Structural Basis of Photosystem Ⅰ and Its Photosynthesis Regulation in Green Plants
Photosynthesis is one of the most important chemical reactions on earth.Oxygenic photosynthetic organisms convert solar energy into chemical energy and release oxygen,thus sustaining almost all life on this planet.Oxygenic phototrophs possess two photosystems,namely photosystem Ⅰ(PSⅠ)and photosystem Ⅱ(PSⅡ).Both photosystems are multi-subunit protein complexes embedded in the thylakoid membrane and bind numerous pigment molecules,thereby can efficiently harvest light energy and transfer it to the reaction center.PSI is one of the most efficient nano-photochemical machineries in nature.Its complex structure and sophisticated regulatory mechanisms are crucial for the high photosynthetic efficiency of oxygenic phototrophs.Eukaryotic PSI consists of a core complex where charge separation occurs and a peripheral antenna system that increases the light absorption cross section of the core.The PSI core possesses approximately 12-15 protein subunits,most of them are conserved during evolution,with only several small transmembrane subunits emerging or disappearing.The peripheral antenna system usually contains a number of light-harvesting complexes(LHCs).In contrast to the core,the protein composition and arrangement of LHC antennae vary considerably among different species of photosynthetic organisms.Previous results showed that in angiosperm plants(such as Pisum sativum and Zea mays),the PSI core binds four LHC proteins arranged as an arc-shaped belt,whereas in green algae,the PSI core is associated with more LHCs,presumably a result of adaption to the low-light aquatic environment.In addition,structures of several green algal PSI complexes indicated that green algae can dynamically regulate their light-harvesting capability by adjusting the size of PSI antennae,thereby better adapting to the changing natural environment.In addition to the light harvesting and energy conversion,PSⅠ is also involved in several photosynthetic regulatory processes,including state transitions and cycle electron flow/transfer(CEF/CET).State transitions represent a short-term regulatory mechanism that balances the energy distribution between the two photosystems.During the process of state transitions,when PSⅡ is preferentially excited,a portion of the PSⅡantenna,the major light-harvesting complex Ⅱ(LHCⅡ),is phosphorylated,and these phosphorylated LHCⅡs bind to the PSI core,forming the PSI-LHCI-LHCⅡ complex.This process is reversible,and when PSI is preferentially excited,LHCⅡ is dephosphorylated,detaches from the PSI and binds to the PSⅡ.Previous reports revealed that although higher plants and green algae possess a similar process of state transitions,their PSI-LHCI-LHCⅡcomplexes exhibit specific characteristics in addition to common conserved features.CEF is another important regulatory process in which the PSI participates.In NDH(NAD(P)H dehydrogenase-like complex)dependent CEF,PSI can form supercomplex with NDH to improve the electron transfer efficiency.Previous reports suggested that the PSI bound to NDH and the PSI not bound to NDH possess different LHC compositions,and the exact protein identity and location were recently unraveled based on high-resolution structures.In the past two decades,a number of structures of PSI and PSI-containing complexes have been determined.These structural data provide important information concerning the protein assembly and pigment arrangement of these complexes,allowing for a deeper understanding of the structure and function of green plant PSI.In this review,we summarize the research progresses on the structure of green plant PSIs and PSI-containing complexes involved in photosynthetic regulation,primarily based on the results obtained in our laboratory,and discuss the current state of knowledge concerning the antenna arrangement and the regulatory mechanisms of plant PSI.
photosystem Ⅰprotein structurephotosynthetic regulationstate transitionscycle electron flow/transfer
万方数据
维普
