Electron Transport in Cytochrome

In organic compounds, iron is often attached to organic ligands by chelation and, thereby, it can undergo oxidation-reduction transformations that are utilized in electron-transport processes. Cytochromes in electron-transport chains contain chelated iron that undergoes such redox transformations. 

Moir, J. W. B., and Ferguson, S. J., 1994, Properties of a Paracoccus denitrificans mutant deleted in cytochrome C550 indicate that a copper protein can substitute for this cytochrome in electron transport to nitrite, nitric oxide and nitrous oxide. Microbiology 140 3989397. 

In the mitochondria of human cells, energy is provided by the oxidation and reduction reactions of the iron ions in the cytochromes in electron transport. Identify each of the following reactions as an oxidation or reduction ... 

Hemoproteins which engage in electron transport — the cytochromes — are much more widely dispersed among living species and occur in microorganisms, plants and animals (13). Again there are two types of iron proteins which can perform the task of electron transport, the heme and the non-heme. The latter term has become practically synonymous... 

The cytochrome (by virtue of its ability to accept and donate electrons during its function in electron transport) can exist in either the oxidised or the reduced state. In reduced-minus-oxidised difference spectra, it has absorption maxima at 426, 530 and 558 nm, typical of many b-type cytochromes. The ease with which the cytochrome can accept and donate electrons is expressed by its redox (reduction-oxidation) potential, which is measured in millivolts. Unlike most mammalian b cytochromes, which have much higher midpoint potentials, that of the cytochrome of the NADPH oxidase is -245 mV. Be-... 

In order to relate structure and function at a more direct level, it is necessary to focus on systems which have better characterized structure than the complex membrane bound proteins like cyt c oxidase. One particularly useful paradigm in this context is the cytochrome c-cytochrome c peroxidase couple [18]. Cep is not involved in electron transport, per se its apparent function [19] is detoxification of hydrogen peroxide via the sequence H2O2 -I- cep Fe(III) -> H2O -I- cep Fe(IV) O (protein) compound ES ... 

Inhibition of cytochromes of electron transport system can be caused by cyanogenic glycosides, such as amygdalin (Fig. 11.14) in bitter almonds, Prunus amygdalus, linamarin and lotaustralin in clover and birdsfoot trefoil, or dhurrin (Fig. 11.14) in Sorghum vulgare. The potent effect of cyanide on cell respiration has given rise to a recent serious conservation problem. In Southeast Asia, divers stun fish on coral reefs with a blast of cyanide to collect them for the aquarium trade. In the process, many fish are killed and the corals bleached, because their symbionts die (e.g. Payne, 2001). 

Surveying our present knowledge about the enzyme activities in PVA biodegradation, a trend toward increasing integration can be seen. There are free enzymes working in the extracellular space of the cells, including also the periplasmatic volume, and there are membrane-associated enzymes that are presumably Unked to the cellular cytochrome-based electron transport chains. 

The discussion to this point has focused on the isolation of intact mitochondria. By various chemical and physical treatments, mitochondria may be separated into their four components. This allows biochemists to study the biological functions of each component. For example, by measuring enzyme activities in each fraction, one can assign the presence of a particular enzyme to a specific region of the mitochondria. Studies of mitochondrial subfractions have resulted in a distribution analysis of enzyme activities in the four locations (Table E10.1). This type of study is often referred to as an enzyme profile or enzyme activity pattern and the enzyme may be considered a marker enzyme. For example, cytochrome oxidase, which is involved in electron transport, is a marker enzyme for the inner membrane. 

The reduction of UQ can be measured by the disappearance of an absorption band at 275 nm. Using this technique, it was shown that adding a substrate such as succinate caused a rapid reduction of essentially all the UQ present in the inner membrane the resulting UQH2 could be reoxidized by the cytochrome system in the presence of 02. To determine whether UQ is a necessary participant in electron transport from succinate to 02, the quinone was removed from mitochondria by selective extraction with an organic solvent. The depleted mitochondria were incapable of respiration but recovered this activity when UQ was added back. 

Flavoproteins play varied roles as coenzymes in biochemical processes. They serve as key intermediates in electron transport between those systems, such as the cytochromes, that carry one electron at a time and the two-electron transfers involved in the redox chemistry of organic-substrate metabolism. Flavins such as FMN or FAD (Figure 2) are designed structurally and electronically to stabilize both one- and two-electron redox intermediates. Thus, the one-electron reduction of... 

Vitamin E deficiency is also associated with impaired mitochondrial oxidative metabolism and impaired activity of microsomal cytochrome P450-dependent mixed-function oxidases, and hence the metabolism of xenobi-ofics. There is no evidence that vitamin E has any specific role in electron transport in mitochondria or microsomes. Again, changes in membrane lipids and oxidative damage presumably account for the observed metabolic abnormalities. 

Axial-ligand combinations found among the low-spin ferricytochromes that are involved in electron transport in a wide variety of cells include two histidine imidazoles (cytochromes the b cytochrome of sulfite oxidase/ the bc mitochondrial protein complex and cytochrome be of chloroplasts/ " cytochrome a of mitochondrial cytochrome c oxidase, and the bacterial tetraheme proteins called the cytochromes as well as other bacterial... 

Figure 20. (A) Ribbon drawing of the interaction between CCP and its redox substrate, cytochrome f determined from X-ray coordinates [ 137]. The hemes are shown for CCP and cytochrome c along with the proximal residues involved in electron transport between the proteins. (B) A close-up of the electron transport chain and their overlapping electron density.

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