Photoprotective sites in the violaxanthin-chlorophyll a binding Protein (VCP) from Nannochloropsis gaditana

Violaxanthin-chlorophyll a binding protein (VCP) is the major light harvesting complex (LHC) of the Heterokonta Nannochloropsis gaditana. It binds chlorophyll a, violaxanthin and vaucheriaxanthin, the last in the form of 19' deca/octanoate esters. Photosynthetic apparatus of algae belonging to this group have been poorly characterized in the past, but they are now receiving an increasing interest also because of their possible biotechnological application in biofuel production. In this work, isolated VCP proteins have been studied by means of advanced EPR techniques in order to prove the presence of the photoprotective mechanism based on the triplet-triplet energy transfer (TTET), occurring between chlorophyll and carotenoid molecules. This process has been observed before in several light harvesting complexes belonging to various photosynthetic organisms. We used Optically Detected Magnetic Resonance (ODMR) to identify the triplet states populated by photo-excitation, and describe the optical properties of the chromophores carrying the triplet states. In parallel, time-resolved EPR (TR-EPR) and pulse EPR have been employed to get insight into the TTET mechanism and reveal the structural features of the pigment sites involved in photoprotection. The analysis of the spectroscopic data shows a strong similarity among VCP, FCP from diatoms and LHC-II from higher plants. Although these antenna proteins have differentiated sequences and bind different pigments, results suggest that in all members of the LHC superfamily there is a protein core with a conserved structural organization, represented by two central carotenoids surrounded by five chlorophyll a molecules, which plays a fundamental photoprotective role in Chl triplet quenching through carotenoid triplet formation.