Isoprenoid quinones, structures

Collins, M.D. and Jones, D. (1 981) Distribution of isoprenoid quinone structural types in bacteria and their taxonomic implication. Microbiological Reviews 45, 31 6-354. 

Figure 1 Structures of major isoprenoid quinones found in E. coli. (1) Q-8, ubiquinone (2) MK-8, menaquinone (3) DMK-8, demethylmenaquinone. in the structure of MK, the A ring and B ring of the naphthoquinone are shown.

The flesh of Chroogomphus helveticus contains considerable quantities of a second isoprenoid quinone, helveticone (221) 606). In the mass spectrum helveticone exhibited characteristic ions arising by fragmentation of a farnesyl side chain and the structure (221) was corroborated by comparison of this and other spectroscopic data with those of the isoprenologue (222). 

The evidence that (- )-shikimic acid plays a central role in aromatic biosynthesis was obtained by Davis with a variety of nutritionally deficient mutants of Escherichia coli. In one group of mutants with a multiple requirement for L-tyrosine, L-phenylalanine, L-tryptophan and p-aminobenzoic acid and a partial requirement for p-hydroxybenzoic acid, (—)-shikimic acid substituted for all the aromatic compounds. The quintuple requirement for aromatic compounds which these mutants displayed arises from the fact that, besides furnishing a metabolic route to the three aromatic a-amino acids, the shikimate pathway also provides in micro-organisms a means of synthesis of other essential metabolites, and in particular, the various isoprenoid quinones involved in electron transport and the folic acid group of co-enzymes. The biosynthesis of both of these groups of compounds is discussed below. In addition the biosynthesis of a range of structurally diverse metabolites, which are derived from intermediates and occasionally end-products of the pathway, is outlined. These metabolites are restricted to certain types of organism and their function, if any, is in the majority of cases obscure. 

The detailed structure of caldariellaquinone (12), a unique benzo[6]thiophen-4,7-quinone from Caldariella acidophila, has been elucidated. Feeding experiments with C-labelled acetate helped to define the nature and biosynthetic origin of the C30 isoprenoid chain. 

Chemistry.—The chemical structures of several bacterial menaquinones (MKs) with partly saturated isoprenoid side-chains have been studied. Spectroscopic (u.v., i.r., m.s., and H n.m.r.) and chromatographic data have been recorded for the tetrahydro-MK8 and -MK9 mixture of some nocardioform and coryneform bacteria.The main component tetrahydro-MK9 has the second and third iso-prene residues from the quinone ring saturated, i.e. has structure (159), 2-... 

The electron donor to Chl+ in PSI of chloroplasts is the copper protein plastocyanin (Fig. 2-16). However, in some algae either plastocyanin or a cytochrome c can serve, depending upon the availability of copper or iron.345 Both QA and QB of PSI are phylloquinone in cyanobacteria but are plastoquinone-9 in chloroplasts. Mutant cyanobacteria, in which the pathway of phylloquinone synthesis is blocked, incorporate plasto-quinone-9 into the A-site.345a Plastoquinone has the structure shown in Fig. 15-24 with nine isoprenoid units in the side chain. Spinach chloroplasts also contain at least six other plastoquinones. Plastoquino-nes C, which are hydroxylated in side-chain positions, are widely distributed. In plastoquinones B these hydroxyl groups are acylated. Many other modifications exist including variations in the number of iso-prene units in the side chains.358 359 There are about five molecules of plastoquinone for each reaction center, and plastoquinones may serve as a kind of electron buffer between the two photosynthetic systems. 

Isoprenoids represent the largest family of natural products, with an exceptional structural diversity. Isoprenoids are present in all living organisms. This group includes essential metabolites, such as sterols 27 (Fig. 6) of the eukaryotic plasma membranes, prenyl chains of the quinones 22 and 23 from electron transport chains, and carotenoids 25 from the photosynthetic apparatus in the plant chloroplasts, or in the phototrophic bacteria. Isoprenoids also include secondary metabolites of a more restricted distribution and with a less obvious physiologic significance. Their carbon skeleton can be derived from the combination of C5 subunits with the branched skeleton of isoprene. 

Ubiquinone (UQ), also known as coenzyme Q, has a benzoquinone structure with a long side chain. The name ubiquinone is for the ubiquitous nature of the quinone. Some bacteria also contain menaquinone (vitamin K2), either in addition to ubiquinone or in place of it. For instance, the reaction center of Rhodopseudomonas viridis contains one ubiquinone and one menaquinone, while in some other bacterial reaction centers both quinones are ubiquinones. Menaquinone has a naphthoquinone structure with a long isoprenoid side chain. The long hydrocarbon side chains in ubiquinone and menaquinone render a high degree of hydrophobicity to these molecules. 

The long side chain of CoQ has 10 of the 5-carbon isoprenoid units, and is sometimes called CoQk,. It is also called ubiquinone (the quinone found everywhere) because quinones with similar structures are found in all plants and animals. CoQ can be synthesized in the human from precursors derived from carbohydrates and fat. The long isoprenoid side chain is formed in the pathway that produces the isoprenoid precursors of cholesterol. CoQio is sometimes prescribed for patients recovering from a myocardial infarction, in an effort to increase their exercise capacity. 

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