Svensson Bengt, Tiede David M, Nelson David R, Barry Bridgette A
Department of Biochemistry, Biophysics, and Molecular Biology, University of Minnesota, Gortner Laboratory, St. Paul, Minnesota 55108, USA.
Biophys J. 2004 Mar;86(3):1807-12. doi: 10.1016/S0006-3495(04)74247-2.
Photosystem II (PSII) is the plant photosynthetic reaction center that carries out the light driven oxidation of water. The water splitting reactions are catalyzed at a tetranuclear manganese cluster. The manganese stabilizing protein (MSP) of PSII stabilizes the manganese cluster and accelerates the rate of oxygen evolution. MSP can be removed from PSII, with an accompanying decrease in activity. Either an Escherichia coli expressed version of MSP or native, plant MSP can be rebound to the PSII reaction center; MSP reconstitution reverses the deleterious effects associated with MSP removal. We have employed Fourier transform infrared (FTIR) spectroscopy and solution small angle x-ray scattering (SAXS) techniques to investigate the structure of MSP in solution and to define the structural changes that occur before and after reconstitution to PSII. FTIR and SAXS are complementary, because FTIR spectroscopy detects changes in MSP secondary structure and SAXS detects changes in MSP size/shape. From the SAXS data, we conclude that the size/shape and domain structure of MSP do not change when MSP binds to PSII. From FTIR data acquired before and after reconstitution, we conclude that the reconstitution-induced increase in beta-sheet content, which was previously reported, persists after MSP is removed from the PSII reaction center. However, the secondary structural change in MSP is metastable after removal from PSII, which indicates that this form of MSP is not the lowest energy conformation in solution.
光系统II（PSII）是植物光合作用反应中心，负责进行光驱动的水氧化反应。水分解反应在一个四核锰簇上催化进行。PSII的锰稳定蛋白（MSP）可稳定锰簇并加速氧气释放速率。MSP可从PSII中去除，随之活性降低。无论是大肠杆菌表达的MSP版本还是天然的植物MSP，都可重新结合到PSII反应中心；MSP重构可逆转与MSP去除相关的有害影响。我们采用傅里叶变换红外（FTIR）光谱和溶液小角X射线散射（SAXS）技术来研究溶液中MSP的结构，并确定重构到PSII前后发生的结构变化。FTIR和SAXS是互补的，因为FTIR光谱检测MSP二级结构的变化，而SAXS检测MSP大小/形状的变化。从SAXS数据中，我们得出结论，当MSP与PSII结合时，其大小/形状和结构域结构不会改变。根据重构前后获得的FTIR数据，我们得出结论，先前报道的重构诱导的β-折叠含量增加在MSP从PSII反应中心去除后仍然存在。然而，MSP从PSII去除后的二级结构变化是亚稳态的，这表明这种形式的MSP不是溶液中能量最低的构象。
