Carotenoids spirilloxanthin

Carotenes can be hydroxylated and otherwise modified in a number of ways.110/128-131 The structure of zeaxanthin, one of the resulting xanthophylls, is indicated in Fig. 22-5. Some other xanthophylls are shown in Eq. 22-10. Lutein resembles zeaxanthin, but the ring at one end of the chain has been isomerized by a shift in double bond position to the accompanying structure. The photosynthetic bacterium Rho-dospirillum rubrum has its own special carotenoid spirilloxanthin, which has the accompanying structure at both ends of the chain. 

Boucher et al. (1977) were the first to report that exogenous carotenoids could be incorporated into RCs from carotenoidless bacteria. Using the G9 strain from Ri. rubrum, these authors demonstrated that four carotenoids, spirilloxanthin, spheroidene, spheroidenone and chloroxanthin could be incorporated with nearly 1 1 mol ratios with respect to the primary donor (P 870). The authors showed that the bound carotenoids protected BChl against photodynamic bleaching. An analysis of the absorption and circular dichroism (CD) spectra of the bound carotenoids lead the authors to conclude that the carotenoids adopted a central mono-c/v configuration, a conclusion later confirmed by X-ray diffraction studies on Rhodopseudomonas viridis and Rb. sphaeroides (Yeates et al., 1988 Amoux, 1989 Deisenhofer and Michel, 1989 Ermler et al. 1994 Chapter 6, Fritsch). 

In purified preparations of antenna complexes obtained from several phototrophic purple bacteria, mixtures of metabolically related carotenoids are usually found (Cogdell and Thomber, 1979). The sole reported exception to this rule seems to be a B880 complex of Rhodo spirillum mbmm in which only spirilloxanthin was detected, although Duysens (in Cogdell and Thomber, 1979, pp. 77) suggested that its uniform carotenoid composition could be due to the use of old bacterial cultures as the source of the... 

We are investigating further the nature of the carotenoids present in the bacterial photosynthetic complexes. In this report we shall present evidence showing that whereas the R. rubrum antenna contains a mixture of carotenoids the composition of which changes with the nutritional state of the culture, only spirilloxanthin and, in minor amounts, monodemethylated spirilloxanthin are found in association with the R. rubrum reaction center. The current progress of our work on the functional bases of the differences in carotenoid composition between the antenna and the reaction center shall also be presented. 

The carotenoid absorption band of each purified preparation of antenna complexes was similar, in shape and location, to that of the membranes from which that particular preparation was derived. Besides, the similitude was kept unchanged after organic solvent extraction of the preparations (not shown). Then, it seems that the incorporation of carotenoid pigments to the antenna complexes in vivo is a nonspecific process. The detection of spirilloxanthin as the sole carotenoid of previously analyzed antenna preparations of the same microorganism seems to be accidental and, as suggested by Duysens (in Cogdell and Thomber, 1979, pp. 77), probably due to the use of old, spirilloxanthin enriched cultures as the starting material for complex solubilization. 

It is known from previous biochemical characterizations that LHl of Rs. rubrum, like the LB2 complexes just discussed, also consists of two types of polypeptides, a and p, containing 52 and 54 amino-acid residues, respectively. Each polypeptide has an a-helix spanning the membrane. Two conserved histidine residues (a-His29 and P-His37) have been implicated in the binding of BChl a, each ap-subunit being associated with 2 BChl-a molecules, as well as 1 carotenoid molecule, spirilloxanthin. 

The Rhodospirillaceae and Chromatiaceae have the spirilloxanthin or the okenone pathway depending on the genus or species. All of the Ectothiorhodospiraceae have the spirilloxanthin pathway. The isorenieratene, the y-and -carotene, and the diapocarotene pathways are found specifically in the Chlorobiaceae, Chloroflexaceae, and Heliobacteriaceae, respectively. Aerobic photosynthetic bacteria mostly have the spirilloxanthin pathway, further most of these species have unusual carotenoids including non-photosynthetic carotenoids, such as carotenoid sulfates and carotenoic acids, which have no photosynthetic functions. 

Most of the aerobic photosynthetic bacteria so far desaibed have the spirilloxanthin pathway, further some also have unusual carotenoids as described below. 

Only the spirilloxanthin pathway is found (Tables 1 and 5). Some species have the unusual spirilloxanthin pathway. Hlr. halochloris and Hlr. abdel-malekii contain in addition carotenoid glycosides and their esters as major components. 

