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ScienceWise - Nov/Dec 2008

The ultimate green energy

Article Illustration
Warwick Hillier in one of the Photobioenergetic labs with an Infrared Spectrometer used to study the molecular structure of light activated proteins.
Article Illustration
Structure of Photosystem II, PDB 2AXT (image by Curtis Neveu)

Harnessing the potential of one of nature’s super-enzymes to create limitless hydrogen fuel

We live in an energy-challenged world. Our demand for energy is rocketing and so to is the environmental cost of our main current source of energy - fossil fuels. Indeed, carbon emissions are now destabilising global climate and changing ocean chemistry. The stakes are high but the solution might be as close as that tree beyond your window. That's because trees, like all plants, possess molecular machinery that efficiently harvests and stores the energy of the Sun. And, according to scientists at the ANU School of Biology, this molecular machinery may hold the key to creating new and innovative energy supplies that will meet our future needs.

Prof Tom Wydrzynski and Dr Warwick Hillier lead the Photobioenergetics Group. Their research aims to understand the molecular mechanisms of photosynthesis, specifically the enzyme system called Photosystem II (or PSII). In so doing they believe it may be possible to develop a clean energy technology that will provide vast quantities of energy into the future without damaging the Earth's environment. That's a big hope but PSII is an amazing molecular process. It's the only enzyme in Nature that can use the energy from the Sun to oxidize water into molecular O2, hydrogen ions and electrons. The enzyme is present in all higher plants, many algae and cyanobacteria, and its action over time has transformed the global environment.

"Photosystem II is incredibly important to life as we know it," explains Warwick Hillier. "It literally created the free oxygen in our atmosphere that has made higher life possible.

"It's believed that PSII evolved about 2.5 billion years ago in cyanobacteria. It's a photosynthetic reaction centre that absorbs solar radiation giving it sufficient oxidising potential to split water and release oxygen. The cyanobacteria evolved into eukaryotic algae like red and brown seaweeds, and then land plants, which appeared around approximately 500 million years ago. That was when the oxygen levels in the atmosphere really began to increase."

So Photosystem II has literally transformed planet Earth by changing its atmosphere. Now scientists are hopeful it might transform human energy systems by providing clean energy technologies. Photosystem II is an oxygen factory which takes in water and photons (solar radiation) at one end and transforms them into free oxygen, hydrogen ions and electrons. If this reaction could be coupled to a hydrogen ion reducing reaction to generate hydrogen gas (H2) then a perfect, unlimited and non-polluting fuel cycle could be created.

But this isn't a big factory. It operates at the nanoscale in the thylakoid membrane of chloroplasts inside plant cells, and unravelling its molecular complexity has proved enormously challenging.

"The Photobioenergetics Group is interested in the molecular mechanism by which light energy gets stored in this PSII enzyme," says Dr Hillier. "The reaction centres in PSII take one photon to generate an oxidised and a reduced species. But to split water you need the equivalent of four oxidised units so the system needs to act as a capacitor that can store the charge. This is a totally unique system in biology"

"At the heart of PSII is an oxygen evolving complex based on a cluster of 4 manganese ions and a calcium ion. We're trying to work out how these ions interact with water molecules but the PSII system is a very complex structure consisting of a massive protein made up of some 25 different polypeptides."

"In order to work out how PSII functions we've developed an artificial system based on a much simpler protein found in bacteria called bacterioferritin. We can manipulate this protein so it has much of the functionality of PSII but without its complexity. Then, by working with bacterioferritin, we hope to be able to reverse engineer PSII."

And, in so doing, the researchers believe that it's possible they could develop a biocatalyst for the solar decomposition of water, and a cheap and environmentally friendly way to generate hydrogen gas for fuel.

"Hydrogen is the simplest fuel you could make," says Dr Hillier. "To generate it you need to split water which can be done thermally using very high temperatures (which is costly) or electrolytically using electrodes. To do that now you need to use platinum electrodes as a catalyst - but there isn't enough platinum on the planet to scale this process up to a level that will meet our energy needs.

"But it looks like Nature may have solved this problem billions of years ago with photosynthesis. It developed enzymes that use metals that are relatively abundant in the ocean; elements like iron, manganese and cobalt. Photosystem II uses a manganese complex to split water, and the efficiency of this reaction is better than our best platinum catalysts.
"Our work with bacterioferritin therefore may lead to the production of biocatalyst with high efficiency and which uses a common metal, and this could drive a whole new generation of energy technology."

And what might this technology look like? The scientists say the simplest system would be a protein that acts as a catalyst that would split water. It would consist of an anode and a cathode. The anode would be an analogue of Photosystem II, that would split water into oxygen, and the cathode would make the hydrogen (the hydrongenase function). You could then either store the hydrogen as a fuel for use elsewhere or you could feed it into a fuel cell and generate electricity.

While there's still much work needed to crack the complexity of Photosystem II, this form of energy generation offers us real hope that future energy needs might be met without generating more greenhouse emissions.

"More energy in the form of sunlight hits the Earth's surface in one hour than humans consume in one year. says Dr Hillier. "I think harvesting this solar energy by using biocatalysts based on PSII is the one form of energy generation that has the potential to be scaled up to meet our future needs."

 

 

 

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