FCPA

Fig. 12. Simple models of various photosystems (A) purple bacteria, (B) green bacteria, (C) cyanobacteria and red algae, (D) bro n algae and (E) green plants. RC=reaction center FCPA=fucoxanthin-chlorophyll-protein assembly LHC=light-harvesting complex PS ll=photosystem II PS l=photosystem I. See text for discussion.

B. The Supramolecular Fucoxanthin-Chlorophyll-Protein Assembly (FCPA).236... 

The FCPA complex is made up of seven protein subunits, each with a molecular weight of 74 kDa and each containing 13 molecules of Chi a, 3 Chi c, 10 fucoxanthin and 1 violaxanthin. The pigment content ofthe FCPA complex is unusually high, with a pigment-to-protein weight ratio of 0.37. 

The intact, orange-brown FCPA complex is readily dissociated into its seven protein subunits by the action of detergents such as LDAO or Triton XI00 at very low concentrations, even as low as 0.005%. The extreme frailty of this supramolecular complex may explain why it had not been isolated earher. 

Fig 6. Absorption and second-derivative spectra of intact fucoxanthin-chlorophyll- a/c protein assembly (FCPA) at room temperature (A, top) and 77 K (A, bottom). Absorption spectra of intact and Triton-treated FCPA (B, top) and their difference spectrum between the two (B, bottom). Figure source Katoh, Mimuro and Takaichi (1989) Light-harvesting particles isolated from a brown alga, Dictyota dichotoma. A supramolecular assembly of fucoxanthin-chlorophyll-protein complexes. Biochim BiophysActa 968 236,237. 

The difference in the fluorescence spectra between intact and dissociated FCPA is rather dramatic, as can be seen in Fig. 7. The main emission band at 677 nm shown in Fig. 7 (A) was excited at 440,460 and 515 nm, wavelengths absorbed predominantly by Chi a, Chi c and fucoxanthin, respectively. The emission spectra indicate that both Chi c and fucoxanthin transfer excitation energy to Chi a with high efficiency. The emission spectrum excited by 460 nm shows a weak emission at 640 nm, originating from Chi c. However, the 640-nm emission is not seen when the sample was excited at 515 nm, which suggests that fucoxanthin transfers energy directly to Chi a without the mediation of Chi c. 

In the dissociated FCPA, excitation at 440, 460 and 515 nm evokes quite different emission patterns [Fig. 7 (B)]. The 460-nm light, preferentially absorbed by Chi c, produces an increase in the emission at 640 nm and much less emission at 670 nm. Similarly, excitation by 515-nm light absorbed primarily by fucoxanthin also produced very little 677-nm emission but no emission at all at 640 nm. These results suggest that the coupling between Chi c and Chi a and that between fucoxanthin and Chi a are both disrupted and they also confirm that transfer of energy from fucoxanthin to Chi a does not go through Chi c. 

An electron microscope examination of the intact FCPA isolated from Petaloniafascia showed the supramolecular assemblies to be uniform in size and shape. They were found to be discoidal, 11.2 nm in diameter and 10.2 nm in height, and with a cavity in the center. Long-term storage of FCPA resulted in the... 

In this model, the function of one remaining fucoxanthin and one violaxanthin has not yet been determined (see Fig. 8). By comparing the fluorescence excitation spectrum with the absorption spectrum of FCPA, it is seen that a decrease of 15-25% in transfer efficiency appears in the 480 nm-region, which would be consistent with an absence ofcoupling by two out of the 11 carotenoid molecules. 

Fig. 8. Schematic representation of energy flow in the FCPA polypeptide. Abbreviations are 669i agegi 3673. bi 3373 SjE, terminai emitter Fuco, fucoxanthin Viola, violaxanthin Cs and Cl for short- and long-wavelength forms of Chi c. See text for details. Figure source Mimuro, Katoh and Kawai (1990) Spatial arrangement of pigments and their interaction in the fucoxanthin-chiorophyil aJc protein assem-biy (FCPA) isolated from the brown alga Dictyota dichotoma. Analysis by means of polarized spectroscopy. Biochim. Biophys Acta 1015 455,...

M Mimuro, T Katoh and H Kawai (1990) Spatial arrangement of pigments and their interaction in the fucoxan-thin-chlorophyll a/c protein assembly (FCPA) isolated from the brown alga Dictyota dichotoma. Analysis by means of polarized spectroscopy. Biochim. Biophys Acta 1015 455. 

FCPA - fucoxanthin-chlorophyll protein assembly FCPC - fucoxanthin chlorophyll ale protein complex Ed - ferredoxin FeCy - ferricyanide Fe Q - iron-quinone complex FeS - iron-sulfur (cluster) see ISP FeS-A / FeS-B - iron-sulfur centers A / B FeS-X - iron-sulfur center X FIAC - field-induced absorption change fMet (or fM) - formylmethionine FMN - flavin mononucleotide FMO (BChl-fl) protein - Fenna-Mathews-Olson protein... 

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