Liina Kangur will defence her doctoral thesis "High-pressure spectroscopy study of chromophore-binding hydrogen bonds in light-harvesting complexes of photosynthetic bacteria" on 26 August at 10 am in the Institute of Molecular and Cell Biology (Riia 23B - 105) in the speciality of biochemistry.
Supervisor: Prof Arvi Freiberg, University of Tartu, Institute of Molecular and Cell Biology
Opponent: Prof Roland Winter, University of Dortmund, Germany
The light-harvesting antenna complexes from purple photosynthetic bacteria are convenient model systems to examine the poorly understood role of hydrogen bonds as stabilizing factors in membrane protein complexes. The non-covalently bound arrays of bacteriochlorophyll chromophores within native and genetically modified variants of light-harvesting complexes were used to monitor local changes in the chromophore binding sites induced by externally applied hydrostatic pressure. A unique combination of optical spectroscopy with genetic and noninvasive physical (high-pressure) engineering applied in this work provides the first demonstration and quantification of the rupture of multiple hydrogen bonds in the bacteriochlorophyll binding pockets of the LH1 and LH2 membrane chromoproteins. While the membrane-bound complexes demonstrate very high resilience to pressures reaching 3 GPa, the detergent-isolated complexes reveal characteristic discontinuities of the absorption band shifts and broadenings around 1.1 GPa and 0.5 GPa in the case of the wild type LH1 and LH2 complexes, respectively, which evidence reversible and cooperative breakage of H-bonds. Genetic manipulations leading to exchange of native carotenoids, partial loss of chromophores, and/or H-bonds that bind the chromophores to the surrounding protein scaffold were found to significantly destabilize the membrane chromoproteins under high pressure. Co-solvents such as glycerol as well as high protein concentration, on the other hand, were able to stabilize not only detergent-isolated, which was known previously, but also the membrane-embedded chromoproteins. The energy required to break the H-bonds in wild type LH1 and LH2 complexes is 10-20 times greater than the average thermal energy at physiological temperatures, which secures their great stability under functional conditions. This study thereby provides important insights into design principles of natural photosynthetic complexes.