Search

Using the mass-measuring capability of scanning transmission electron microscopy, we demonstrate that membrane crystals of the main light-harvesting complex of plants possess the ability to undergo light-induced dark-reversible disassociations, independently of the photochemical apparatus. This is the first direct visualization of light-driven reversible reorganizations in an isolated photosynthetic antenna. These reorganizations, identified earlier by circular dichroism (CD), can be accounted for by a biological thermo-optic transition: structural changes are induced by fast heat transients and thermal instabilities near the dissipation, and self-association of the complexes in the lipid matrix. A comparable process in native membranes is indicated by earlier findings of essentially identical kinetics, and intensity and temperature dependences of the ΔCD in granal thylakoids.

Catalase from the facultatively psychrophilic bacterium Vibrio rumoiensis S-1 T , which was isolated from an environment exposed to H 2 O 2 and exhibited high catalase activity, was purified and characterized, and its localization in the cell was determined. Its molecular mass was 230 kDa, and the molecule consisted of four identical subunits. The enzyme, which was not apparently reduced by dithionite, showed a Soret peak at 406 nm in a resting state. The catalytic activity was 527,500 U · mg of protein −1 under standard reaction conditions at 40°C, 1.5 and 4.3 times faster, respectively, than those of the Micrococcus luteus and bovine catalases examined under the same reaction conditions, and showed a broad optimum pH range (pH 6 to 10). The catalase from strain S-1 T is located not only in the cytoplasmic space but also in the periplasmic space. There is little difference in the activation energy for the activity between strain S-1 T catalase and M. luteus and bovine liver catalases. The thermoinstability of the activity of the former catalase were significantly higher than those of the latter catalases. The thermoinstability suggests that the catalase from strain S-1 T should be categorized as a psychrophilic enzyme. Although the catalase from strain S-1 T is classified as a mammal type catalase, it exhibits the unique enzymatic properties of high intensity of enzymatic activity and thermoinstability. The results obtained suggest that these unique properties of the enzyme are in accordance with the environmental conditions under which the microorganism lives.

We investigated the temperature dependence and inhibitor sensitivity of the light-induced reversible changes in the circular dichroism (ΔCD) of chloroplast thylakoid membranes. Earlier, these changes, which originate from structural changes affecting the chiral macroorganization of the chromophores, were thought to be driven by photochemically generated proton and/or ion gradients in the thylakoids [Garab et al. (1988) Biochemistry 27: 2430]. However, more recently, these changes have been shown to be largely independent of the photochemical activity of thylakoids, and ΔCD has been observed in lamellar aggregates of the light harvesting chlorophyll a/b complex (LHC II) of Photosystem II [Barzda et al. (1996) Biochemistry 35: 8981]. Here, we show that in thylakoids (i) ΔCD is gradually and substantially decelerated upon gradually decreasing the temperature from 33 °C to 2 °C, and abruptly disappears above 35–37 °C; (ii) ΔCD is inhibited with nigericin with I50≈ 1 μM, which is about 10 times higher than the I50for the transmembrane ΔpH; (iii) ΔCD can be inhibited with dicyclohexylcarbodiimide that blocks proton binding at the lumenal side of LHC II; (iv) quinone antagonists, such as antimycin-A and myxothiazol, inhibit ΔCD without noticeably affecting the electron and proton transport, and the chiral macroorganization of the chromophores in the dark. We conclude that ΔCD is conditioned but not driven by the photochemical activity of the membranes, and the structural changes are given rise by a physical mechanism previously unrecognized in thylakoids, thermooptic effect described for liquid crystals. We discuss the possible link between the deactivation(s) of the excess excitation energy and ΔCD, the light-induced changes in the chiral macroorganization of the chromophores of the photophysical apparatus in thylakoids.

In this paper, we show that stacked lamellar aggregates of the purified chlorophyll a/b light-harvesting antenna complexes (LHCII) and granal thylakoid membranes are capable of undergoing light-induced reversible changes in the chiral macroorganization of the chromophores as well as in the photophysical pathways. In granal thylakoids, the light-induced reversible structural changes, detected by circular dichroism (CD) measurements, are accompanied by reversible changes in the fluorescence yield that indicate an increased dissipation of the excitation energy. These changes become gradually more significant in excess light compared to nonsaturating light intensities, and can be eliminated by suspending the membranes in hypotonic, low-salt medium in which the chiral macroaggregates are absent. In lamellar aggregates of LHCII, the light-induced reversible changes of the main, nonexcitonic CD bands are also accompanied by reversible changes in the fluorescence yield. In small aggregates and trimers, no light-induced delta CD occurs, and the fluorescence changes are largely irreversible. It is proposed that the structural changes are induced by thermal effects due to the excess light energy absorbed by the pigments. Our data strongly suggest that the structure and function of the antenna system of chloroplasts can be regulated by the absorption of excess light energy with a mechanism independent of the operation of the photochemical apparatus.