Jian-Ren Shen

Mechanism of photosynthetic water oxidation based on high resolution, damage-free structure of photosystem II revealed by XFEL
Presenter Jian-Ren Shen, Okayama University
Session Title New Discoveries in Biology with XFELS
Abstract

Photosystem II (PSII) is a large membrane-protein complex consisting of 20 subunits with a total molecular mass of 350 kDa for a monomer, which catalyzes light-induced water-splitting leading to the evolution of molecular oxygen indispensible for oxygenic life on the earth. We have solved the crystal structure of PSII from a thermophilic cyanobacterium Thermosynechococcus vulcanus at a resolution of 1.9 Å (1) using synchrotron radiation (SR) X-rays, which revealed a clear picture of the catalytic center for water-splitting, namely, a Mn4CaO5-cluster organized as a distorted chair form. This feature suggested a remarkable flexibility in its structure, which would be needed for the structural changes expected to occur during the catalytic water-splitting cycle (S-state cycle). Some of the inter-atomic distances within the metal cluster revealed by the structural analysis using SR, however, were shown to be slightly longer than those obtained by EXAFS and theoretical studies, presumably due to radiation damage caused by the SR X-rays. In order to avoid possible radiation damage and eliminate the uncertainties in the inter-atomic distances within the cluster, we used femtosecond X-ray pulses provided by an X-ray free electron laser (XFEL) facility SACLA, Japan, to solve the structure of PSII. In order to obtain a high resolution structure, we used large PSII crystals (1.0 mm x 0.5 mm x 0.2 mm), and adopted an approach where every point of the crystal was shot by 1 XFEL pulse, and every two XFEL pulses were separated at least by 50 μm on the crystals. This approach required a huge number of large, isomorphous PSII crystals, but allowed us to collect damage-free, high resolution diffraction data, enabling us to solve the PSII structure at 1.95 Å resolution (2). This structure showed that most of the Mn-Mn and Mn-ligand distances were 0.1-0.3 Å shorter than those observed in the previous SR structure. However, the bond distances of O5, a unique oxo-bridged oxygen, with two of its nearby Mn ions Mn1 and Mn4, are still unusually longer (2.3 and 2.7 Å, respectively) compared with typical Mn-O bond distances. This unusual property implied that this oxo-bridge binds weakly to its nearby metal ions, and therefore may participate in the O-O bond formation during O2 release. Based on these results, I will discuss the possible mechanisms for photosynthetic water-splitting.

 

References

(1) Umena Y., Kawakami K., Shen J.-R., Kamiya N. (2011) Nature, 473, 55-60.

(2) Suga M., Akita F., Hirata K., Ueno G., Murakami H., Nakajima Y., Shimizu T., Yamashita K., Yamamoto M., Ago H., Shen, J.-R. (2015) Nature 517, 99-103.