Vitamin Electron transport

The term vitamin K2 was applied to 2-methyl-3-difarnesyl-l,4-naphthoquinone, m.p. 54 C, isolated from putrefied fish meal. It now includes a group of related natural compounds ( menaquinones ), differing in the number of isoprene units in the side chain and in their degree of unsaturation. These quinones also appear to be involved in the electron transport chain and oxidative phosphorylation. 

Quinones are widespread in natnre (Thomson, 1971) and have a variety of functions in the life cycles of most kinds of living organisms. These diketones are fonnd in higher plants, fungi, bacteria, and throughout the animal kingdom, and they play a central role in many biosynthetic processes that involve electron transport, such as cellular respiration (nbiqninone) and photosynthesis (plastoqninone). Vitamin K is an important factor in blood... 

Decreased metabolism of lipids. Decreased mitochondrial oxidation of fatty acids is another possible cause of ethanol-induced steatosis. Other possible causes are vitamin deficiencies and the inhibition of the mitochondrial electron transport chain. 

It has been reported that vitamin Kj and several of the vitamin K2 homologues are capable of restoring electron transport in solvent-extracted or irradiated bacterial and mitochondrial preparations. Other reports suggest that vitamin K is concerned with the phosphorylation reactions accompanying oxidative phosphorylation The capacity of these compounds to exist m several forms, e.g., quinone, quinol. chromanol, etc., appears to strengthen the proposal that links them to oxidative phosphorylation. Information has suggested that vitamin K acts to induce prothrombin synthesis. Since prothrombin has been shown to be synthesized only by liver parenchymal cells m the dog, it would appear that the proposed role for vitamin K is not specific for only prothrombin synthesis, but applicable to other proteins. 

Figure 4. Scheme for proton transfer by plastoquinone as a mobile carrier in membrane lipid. Electrons are transferred one by one to a bound plastoquinone A (PQA) which in turn reduces external plastoquinone. When reduced, the anionic plastoquinone takes up protons to become a hydroquinone which is oxidized by the cytochrome bb f complex on the inside of the membrane to release protons. A second quinone, vitamin K, (KQ) is also involved in chloroplast electron transport, but its role in proton movement is not known. 

Now that it is established that cestodes possess all the components of a electron transport system, is the latter functional Weinbach von Brand (952) failed to demonstrate either respiratory control or oxidative phosphorylation in T. taeniaeformis, although they regarded this as a technical rather than a physiological problem. However, there is good evidence that isolated mitochondria from M. expansa (124-127) and H. diminuta (663, 978) are capable of oxidative phosphorylation and respiratory control. The demonstration that a preparation of H. diminuta mitochondria will oxidise a range of substrates, exhibiting respiratory control, is shown in Table 5.14. Similarly, mitochondria from Diphyllo-bothrium latum can oxidise NADH (728) and succinate (729). It is likely that the classical mammalian-type part of the cytochrome chain in cestodes is capable of oxidative phosphorylation, but there is no evidence for ATP synthesis occurring on the alternative branch from the quinone or vitamin K/cytochrome b complex to cytochrome o. 

Vitamins C and K Facilitate electron transport by respiratory chain... 

Riboflavin (vitamin B2) has been reported to improve the exercise capacity of a patient with Complex I deficiency. After conversion to flavin monophosphate and FAD, riboflavin functions as a cofactor for electron transport in Complex I, Complex II, and electron transfer flavopro-tein. Nicotinamide has been used because Complex I accepts electrons from NADH and ultimately transfers electrons to Q10. 

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. 

In green plants, vitamin K (phyUoquinone) functions as a secondary electron acceptor in photosystem I, and in bacteria a variety of menaquinones (which also have vitamin K activity) have a role in the plasma membrane in electron transport, where they serve the same role as ubiquinone (Section 14.6) in mitochondrial electron transport. There is no evidence that vitamin K has any role in electron transport in animals. 

Ubiquinone functions as a carrier in the mitochondrial electron transport chain it is responsible for the proton pumping associated with complex I (Brandt, 1999) and is directly reduced by the citric acid cycle enzyme succinate dehydrogenase (Lancaster, 2002). As shown in Figure 14.8, it undergoes two single-electron reduction reactions to form the relatively stable semiquinone radical, then the fully reduced quinol. In addition to its role in the electron transport chain, it has been implicated as a coantioxidant in membranes and plasma lipoproteins, acting together with vitamin E (Section 4.3.1 Thomas etal., 1995, 1999). 

In addition to vitamin E, the isoprenoids and their derivatives up to cholesterol give rise to all the fat-soluble vitamins (A,D,E,K) and to coenzyme Q (used in electron transport in the Ferris Wheel Generator). Dolichol is an isoprene polymer that participates in the transfer of oligosaccharides during glycoprotein synthesis. 

Eioosapentanoic acid (EPA), 638, 643 Elaidic acid, 365 Rlasta.se, function, 38 Elastin,88,622 Elderly populatioiu constipalicm in, 144 ostcomalada, 576 ostecipnrosis, 583- 34,774-778 vitamin B12 deficiency, 521, 553 Electrolytes, 698 Electron transport ATP production and, 278-232 defifi, 279... 

Figure 5. Reversal of decreased electron transport components in vitamin C deficient guinea pigs with ascorbic acid (15).

NAD+ (derived from the vitamin niacin) and FAD (derived from the vitamin riboflavin) pass electrons to the electron transport chain. FMN and... 

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