Photosynthesis secrets exposed


Rebecca Pool

Wednesday, May 29, 2019 - 19:30
Cryo-EM image of PSI-IsiA supercomplex. [Yuval Mazor]
Researchers from Arizona State University, US, have unveiled a stunning image of the PSI-IsiA supercomplex, a massive molecular machine responsible for light reactions in oxygenic photosynthesis.
Captured using single-particle cryo-electron microscopy, the overall structure comprises more than 700 different molecules as well as 591 chlorophylls, by far the largest number of bound pigments in any of the photosynthetic super-complexes with known structures.
"Supercomplexes are associations between antennae proteins and photochemical reaction centres that exist in all photosynthetic organisms," explains Professor Yuval Mazor from the School of Molecular Sciences and the Biodesign Institute's Center for Applied Structural Discovery.
"This particular one comes from cyanobacteria, the class of bacteria in which oxygenic photosynthesis first appeared a few billion years ago and later evolved, into all types of oxygenic photosynthesis that we know today," he adds.
Overall structure of the PSI-IsiA supercomplex just published in Nature Molecular and Structural Biology. [Yuval Mazor]
The supercomplex is the biggest photosynthetic supercomplex with a known molecular structure and represents a large class of antennae that are very common in marine cyanobacteria.
These marine cyanobacteria are responsible for up to 25% of the total global photosynthesis output.
The ability of cyanobacteria to express the PSI-IsiA supercomplex when under stress is crucial to the photosynthetic microbes' survival.
As Mazor writes in Nature Structural & Molecular Biology: “In low-iron environments, cyanobacteria express IsiA, a PSI antenna, critical to their survival.”
Importantly, the latest results uncover crucial details of this enormous machine and point the way to a better understanding of the light-harvesting and photoprotection mechanism in cyanobacteria.
“This 2-MDa photosystem-antenna supercomplex structure reveals more than 700 pigments coordinated by 51 subunits, as well as the mechanisms facilitating the self-assembly and association of IsiA with multiple photosystem I assemblies,” highlights Mazor.
The researchers believe that understanding the complexity and functions of the IsiA photosynthetic supercomplex will ultimately help to ensure a stable energy supply on Earth.
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