Isoprenoid chains

Methyl coenzyme M reductase plays a key role in the production of methane in archaea. It catalyzes the reduction of methyl-coenzyme M with coenzyme B to produce methane and the heterodisulfide (Figure 3.35). The enzyme is an a2P2Y2 hexamer, embedded between two molecules of the nickel-porphinoid F jg and the reaction sequence has been delineated (Ermler et al. 1997). The heterodisulfide is reduced to the sulfides HS-CoB and HS-CoM by a reductase that has been characterized in Methanosarcina thermoph-ila, and involves low-potential hemes, [Fe4S4] clusters, and a membrane-bound metha-nophenazine that contains an isoprenoid chain linked by an ether bond to phenazine (Murakami et al. 2001). 

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. 

Most of the natural 2,2-dialkylchromenes arc derived from a single C5 isoprenoid chain (3, R = R = Me). Recently, however, some poly-isoprenoid cliromencs have been isolated gambogic acid (13),74 flemingins (14,)75 cannabichromene (15) and cannabichromenic acid... 

The pioneering work of Evans, Emerson and Emerson led to the isolation of a-tocopherol, a phenol exhibiting vitamin-E-like activity in biological systems. Now, eight compounds, usually oils, have been identified from plant sources, with similar vitamin-E-like actions. The tocopherols differ only in the number of methyl groups in the aromatic ring and therefore can be regarded as derivatives of 2-methyl-6-chromanol, onto which a 16-carbon isoprenoid chain is attached at C-2 and which is methylated at positions C-5, C-7 and C-8, respectively. 

It has long been established that the functional unit of cytochrome oxidase contains four redox-active metal centres. Two of these, cytochromes a and a3, contain heme A (Fig. 5-23) coordinated in different ways in subunit I. The heme group is not covalently linked to the protein. The main structural features are the carbonyl group at position 8 and the isoprenoid chain at position 2 of the porphyrin ring. 

Isoprenylated Quinones.—Chemistry. An efficient method has been described for the preparation of ubiquinone-1 (221) and plastoquinone-1 (223) from the parent quinone and allyltributyltin. The synthesis of ubiquinone-10 by isoprenoid chain-elongation of a ubiquinone-1 derivative has been reported.The sul-phone derivative of the protected ubiquinol-1 (224) on reaction with solanesyl bromide (225) and McjCOK gave the sulphone (226) in 90% yield. Benseker reduction to remove the PhCH2- and PhS02-groups, followed by oxidation in air, afforded ubiquinone-10 (222). 

Table 11 lists several scaled calculated vibrational frequencies of UQ and UQ-, along with experimentally measured frequencies.(S/, 89, 90, 99-101) The table focuses on C=0 and C=C stretching and methoxy torsional frequencies because these frequencies have been experimentally measured and are key to inferences concerning the influence of the protein on ubiquinone structure. For neutral UQ, the mode calculated at a scaled frequency of 1683 cm is a stretching vibration concentrated at the C=0 bond located meta to the isoprenoid chain and corresponds most closely to the recently observed UQ-1 C=0... 

Vitamin E is the nutritional term for the group of naturally occurring tocopherols and tocotrienols that have biological activity similar to RRR-a-tocopherol (formerly d-a-tocopherol). Both groups have a common 6-chromanol nucleus substituted with methyl groups at positions 2 and 8 and with a phytyl tail of isoprenoid units at position 2. The isoprenoid chain is saturated in the tocopherols, but unsaturated at positions 3", and iV for tocotrienols (Figure 30-... 

Another possibihty is that the first membranes arose from phospholipid-like molecules produced by primitive metabolism. Phosphate, with or without additional components, could have provided the hydrophilic heads of the molecules. As to their hydrophobic tails, the French chemist Guy Ourisson has built a strong case in favor of polyisoprenoid molecules, the building blocks of ether lipids, as opposed to long-chain fatty acids, the main constituents of ester lipids (see Ourisson and Nakatani, 1994 Chapter 19, this volume). In nature, isoprenoid chains arise from isopentenyl pyrophosphate, a relatively simple precursor. 

The formation of ether-type polar lipids that occur in the membranes of Archaea remains elusive. The enantiomeric glycerol-phosphate backbone, ether linkages, and isoprenoid chains are distinguishing features of archaeal lipids. Carbonaceous meteorites contain up to several percent of their mass as organic carbon, mainly polycyclic aromatic hydrocarbons. Material extracted from the Murchison meteorite by organic solvents contains amphiphiles that form membrane-like vesicles in aqueous solution. 

Fig. 5.13 Some diagenetic products of carotenoids (R = isoprenoid chain).

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