Fujii, 1980). It resembles mitochondrial cytochrome c in that it... 

A key question to keep asking during any discussion of bacterial cytochrome function, including that which follows, is To what extent does the primary evidence support the proposed conclusions, and how heavy is the dependence on analogy with known eukaryotic pathways ... 

CYPedia Cytochrome Functions of Arabidopsis P450, large scale expres- ibmp.u-strasbg. 

L-Tyrosine metabohsm and catecholamine biosynthesis occur largely in the brain, central nervous tissue, and endocrine system, which have large pools of L-ascorbic acid (128). Catecholamine, a neurotransmitter, is the precursor in the formation of dopamine, which is converted to noradrenaline and adrenaline. The precise role of ascorbic acid has not been completely understood. Ascorbic acid has important biochemical functions with various hydroxylase enzymes in steroid, dmg, andhpid metabohsm. The cytochrome P-450 oxidase catalyzes the conversion of cholesterol to bUe acids and the detoxification process of aromatic dmgs and other xenobiotics, eg, carcinogens, poUutants, and pesticides, in the body (129). The effects of L-ascorbic acid on histamine metabohsm related to scurvy and anaphylactic shock have been investigated (130). Another ceUular reaction involving ascorbic acid is the conversion of folate to tetrahydrofolate. Ascorbic acid has many biochemical functions which affect the immune system of the body (131). 

Hydroxy vitamin D pools ia the blood and is transported on DBF to the kidney, where further hydroxylation takes place at C-1 or C-24 ia response to calcium levels. l-Hydroxylation occurs primarily ia the kidney mitochondria and is cataly2ed by a mixed-function monooxygenase with a specific cytochrome P-450 (52,179,180). 1 a- and 24-Hydroxylation of 25-hydroxycholecalciferol has also been shown to take place ia the placenta of pregnant mammals and ia bone cells, as well as ia the epidermis. Low phosphate levels also stimulate 1,25-dihydtoxycholecalciferol production, which ia turn stimulates intestinal calcium as well as phosphoms absorption. It also mobilizes these minerals from bone and decreases their kidney excretion. Together with PTH, calcitriol also stimulates renal reabsorption of the calcium and phosphoms by the proximal tubules (51,141,181—183). 

Cytochromes c (Cyt c) can be defined as electron- transfer proteins having one or several haem c groups, bound to the protein by one or, more commonly two, thioether bonds. Cyt c possesses a wide range of properties and function in a large number of different redox processes. 

The electron transport protein, cytochrome c, found in the mitochondria of all eukaryotic organisms, provides the best-studied example of homology. The polypeptide chain of cytochrome c from most species contains slightly more than 100 amino acids and has a molecular weight of about 12.5 kD. Amino acid sequencing of cytochrome c from more than 40 different species has revealed that there are 28 positions in the polypeptide chain where the same amino acid residues are always found (Figure 5.27). These invariant residues apparently serve roles crucial to the biological function of this protein, and thus substitutions of other amino acids at these positions cannot be tolerated. 

Thus, Og and cytochrome c oxidase are the final destination for the electrons derived from the oxidation of food materials. In concert with this process, cytochrome c oxidase also drives transport of protons across the inner mitochondrial membrane. These important functions are carried out by a transmembrane protein complex consisting of more than 10 subunits (Table 21.2). 

FIGURE 21.21 A model for the electron transport pathway in the mitochondrial inner membrane. UQ/UQH9 and cytochrome e are mobile electron carriers and function by transferring electrons between the complexes. The proton transport driven by Complexes I, III, and IV is indicated. 

Another important group of cytochromes, found in plants, bacteria and animals is cytochrome P-450, so-called because of the absorption at 450 nm characteristic of their complexes with CO. Their function is to activate... 

Cytochrome c oxidase contains two, or possibly three, copper atoms referred to as Cua and Cub since they do not fit into the usual classification. The former (possibly a dimer) is situated outside the mitochondrial membrane, whereas the latter is associated with an iron atom within the membrane. Both have electron transfer functions but details are as yet unclear. 

Many examples of microbial hydroxylation of sterols/steroids have been reported. These hydroxylations usually involve mixed function oxidases which utilise molecular oxygen and cytochrome P-450. The reaction can be represented by ... 

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