LIGHT ENERGY CONVERTING BIOLOGICAL STRUCTURES, PHYSIOLOGICAL PROCESSES, PHYSICAL MECHANISMS

Our laboratory focuses on light energy converting biological systems and similar artificial assemblies. The main goal of our research is to understand the structure and function of photosynthetic membranes; furthermore we participate in the research and development of environmentally friendly self-assembling systems for solar energy conversion, and we also design and construct scientific instruments.

Photosynthesis is the energetic basis of life on Earth. The atmospheric oxygen, and thus also the ozone shield, are of photosynthetic origin. The fossil energy carriers are ‘solar-energy deposits’ of the past, and the greenhouse gas CO2 that is released during their combustion can only be recycled by photosynthetic organisms. Hence, a better understanding of photosynthesis is of paramount importance, which can also help in designing artificial solar energy utilizing systems.

In recent years, we have (i) revealed the three dimensional ultrastructure of the photosynthetic membrane system of plants – the most abundant and one of the most complex membrane systems of the biosphere, (ii) offered an explanation for the (enigmatic) role of non-bilayer lipids in the organization and dynamics of the bilayer biological membrane, (iii) have shown that pigment-protein complexes are assembled into highly organized but structurally flexible macrodomains, and (iv) discovered a novel, biological thermo-optic mechanism, which appears to play important roles in the regulation of physiologically relevant processes. We also design and construct patented innovative instruments that provide unique physical information on the molecular architecture of microscopic samples.

The structure and flexibility of granal thylakoid membranes; light harvesting systems

During photosynthesis the absorbed light energy is converted into chemical energy. Since sunlight is quite ‘dilute’ (the photon flux density is low), photons must be harvested by an ‘antenna’ system. The high efficiency of this system, which captures photons and transfers light (excitation) energy into the photochemical reaction centers, can only be ensured by a highly organized molecular assembly, the (macro-)organization of which is still largely unknown. The main goal of our research is to reveal the structure and function of these self-assembling antenna and membrane systems, and to construct similar artificial molecular devices.

Our electron tomographic investigations and earlier data obtained with ‘conventional’ electron microscopy, supervised in part by László Mustárdy, revealed the 3-dimensional ultrastructure of the granum-stroma thylakoid membrane system, a quasi-helical assembly (Figure 1) [1,2].

Granal chloroplasts are a rather late but immensely successful product of evolution. They appeared ‘only’ about half a billion years ago, but have ‘conquered’ all green plants and thus have become the most abundant membrane system in nature. Concerning the structure and dynamics of this multilamellar membrane system under physiologically relevant conditions, non-invasive neutron and X-ray scattering measurements are performed, mostly on European large scale facility instruments. Spectroscopic techniques are used to reveal self-assembly, molecular organization and structural flexibility of this hierarchically organized complex membrane system.

By using NMR and optical spectroscopic techniques, we have recently shown that thylakoid membranes contain significant amounts of non-bilayer lipid phase(s), which appear to be associated with the bilayer membrane [3]. The importance of this question is signified by the fact that in energy-converting membranes non-bilayer lipids constitute half or more of the total lipid content; these lipids, their interactions with proteins, and non-bilayer phases might play important roles in determining the structure and dynamics of membranes [4,5].

Polarization spectroscopic investigations revealed that these membranes contain highly ordered, but structurally flexible units, the chiral macrodomains [6 and references therein]. We would like to understand the roles of different light harvesting components and different lipid classes in macro-organization [7], a project co-supervised by László Kovács. Granal thylakoid membranes and lamellar aggregates of isolated light-harvesting antenna complexes display very interesting spectroscopic features; they are non-recomposable from their constituents („the whole is more than the sum of its parts”). Their investigation as models is justified by the fact that similar features are exhibited by more complex entities, such as nuclei, chromosomes, protein aggregates, and by observations suggesting that most regulatory processes occur at this level of complexity, rather than at lower levels of the structural hierarchy.

While studying the structural flexibility of chiral macrodomains, we have discovered a novel, biological thermo-optic mechanisms [8]. During thermo-optical reorganizations ultrafast local heat-transients, originating from the dissipation of excess photon energy, cause elementary structural changes due to the “built-in” thermally instable structural units in the close vicinity of the dissipation (Figure 2). In this way, the excess energy, which cannot be used in photosynthesis, induces structural changes. According to other data, these reorganizations play important roles in the regulation of photosynthesis, especially in high-intensity light and at elevated temperatures [9, 10]. Similar reorganizations have also been observed in diatoms [11]. (These algae deserve special attention for their ability to remove the atmospheric CO2 from the fast carbon cycle, and sequester the fixed CO2 into the sediments of deep oceans.)

Self-assembling light harvesting natural and reconstituted multilamellar systems [11,12] are of great potential interest with regard to solar energy converting systems; they are ideally suited to supply energy for photochemical reactions located on catalytic surfaces to produce hydrogen or other fuel. We also participate in national and international collaborations in this field.

Heat stress - replacing the oxygen evolving complex in vivo

Plants, algae and cyanobacteria are capable of water oxidation. During this process, molecular oxygen is released to the atmosphere and the electrons are used in a multistep electron transport process to produce carbohydrates from CO2. This process also depends on the activity of the water-oxidizing complex (WOC), which supplies electrons for carbon fixation. However, WOC is very sensitive to environmental stresses, especially to heat stress, which can lead to its full inactivation. Our studies on different plant species and algae have shown that WOC can be replaced by ascorbate (vitamin C). By this means, ascorbate can transiently maintain photosynthetic activity, and thus protect the machinery (Figure 3) [13]. The aim of our studies – led by Szilvia Z. Tóth - is to explore the physiological significance and the potential biotechnological applications of this mechanism.

Order or disorder in biological samples? DP-LSM: a novel tool for mapping anisotropy

Highly organized molecular macroassemblies are found in many hierarchically organized biological systems, e.g. in nuclei, chromosomes, viruses, stacked membranes, tissues, actin-based structures. However, our understanding of the self-assembly, molecular organization, structural dynamics and physiological functions of these complex molecular assemblies is still rudimentary, mainly because in most samples these features can hardly be discerned macroscopically. In recent years we have elaborated new imaging methods to measure and map the anisotropy in microscopic samples. In collaboration with spin-off companies and the Carl Zeiss Jena GmbH, we have equipped our laser scanning confocal microscope (LSM) with differential polarization (DP) attachments. With the DP-LSM, 8 independent physical parameters can be mapped, which all carry unique information on the anisotropic molecular architecture of the samples. DP images revealing important novel features have been recorded on different biological systems, such as granal chloroplasts, plant cell walls (Figure 4.), cell membranes, amyloid filaments, actin-based structures, and also on intelligent materials, such as liquid crystals and pigment nanorods [14, 15 and references therein]. The DP-LSM has been awarded gold and silver medals at major international exhibitions.