Book of Abstracts: Albany 2003
June 17-21 2003
Crystal Structures of Photosystems I and II from the Cyanobacterium Synechococcus Elongatus
Oxygenic photosynthesis in plants, green algae and cyanobacteria is performed by photosystems I and II (PSI and PSII) embedded in the thylakoid membrane. They function in series, PSII oxidizing water to molecular (atmospheric) O2, H+ and electrons; the latter are transferred to PSI and from there via ferredoxin to NADP-reductase to provide NADPH+. The H+ released at PSII form a pH-gradient over the thylakoid membrane that drives ATP synthetase. ATP and NADH+ serve in dark reactions to reduce CO2 to carbohydrates. PSI and PSII from Synechococcus elongatus were solubilized from the thylakoid membrane using dodecylmaltoside as detergent and crystallized; they are fully active in the crystals. X-ray diffraction data were collected at synchrotrons DESY/Hamburg, ESRF/Grenoble, LURE/Paris and the structures determined at 2.5 Å (PSI, (1)) and 3.8 Å (PSII, (2)) resolution. In PSI, the amino acid sequences of the 12 polypeptides were fitted to the electron density, and 96 chlorophyll a, 2 phylloquinones, 3 Fe4S4 clusters, 22 carotenoids and 4 lipid molecules could be located. In PSII, 12 of the 17 subunits were identified in the electron density and the Mn-Cluster has been located (where water oxidation occurs) as well as 31 chlorophyll a, 4 Fe2+, plastoquinone QA, 2 pheophytins. Of particular interest is the architecture of the electron transfer chains including the chlorophyll a "special pairs" P680 (PSII) and P700 (PSI). They are at the heart of the photosystems and drive photosynthesis, fueled by photons from sunlight that is collected by large antenna systems containing chlorophyll a and carotenoids.
These studies were supported by DFG-Sonderforschungsbereich 312 and 498 and by Fonds der Chemischen Industrie. We are grateful to the mentioned synchrotrons for providing X-ray beam time.
1Institut für Kristallographie