Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organisms’ activities. It is largely responsible for producing and maintaining the oxygen content of the Earth’s atmosphere, and supplies most of the energy necessary for life on the Earth.
The reaction center (RC)—the core of the photosynthetic apparatus where charge separation occurs—is thought to have originated a single time and diverged, yielding new kinds of complexes adapted to different tasks and environments. RCs are classified as type I (Fe-S type) or type II (quinone type) on the basis of their terminal electron acceptors, and all oxygenic photosynthetic organisms contain both type I and II RCs. Most extant RCs are heterodimers represented by photosystems I and II (PSI and PSII), and they have evolved from homodimeric RCs similar to those seen in green sulfur bacteria (GSB) and heliobacteria. PSI absorbs light energy and converts it into chemical energy to fix carbon dioxide and produce food whereas PSII produces atmospheric oxygen to catalyze the photo-oxidation of water by using light energy. The photosynthetic reaction is an extremely sophisticated process and the architecture of proteins in RCS is also incredibly complex, so they have evolved only once over eons throughout history. All RC proteins on the Earth are thus deemed to have descended from the same ancestral protein.
GSB are part of the family of photosynthetic bacteria that can perform anaerobic photosynthesis by acquiring electrons from such substances as hydrogen sulfide, colloidal sulfur, and sodium thiosulfate. Although the green sulfur bacterium was discovered decades ago, the detailed structure of its internal photosynthetic system has remained obscure for scientists. It is the only phylum of photosynthetic bacteria whose RC structure has not yet been decoded. There are two main factors which contribute to the tremendous difficulty in resolving the structure of the RC of GSB. For one thing, it is a formidable challenge to fabricate samples in the RC of GSB because GSB are strictly anaerobic and can grow under extremely low light intensities. For another, it is extremely hard to meet the demanding requirement of collecting a large number of samples with high purity and homogeneity when the structure of biomolecules was principally analyzed by X-ray crystallography in early studies.
The research team led by Prof. ZHANG Xing from Sir Run Run Shaw Hospital of Zhejiang University School of Medicine and Center of Cryo-Electron Microscopy unraveled this mystery from the GSB Chlorobaculum tepidum at 2.7-Å resolution by cryo–electron microscopy. It is the first time that the structure of the FMO-GsbRC supercomplex has been solved in the world. Their findings were published in the journal of Science on November 20.
Their study indicates that the structure contains one FMO trimer attached to a distal end of the homodimeric GsbRC, and the edge-to-edge distance between the bacteriochlorophylls of FMO and GsbRC is >21 Å. “This distance is considerably longer than that has been observed in other photosynthetic systems and can thus account for the reduced efficiency of energy transfer between these two complexes,” said CHEN Jinghua, a postdoc fellow and lead author of this study.
The structure also reveals a set of features that may represent an ancestral form of both type I and II RCs. These features include a homodimeric core with fewer pigments associated, a conserved core protein structure, distinctive pigment arrangement similar to that seen in PSII instead of PSI, and a chlorobactene derivative located between the two chlorophyll layers within the membrane.
“With this structure, we now have a complete set of structures from different groups of photosynthetic organisms, allowing them to examine the evolution of photosystems in greater detail. By revealing the arrangement of proteins and pigments, including features of both type I and II RCs, the structure provides valuable insight into how extant members of this family of proteins diverged from a common RC ancestor,” said ZHANG Xing.
The research team will strive to collect more supporting data in their follow-up research. It is expected that in the future, bionic photosensitive devices will be designed via artificial simulations of photosynthesis, and crop yields will be improved so as to alleviate the growing food and energy problems by modifying the plant photosynthetic reaction system and improving the utilization rate of light energy.