Trehalose Interferes with the Photosynthetic Electron Transfer Chain of Cereibacter (Rhodobacter) sphaeroides...

Disaccharide trehalose has been proven in many cases to be particularly effective in preserving the functional and structural integrity of biological macromolecules. In this work, we studied its effect on the electron transfer reactions that occur in the chromatophores of the photosynthetic bacterium Cereibacter sphaeroides. In the presence of a high concentration of trehalose, following the activation of the photochemistry by flashes of light, a slowdown of the electrogenic reactions related to the activity of the photosynthetic reaction center and cytochtome (cyt) bc₁ complexes is observable. The kinetics of the third phase of the electrochromic carotenoid shift, due to electrogenic events linked to the reduction in cyt b_H heme via the low-potential branch of the cyt bc₁ complex and its oxidation by quinone molecule on the Q_i site, is about four times slower in the presence of trehalose. In parallel, the reduction in oxidized cyt (c₁ + c₂) and high-potential cyt b_H are strongly slowed down, suggesting that the disaccharide interferes with the electron transfer reactions of the high-potential branch of the bc₁ complex. A slowing effect of trehalose on the kinetics of the electrogenic protonation of the secondary quinone acceptor Q_B in the reaction center complex, measured by direct electrometrical methods, was also found, but was much less pronounced. The direct detection of carbohydrate content indicates that trehalose, at high concentrations, permeates the membrane of chromatophores. The possible mechanisms underlying the observed effect of trehalose on the electron/proton transfer process are discussed in terms of trehalose's propensity to form strong hydrogen bonds with its surroundings.