Ubiquinones substituting

Two groups of substituted l,4-ben2oquiaones are associated with photosynthetic and respiratory pathways the plastoquinones, eg, plastoquinone [4299-57-4] (34), and the ubiquinones, eg, ubiquinone [1339-63-5] (35), are involved in these processes. Although they are found in all living tissue and are central to life itself, a vast amount remains to be learned about their biological roles. 

The excellent resolution of the 0-tensor components at W band has been used to measure the relaxation properties of QA in the Zn-substituted bRC of R. sphaeroides.m The experiment showed, in contrast to the respective ubiquinone radical in organic solution, an anisotropic relaxation behavior in the pulse high field ESE experiments. From the analysis of the T2 experiments a motional anisotropy of Q% in the protein pocket was deduced with a preferred libration about the C-O symmetry axis. Recently, similar experiments were also performed on Qb- in ZnbRCs. Compared to QA different echo decay time constants were found. A model was proposed in which the relaxation is related to reorientational fluctuations around the quinones specific H-bonds to the protein.142... 

Components of the electron transport chain in bacteria have been shown to include b- and c-type cytochromes, ubiquinone (fat-soluble substitute quinone, also found in mitochondria), ferredox (an enzyme containing nonheme iron, bound to sulfide, and having the lowest potential of any known electron-canying enzyme) and one or more flavin enzymes. Of these a cytochrome (in some bacteria, with absorption maximum at 423.5 micrometers, probably Cj) has been shown to be closely associated with the initial photoact. Some investigators were able to demonstrate, in chromatium, the oxidation of the cytochrome at liquid nitrogen temperatures, due to illumination of the chlorophyll. At the very least this implies that the two are bound very closely and no collisions are needed for electron transfers to occur. 

Other studies have shown biological actions from these kinds of structure and indicate interrelationships between the chemicals in terms of their potencies. Tomita [229] found that the substances, vitamin A, vitamin K, vitamin E, /1-carotene, ubiquinone (15), phytol and squalene (16), from green-yellow vegetables could suppress the growth of tumour cells and enhance T-cell cytotoxicity, but /1-carotene, which does have both ends of the chain substituted with a bulky / -ionone ring on each end-group did not. Hydrophobic chain... 

Fig. 9. Substitution of exotic quinones for native ubiquinone at the Qa site or reaction centers from Rba. sphaeroides shows that charge recombination from the BChl2+QA state proceeds uphill via a thermally activated intermediate X near the energy level of BPh (Woodbury et al., 1986). Figure after Gunner (1988).

Cytochrome c carries electrons from the cytochrome bc complex, but also from auxiliary redox enzymes (see e.g., Refs. 1,2) to cytochrome oxidase. Ubiquinone is the only non-protein member of the chain. This highly hydrophobic substituted... 

Ubiquinone is a substituted (2,3-dimethoxy-5-methyl-(l,4)-)benzoquinone with a long isoprenoid side chain in position 6 (see Ref. 238). The fact that ubiquinol is a donor of two reducing equivalents, while cytochrome c is a one-electron acceptor, requires special arrangements of electron transfer (cf., the analogous but opposite problem in cytochrome oxidase). Although ubisemiquinone is very unstable in most circumstances, it can be stabilised by specific binding to a catalytic site. Two such sites have been identified in Complex III [236,239-244]. Quinone-binding proteins have also been described [194-196,245]. 

Fig. 4.2. (C and D) Intermediate electron acceptor (A,) hacteriopheophytin. (C) Flash-induced optical changes compared to the sum of the spectra of D, oxidation and A, reduction (from Ref. 18) ESR spectrum (recorded at 2.1 °K and 10 W power) of the hacteriopheophytin anion radical trapped in RC in which ubiquinone was substituted by menaquinone. The doublet disappears if the spectrum is recorded at low power (10 W) (from Ref. 23).

Tocopherols. Name for 3,4-dihydro-2//-l-benzopy-ran-6-ols (6-chromanols) substituted in the 2-position by a saturated or unsaturated 4,8,12-trimethyltridecyl residue. Oxidation of T. furnishes the tocoquinones. The latter, like the plastoquinones, ubiquinones, " boviquinones, and K vitamins (see vitamin K ) belong to the polyprenylated p-benzo- or p-naphthoqui-nones, which llfil important biological functions as redox pairs with the corresponding hydroquinones (so-called bioquinones). 

