To create chemicals and fuels from solar energy, scientists often look to Mother Nature and the process of photosynthesis for inspiration. Now, a team has gotten an incredibly detailed view of a reaction center, the pigment-protein complexes that act as the heart of photosynthesis, in a primitive organism. The primitive reaction center is symmetrical. It contains two very similar central proteins and two smaller proteins that bind pigments and other molecules that pass electrons to mobile carriers. The team believes the reaction center (a.k.a. photosystem) is similar to the earliest ones on Earth.
With these results, scientists can compare the primitive reaction center to more advanced centers in other photosynthetic organisms. Such comparisons help them better understand how reaction centers optimally gather light and convert it into chemical energy that the organism can use. Scientists could then use this knowledge—even potentially mimic parts of a reaction center—to develop artificial systems that use sunlight to produce fuels or other chemicals. Also, the structure of the primitive reaction center helps scientists see how photosynthesis has evolved.
By studying photosynthesis, scientists can uncover key details about how organisms use light to drive their metabolism. This fundamental understanding of how photosynthesis takes energy from sunlight and converts it into chemical energy can provide inspiration for new strategies and systems to use sunlight for sustainable energy. Of particular interest are reaction centers, specialized protein complexes that gather light and use it to produce chemical energy. Scientists believe that photosystems, which vary across plants, algae, and bacteria, evolved from a single primitive reaction center. To find the closest relative to this ancestral photosystem, researchers examined the reaction center in a primitive photosynthetic bacterium, Heliobacterium modesticaldum. Using advanced X-ray crystallography, the team determined the structure of the reaction center. They showed the center is symmetrical, unlike the centers in more advanced organisms. The structure suggests how the ancestral reaction center may have looked and, interestingly, how very similar it is in structure to photosystems in plants, algae, and “modern” bacteria. Yet there are important differences in the chemistry, and the primitive reaction center seems to behave like a hybrid of two different modern photosystems, photosystem I and photosystem II. From this structure, scientists have gained new insights into how nature optimized the use of light to drive chemical reactions.
This work was funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, Office of Science, Department of Energy (DOE) (K.E.R., R.F., and J.H.G.). X-ray crystallographic equipment and infrastructure were provided by P. Fromme of the Biodesign Center for Applied Structural Discovery at Arizona State University. Resources were also used at the Berkeley Center for Structural Biology, which is supported in part by the National Institutes of Health, National Institute of General Medical Sciences, and the Howard Hughes Medical Institute, along with the Advanced Light Source and Advanced Photon Source (APS), both DOE Office of Science user facilities. At APS, work was performed at the Structural Biology Center, which is funded by the DOE Office of Science, Office of Biological and Environmental Research.
C. Gisriel, I. Sarrou, B. Ferlez, J.H. Golbeck, K.E. Redding, and R. Fromme, “Structure of a symmetric photosynthetic reaction center-photosystem.” Science 357, 1021 (2017). [DOI: 10.1126/science.aan5611]