X-rays Reveal PSII Secret

Water seems like the perfect energy source, packed, as it is, with two atoms of hydrogen per molecule. There is a problem though - how to extract the hydrogen as a fuel without using more resources than energy released. Today, it costs ten times as much as natural gas to split water electrolytically into oxygen and hydrogen and three times as much as gasoline. Certain bacteria solved the problem of splitting water with sunlight billions of years ago, thus generating the oxygen in the atmosphere we breathe.

Now, the possibility of using the endless supply of solar energy and the Earth's abundant amounts of water as a cheap source of hydrogen, as well as oxygen, is a step closer thanks to British researchers who have discovered how the photosynthetic centre in a cyanobacterium works.

Jim Barber

Jim Barber and his colleagues at Imperial College London have used X-ray crystallography to reveal the precise structure of the protein complex that drives photosynthesis II (PSII in the bacterium Thermosynechococcus elongates). Their results reveal for the first time the details of the catalytic site that underpins the water-splitting photosynthetic reaction. Barber suggests that emulating this process on an industrial scale could one day be possible, allowing hydrogen fuel to be released from water using sunlight.
 

"The splitting of water is driven by sunlight in photosynthetic organisms," Barber told Reactive Reports. "The oxygen is released into the atmosphere while the 'hydrogen' is combined with carbon dioxide to form organic matter; we want to sidestep the latter and tap the 'hydrogen' directly."

Green plants have  been splitting water easily using sunlight for millions of years.

 Earlier models of photosynthesis have merely sketched out the details of how the water-splitting center is organized. Barber's team worked with membrane protein crystallographer So Iwata of the ATP Systems Project, of ERATO, the Japan Science and Technology Corporation, to study the system with an unprecedented resolution. "Results by other groups, including those obtained using lower resolution X-ray crystallography at 3.7 angstroms have shown that the splitting of water occurs at a catalytic center that consists of four manganese atoms (Mn)," Iwata explains. "We've taken this further by showing that three of the manganese atoms, a calcium atom, and four oxygen atoms form a cube-like structure, which brings stability to the catalytic centre. The forth and most reactive manganese atom is attached to one of the oxygen atoms of the cube. Together this arrangement gives strong hints about the water-splitting chemistry."

Click image to magnify

The particular photosynthetic system studied, PSII, is the "engine of life", Barber adds, and finding out how it works has been a major challenge. The ability to use sunlight to extract hydrogen from the 326 million cubic miles of water on the planet as efficiently as cyanobacteria and green plants could address the world's desperate need for a new and environmentally friendly energy source.