Substituting the computed LUMO energy for compound 6 (a model of ubiquinone) yields = [-(-1.446+ 1.62D/I.435]... 

The fact that synthetic, substituted benzoquinones, and also substances of the vitamin K series and ubiquinones are similar in activity to vitamin E or its oxidation products constitutes another indication against the antioxidant hypothesis of vitamin E action. It would hardly be feasible to class all the active compounds as antioxidants. 

A comparison of the 1 resonances of the two spectra suggests differences between the orientation of PJ q relative to Q" and P qq relative to Aj. An ultimate assessment of these structural differences must of course consider the differences in the chemical natures of the donors and acceptors of PSI and photosynthetic bacteria. For example, Q is ubiquinone, a substituted benzoquinone and Aj may be vitamin Kj, a reduced naphthoquinone. (1) We have shown that the g-tensor of reduced vitamin Kj is shifted to lower values compared to reduced ubiquinone. (13) If Aj is vitamin Kj, the g shifts may account for some of the differences at low field and the shift between the two spectra at high field. 

We showed previously that vitamin K3 (2-methyl 1-4, naphthoquinone) would not substitute for phylloquinone in the reconstitution of PSI, and concluded that the 3-phytyl tail may be essential to provide the correct interaction for binding the quinone in the active site on the PSI core (6). We have now extended this study and examined additional naphtho- and ubiquinone derivatives for possible reconstitution of phylloquinone-extracted PSI preparations. Table 1 shows the structures of the quinones tested so far. 

The basic skeleton of isoprenoids may be modified by the introduction of a wide variety of chemical groups, by isomerization, shift of double bonds, methyl groups, etc. Hence a bewildering number of chemical structures arises. In addition compounds derived from other biogenic pathways may contain isoprene residues. For instance the K vitamins (D 8.1), ubiquinones (D 8.3), chlorophylls (D 10.1), plastoquinones, and tocopherylquinones (D 22.4) have isoprenoid side chains with up to ten isoprene units. Polyketides (D 3.3), alkaloids (D 8.4.2), and coumarins (D 22.2.2) may be substituted by dimethylallyl groups. The terpene residues are attached to nucleophilic sites, such as active methylene groups and phenolic oxygen atoms. 

Dehydroquinic acid, shikimic acid, and chorismic acid are carboxylated compounds containing a six-membered carbocyclic ring with one or two double bonds (Fig. 143). The secondary products derived from these substances either still contain the ring and the C -side chain of the acids (see the structure of the benzoic acid derivatives, of anthranilic and 3-hydroxyanthranilic esters, D 8, D 8.2, D 8.4 and D 8.4.1) or have additional rings (see the formulae of naphthoquinones and anthraquinones, D 8.1, of quinoline, acridine, and benzodiazepine alkaloids, D 8.3.2). The carbon skeletons may be substituted by isoprenoid side chains (see the structure of ubiquinones, D 8.3) and may carry different functional groups, e.g., hydroxy, carboxy, methoxy, and amino groups. 

The iron-sulphur centres then pass electrons to ubiquinone (UQ). Ubiquinone (otherwise known as Coenzyme Q) is a substituted quinone which can be readily oxidized and reduced ... 

Approaches such as those described above which rely on the isolation and detection of polyprenyl substituted phenols and quinones in lipid extracts are restricted by the minute quantities of compounds present and the difficulties, frequently encountered, in establishing a precursor relationship between the suspected intermediate and ubiquinone. Gibson and his colleagues approached the problem of ubiquinone biosynthesis in Escherichia coli K-12 by the isolation and examination of mutants unable to form ubiquinone. Five classes of ubiquinone deficient mutants were... 

Plastoquinone, isolated from chloroplasts, chemically falls between ubiquinone and 7-tocoquinone. The quinone ring is substituted with methyl groups, save for one position, just as in the tocopherol series. The side chain is an unsaturated poly-isoprenoid with 45 carbon atoms. 

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