See Table 1 Acid carotenoic acid C3Q 4,4 -diapocarotene derivatives Erb-type carotenoid sulfate(s), )3-carotene and its hydroxyl derivatives, y-carotene and its cross-conjugated aldehyde, and spirilloxanthin type. 

In Methylobacterium radiotolerans (previously, Pseudomonas radiora), spirilloxanthin is the dominant component in the RC-LHI complex (Saitoh et al., 1995), while carotenoic acids are found in the outer membranes accompanied by no BChls and have nophotosynthetic functions (S. Saitoh, personal communication). Two species of Methylobacterium have also similar carotenoic acids with M radiotolerans, and two species of the forth group described above have polar carotenoids, diapocarotenoic acid derivatives. These highly polar carotenoids in the third, the forth and the fifth groups may be nonphotosynthetic carotenoids, which are not bound to the photosynthetic pigment-protein complexes (Shimada, 1995). Although their functions are not known, there is the possibility that they protect the photosynthetic apparatus from the outside aerobic conditions. 

Heterologous reconstitution experiments show some structural restraints Spheroidene from Rb. sphaeroides can be reconstituted into a complex with LHl polypeptides from Rhodospirillum rubrum, restoring spectral properties of native Rs. rubrum LHl and carotenoid-BChl energy transfer whereas spirilloxanthin is unable to incorporate into a reconstituted complex with LHl apoproteins from Rb. sphaeroides (Davis et al., 1995). 

Fig. I. Chemical structures of carotenoids, the dependence ofthe ground-state properties on their m-tra/ij configurations being described in Sec. II, (a) p-Carotene, (b) canthaxanthin, (c) zeaxanthin, (d) lutein, (e) /3-apo-8 -carotenal, (f) lycopene, (g) neurosporene, (h) spheroidene, (i) spirilloxanthin and (j) okenone. For each carotenoid, the number of conjugated C=C plus C=0 bonds is shown.

Figure 17 summarizes the 15-cf carotenoids identified in the RCs so far (see above for references). The 5-cis isomers of neurosporene, spheroidene and spirilloxanthin have been identified in the quinone-type RCs of purple bacteria, i.e., Rb. sphaeraides GIC, 2.4.1 and Rs. rubrum SI, respectively. 15-CA-y-carotene and 15-crx-chloro-bactene are found in the iron sulfur-type RC of a green sulfur bacterium, Cb. tepidum. l5 Cis-P-carotene have been identified in the quinone-type PS II RC of spinach, and also in the iron sulfur-type PS I RCs of a cyanobacterium, Sc. vulcanus and from spinach. 

Resonance Raman studies have been conducted on carotenoid molecules bound to light-harvesting proteins (the core and peripheral complexes), isolated from a large number of bacterial species including those either synthesizing carotenoids from the spheroidene or the spirilloxanthin series. In general, Raman spectra of LH-bound carotenoids are extremely similar to those of all-trans carotenoid in... 

Agalidis et al. (1980) showed that the carotenoidless RCs from Rb. sphaeroides R-26 were able to bind either spheroidene or spheroidenone in nearly 1 1 mol ratios with respect to P-870. Neither j3-carotene nor spirilloxanthin could be bound in appreciable amounts, however, suggesting steric interactions are important in determining the type of carotenoids that... 

Krinsky NI (1971) Function. In IslerO, GuttmanG andSolms U (eds) Carotenoids, pp 669-716. Birkhauser Verlag, Basel Kuki M, Namse M, Kakuno T and Koyama Y (1995) Resonance Raman evidence for 15-cw to ail-trans photoisomerization of spirilloxanthin bound to a reduced form ofthe reaction center of Rhodospirillum rubrum SI. Photochem Photobiol 62 502-508... 

Carotenoid pigment isolated from Rhodovibrio and Thioceptis bacteria Karrer, Solmssen, Helv. Chim, Acta 18, 1306 (1935). Identity with spirilloxanthin Zechmeister et al, Arch. Biochim. 5, 243 (1944). Structure Karrer, Koenig. ffeiv. Chim. Acta 23, 460 (]940) Barber ei a(., Proc. Chem. Soc. 1959, 96. Synthesis Surmatis, Ofner, J. Org. Chem. 28, 2735 (1963) Surmatis, U.S. pat. 3,168,658 (1964 to Hoffman-La Roche). 

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