Molecule of the Month: Photosynthetic Supercomplexes

Several recent experimental advances have opened a new window on the detailed mechanisms of light capture during photosynthesis. Electron microscopy of chloroplast membranes revealed that photosystems are at the center of large supercomplexes, arrayed closely together and surrounded by antenna systems. Scientists are now developing new, gentler ways to coax these supercomplexes out of living cells, so that they don't break up in the process. Then, new techniques of single particle cryoelectron microscopy allow researchers to observe images of many, many copies of these purified supercomplexes, and combine these images to determine a detailed structure.

Photosynthetic supercomplexes are composed of photosystems, which do the heavy lifting of the photosynthetic energy transformations, surrounded by antenna complexes, which harvest light and funnel energy into the photosystems. Light-harvesting complex II (LHCII) is a major antenna complex for plants and green algae--it is a triangular assembly of proteins filled with light-absorbing cofactors like chlorophyll and carotenoids. Several minor antenna complexes help to link LHCII with the photosystem. The supercomplex shown here (PDB entry 5xnl) is PSII from pea plants.

Plants and algae continually customize their supercomplexes to make most efficient use of the available light. For example, PSII is most efficient with red light, and PSI is most efficient with light that is pushed more towards far-red. Also, plants need to protect their photosystems from damage when there is too much light. Plant and algal cells dynamically shift the location of antenna complexes based on the amount and type of light that is available. These two structures (PDB entries 6kac and 6kad) show two forms of a photosystem from green algae, one with many antenna complexes for conditions with low light, and one with fewer for bright light.

Photosystem I (PSI) commonly forms supercomplexes with two types of light-harvesting complexes. PDB entry 5zji shows the supercomplex from maize plants. Four light-harvesting complex I (LHCI) molecules line one side of the photosystem. If light conditions are right for PSI, LHCII is also bound in the supercomplex, as seen here. Looking at other photosynthetic organisms, there are many variations on this theme. One of the largest supercomplexes is found in cyanobacteria, the most ancient photosynthetic organisms. Under stressed conditions, such as when they are starved for iron, cyanobacterial PSI forms a huge supercomplex with a trimer of the photosystem surrounded by one or two rings of antenna complexes (PDB entry 6nwa).