Estudio de impacto ambiental

surface Iodination (Peters & Drews, 1983b) and protease digestion studies (Oelze, 1978) indicate that the central hydrophobic region of these polypeptides spans the membrane, and it probably has an CT-helical structure (Drews, 1985).

The interaction of Bchl with the light-harvesting polypeptides causes a pronounced red-shift in the near IR absorption maximum from 770 nm to 800-900 nm and is thought to result from electrostatic interactions between Bchl and charged amino-acids on the protein (Eccles & Honig, 1983). The electronic structure of these IR peaks has been extensively Investigated in order to probe the organization of the pigment molecules within the complexes (Saeur & Austin, 1978; Kramer £t fil., 1984).

Thornber ¿1. (1983) concluded that two subclasses of LUI type complexes exist based on such parameters as polypeptide composition, carotenoid stoichiometry, absorption and circular dichrolsm (CD)

spectra. These are represented by the B890 complex from R s . rubrum, and the B870 complex from Rb. sphaeroldes and Rb. caosulatus respectively. The B890 complex contains 2 polypeptides, 2 with Bchl, 1 mol carotenoid and exhibits an intense CD spectrum around the 890 nm IR peak. This Is indicative of strong exclton coupling within a Bchl dimer (Saeur & Austin, 1978). In contrast, the B870 class contains 2 mol carotenoid per pair of polypeptides and exhibits a relatively weak CD spectrum. However, studies using resonance raman spectroscopy (Robert & Lutz, 1985) indicate that these differences are not reflected in the microenvironments of the Bchl molecules within the complexes.

The electronic structure of the pigment within the LHII type complexes also indicates at least two sub-classes to exist (Thornber a l . , 1983). Type I B800-850 complexes are typified by those of R b .

surface iodination (Peters & Drews, 1983b) and protease digestion studies (Oelze, 1978) indicate that the central hydrophobic region of these polypeptides spans the membrane, and it probably has an Of-helical structure (Drews, 1985).

The interaction of Bchl with the light-harvesting polypeptides causes a pronounced red-shift in the near IR absorption maximum from 770 nm to 800-900 nm and is thought to result from electrostatic interactions between Bchl and charged amino-acids on the protein (Eccles & Honig, 1983). The electronic structure of these IR peaks has been extensively investigated in order to probe the organization of the pigment molecules within the complexes (Saeur & Austin, 1978; Kramer £t al . , 1984).

Thornber ££ ¿1. (1983) concluded that two subclasses of LHI type complexes exist based on such parameters as polypeptide composition, carotenoid stoichiometry, absorption and circular dichrolsm (CD)

spectra. These are represented by the B890 complex from R s . rubrum, and the B870 complex from R b . sohaeroldes and R b . cansulatus respectively. The B890 complex contains 2 polypeptides, 2 with Bchl, 1 mol carotenoid and exhibits an intense CD spectrum around the 890 nm IR peak. This is indicative of strong exciton coupling within a Bchl dimer (Saeur & Austin, 1978). In contrast, the B870 class contains 2 mol carotenoid per pair of polypeptides and exhibits a relatively weak CD spectrum. However, studies using resonance raman spectroscopy (Robert & Lutz, 1985) indicate that these differences are not reflected in the microenvironments of the Bchl molecules within the complexes.

The electronic structure of the pigment within the LHII type complexes also indicates at least two sub-classes to exist (Thornber &£ a l ., 1983). Type I B800-850 complexes are typified by those of R b .

capsulatus. R b . sphaeroides and possibly R p . palustrls (Feick & Drews, 1978; Thornber e£ a l . , 1983). Cogdell & Thornber (1980) discussed a model of the B800-850 complex of R b . capsulatus in which 1 carotenoid molecule and 1 Bchl molecule responsible for the 800 nm absorption band were bound to the 8,000 (a) polypeptide and two Bchl molecules

responsible for the 850 nm absorption band were bound to the 10,000 (/3) polypeptide. The 850 nm band exhibits a strong CD spectrum indicating exciton coupling between these two Bchl molecules while the 800 nm band does not, consistent with the proposed monomeric structure (Picorel e£ a l - , 1984). The peak height of the 850 nm band is usually 1.5 times as intense as the 800 nm band in type I B800-850 complexes (Thornber et a l - . 1983). The type II B800-850 complex is present in some R p , acldophila strains under certain growth conditions (Cogdell et a l ., 1983) and in the purple sulphur bacterium Chromatium vinosum. Here the 850 nm absorption band is of equal or lower intensity than the 800 nm band. The resonance Raman study of Robert & Lutz (1985) also revealed a difference between the B800-850 complexes of the

Rhodospirillaceae and Chromatiaceae in the type of "environment" in which the Bchl is located but this Included differences between R p . acldophlla and Ch r . vinosum.

Clearly, the classification of the various light-harvesting complexes depends upon which parameters are used. Although the Bchl environments may be very similar in the B800-850 complexes from the Rhodospirillaceae

for example, there still exists marked differences in absorption spectra, pigment stoichiometry and polypeptide composition. This is especially true for organisms such as the Bchlb containing R p . virldls. in which a single B1020 LH complex is thought to exist (Jay a l . , 1984) • how does this compare in properties with those of Bchla

containing LH complexes? New approaches, such as the recently reported crystallization of the B800-850 complex from R b . capsulatus by Welte e£ al. (1985) should allow detailed studies by X-ray crystallography of the structure of LH complexes in general, and give new insights into how energy is transferred within them.

1.3.1.2 The photochemical reaction centre. That the photochemical reaction centre existed as a defined structure in photosynthetic bacteria was first proposed by Clayton (1963). This followed the observation that when the light-harvesting Bchl of the carotenoidless mutant R-26 of R b . sohaeroldes was destroyed chemically, light induced bleaching of a component in the absorption spectrum (at 870 nm) could still be observed. The isolation of the RC as a pigment-protein complex from R b . sohaeroldes (Reed & Clayton, 1968) precipitated intense

research, both on the mechanism of primary photochemistry and susbsequent electron transport (section 1.3.1.4) and on structure- function relationships within the complex.