Copying plants’ ability to split water
By Robyn Williams on The Science Show
Photosystem 2 is the first stage of photosynthesis. It’s where light breaks water molecules, providing oxygen and starting many reactions. The secret is a metal centre composed of manganese and calcium. This is the catalyst that drives the reaction. Understanding the chemical pathway is a great challenge. If the process can be reproduced, it will provide hydrogen which is a low pollution fuel source.
Associate Professor of Biochemistry
University of Otago
Dunedin New Zealand
Robyn Williams: People all over the world are looking for alternative power sources. Professor Julian Eaton-Rye is a biochemist at the University of Otago in Dunedin in New Zealand and he’s interested in adapting photosynthesis to provide power, hydrogen power. He is working on Photosystem II.
Julian Eaton-Rye: Photosystem II is the start of photosynthesis, it's where light is initially absorbed and is used to break water molecules, and that provides the oxygen we breathe, and electrons from the water which are used to help change the carbon dioxide in the atmosphere into sugars. So it is the starting step.
Robyn Williams: And it normally happens at normal temperatures.
Julian Eaton-Rye: It does and I guess that's the trick of Photosystem II, that it can break water molecules down at ambient temperatures, whereas if you wanted to break water you would have to heat it to about 2,000°C in order to break it down, so you're not boiling the water, it's not steam you want, you want to actually decompose the water.
Robyn Williams: So if you weren't using an electric current, if you just wanted to break it down into hydrogen and oxygen, 2,000°C is a hell of a hot temperature, isn't it.
Julian Eaton-Rye: Yes, it is. I guess perhaps that's good for us because it means water is very stable and we need a lot of it and we've got a lot of it.
Robyn Williams: And we don't blow up.
Julian Eaton-Rye: Right, exactly, yes.
Robyn Williams: And so the secret is presumably an enzyme or more than one enzyme or what?
Julian Eaton-Rye: The secret is a metal centre composed of manganese and calcium, four manganese and one calcium, and they are put together in an architecture and a protein environment that holds them in a certain configuration, which simply allows that to act as the catalyst to drive this reaction. And one of the great challenges is to understand that architecture so that it can be mimicked so that we can make machines that will split water, because we talked about the electrons and the oxygen but of course you also have those protons, and those protons can become hydrogen, and hydrogen is a fuel.
Robyn Williams: And hydrogen would be great to have, but the great thing about enzymes is that when you've had the reaction, they are still there at the end, which is nice, isn't it.
Julian Eaton-Rye: It is nice. In the case of Photosystem II, there's a slight twist because this is a machine that works at the edge and there's quite a bit of energy involved from the light that is absorbed, and while that's splitting water it also causes damage to the actual protein architecture that is essential to hold everything in place to make it work. And what has happened in evolution is that the organisms that house the enzyme have developed a system to constantly repair the photosystem to sustain this reaction, and therefore Photosystem II offers an interesting model to look at how you might design a machine that repairs itself in a chemical way that would sustain this reaction in a synthetic operation.
Robyn Williams: So instead you've got a very delicate machine here which can easily break down. In other words, the enzyme can go bust, and you're trying to see whether there is a way in which you can harness all this and maintain a vigour...the enzyme or something else, so that you don't have the whole thing break down in about two seconds.
Julian Eaton-Rye: The enzyme doesn't go bust, and I think that's its success, is that this machinery that repairs itself is so efficient that the enzyme continues to work seamlessly, and therefore the mechanism involved is a very intriguing one, and if we were able to reproduce it in biomimetic systems then we would be able to have sustained water oxidation panels on our roof that lights shines on that continue to make us hydrogen and electrons as a fuel source.
Robyn Williams: You imply you haven't done it yet.
Julian Eaton-Rye: That's perhaps a little bit out there in the future, but the understanding of the chemistry to split water has received a big boost in just the last 12 months with an important X-ray crystal structure of the photosystem from a group in Japan, and there are many people around the world working on the manganese operation. Our own lab works on the protein environment that surrounds that manganese system, and the polypeptides that are necessary to continually turn over the photosystem. I think that the future is promising and the biochemistry is very interesting, and we hope that in the next five to ten years...perhaps just five actually, the real chemistry of the reaction will be understood, and to therefore reproduce it in a sustained way becomes a reasonable goal.
Robyn Williams: What would the payoff be then if you were successful?
Julian Eaton-Rye: The payoff is clearly that if you can actually make hydrogen from water, you would have solved many, many issues surrounding pollution and sustained energy production. There is no pollution from using water as a source of fuel. So in that sense it's a very wonderful opportunity that we might be able to pursue.
Robyn Williams: Why pursue that route instead of using solar panels and putting an electric current through water? Isn't that simpler?
Julian Eaton-Rye: I don't know if it's necessarily simpler. It is obviously done. The technology requires the use of a number of metals that are in limiting supply in nature, and so it's less clear that that is going to be, with the current technology, the single answer to our energy needs because there just aren't enough components out there to be mined to make enough of them for us to do that. So we need more than one answer.
Robyn Williams: So yours could be augmenting the other one.
Julian Eaton-Rye: Yes, and I think that there will be many modern technologies that we use to sustain our growing population. Another one in our lab that we're interested in is diverting the electrons that come from water. Normally they would be on their way to carbon dioxide to make sugars, but you can also divert the electrons into other compounds, and we've been working on putting them into butanol, and for that we use an organism called cyanobacteria...
Robyn Williams: Blue green algae.
Julian Eaton-Rye: The blue green algae. And they are then able to divert electrons to butanol. We can then simply shine light on these organisms and extract butanol. Butanol can go into petrol engines that we have now, you don't actually have to do anything to them, you can run cars on butanol. So it's a direct fuel substitute.
Robyn Williams: It's a form of alcohol, is it?
Julian Eaton-Rye: It is. That has also tremendous potential. You have to be able to grow the organisms and collect the butanol in an efficient way that makes it economical, so there are scale-up issues. But the proof of concept is there, it works, and it's simply a trick of bringing in some genes from an organism that makes butanol and putting them into the cyanobacteria. So it's molecular biology and that's not everybody's cup of tea, but in the laboratory it works.
Robyn Williams: Well, you're not going to be eating the bacteria, are you, the GM bugs?
Julian Eaton-Rye: No, not at all, they would be contained, they just grow them in large vats and then you would just collect the butanol. The non-photosynthetic version of that is used in other parts of the world, in South Africa for example they use butanol from an organism called Clostridium. But there you've got to feed the organisms some input. The advantage to having put that chemistry into cyanobacteria is that you just have to shine the sun.
Robyn Williams: The inexhaustible sun. Julian Eaton-Rye is an associate professor of biochemistry at the University of Otago in Dunedin, New Zealand.