Directed evolution of hydrogenase for efficient light-driven hydrogen production via quantum dot-enzyme hybrid systems.
The ultimate sustainable energy source is the sun, Stefan Lutz points out, adding that plants have learned how to use it to create fuel – storable chemical energy. They do this through a process called photosynthesis. But plants are smarter than humans. Or, as Lutz puts it: “If you look at Nature’s photosynthetic machinery, it’s humongous, it’s complex. And for any kind of application that we have in mind, playing around with the photosynthetic machinery is just too complex.” We humans need to keep things simple. “What type of storage system do we really like?” Lutz asks, somewhat rhetorically. “Hydrogen gas is the one we really like.” That’s because when it comes to simple, hydrogen is about the simplest bit of matter you can find – the hydrogen atom is one proton and one electron. Real simple. What’s more, Lutz points out, Nature has already developed perfectly workable schemes to produce hydrogen gas. One way Nature does this is with hydrogenases, iron-sulfur containing proteins, or enzymes. Basically, these enzymes take protons – in any atom, the proton is the stable particle with the positive charge that offsets the much smaller, negatively charged electrons—and converts them into molecular hydrogen gas. And so, in hydrogenases, we have a perfectly fine catalyst for the production of hydrogen. The question now becomes, how can we get the energy that’s required to catalyze this reaction into the enzyme? Nature uses photosynthesis – sorry, too complicated. Not our style. “But it turns out that quantum dots can do the same trick that photosynthesis does,” Lutz says. Quantum dots, also known as nanocrystals, are a special class of semiconductor. They range in size from 2-10 nanometers in diameter. A nanometer is one billionth of a meter – vanishingly small, and a quantum dot is so small that the addition or removal of an electron changes its properties in some useful way. “Quantum dots can capture photons,” Lutz says. Photons are tiny packets of electromagnetic energy. By “capturing” photons, Lutz means basically that once an electron absorbs a photon the electron grows more energy rich, “and at that point it could actually leave the quantum dot if there’s something there to accept the electron. And in our case that would be the hydrogenase. So, basically as soon as that electron gets excited and ready to leave, the hydrogenase is right there to pick it up.” In theory anyway, with the help of this energy boost from the exited electron, the resulting enzymatic reaction generates molecular hydrogen gas. And, if you remember the Hindenburg, that gigantic exploding zeppelin, you know that hydrogen gas burns when mixed with oxygen and a tiny spark. That’s called combustion, and that’s exactly what we want our fuels – chemically stored energy – to do; although, unlike the Hindenburg disaster, we want our fuels to burn in ways that we can control. But Lutz isn’t worried about that part yet; his Scialog grant is all about producing the hydrogen in the first place. “The trick is that we want to physically attach this hydrogenase onto the surface of the quantum dot, so that it’s right there at the right place at the right time when that photon strikes,” he says. The other part of his project involves modifying the enzyme, a product of millions of years of evolution, so that it works well in an artificial environment. “What we’re really trying to is make sure that the enzyme and the quantum dot form effective pair this new environment,” Lutz says. An enzyme has a limited lifetime inside its natural environment,the cell – “as short as a couple of minutes, maybe sometimes an hour, depending on the nature of the protein and the environment,” Lutz says. While other chemists and materials scientists have already tried to make enzymes thrive on artificial substrates, with much tinkering over the artificial materialsbut not much ultimate success, Lutz points out: “Nobody has really looked at the enzyme itself and its ability to evolve, to adjust to a new working environment. I see that as a key towards making this successful, because with materials you can only tinker around so much. But by the power of evolution, which has certainly created some amazing stuff out there, and with some of the protein-engineering techniques available nowadays in the laboratory, we can really speed up that evolutionary process, thereby giving the enzyme the opportunity to adjust to the solid support, rather than the other way around. In any artificial environment Lutz and his team will be able to create, however, over time some of the protein will fall apart, despite their best efforts at forced evolution. “To be honest, I don’t know how long it’s going to last, but we’re predicting that it should certainly last on the order of days, maybe even weeks, because there’s nothing there that would actively degrade it, like proteases, which you find in nature.” Obviously, he’s going to try to keep his experiments really clean to avoid contaminating the enzyme. What would Lutz’s energy-producing process look like, assuming it’s 20 years in the future and the experiments have worked beyond his wildest dreams? Would it look like some kind of science-fiction device with a glowing, pulsing, energy-rich liquid streaming through some kind of elaborate system of clear tubes? Lutz laughs at the suggestion. “I wouldn’t take it that far,” he says. “But if we were to succeed in every aspect of this project, the idea would be to have these nanoparticles, the quantum dot hydrogenase hybrids, floating in a reaction media. Basically you could shine light on it and out would bubble hydrogen gas.” At this point in the development of renewable energy technology, even that sounds pretty much like wild science fiction. “There are certainly a lot of technical issues that are currently unsolved,” Lutz admits. “But at same time, I think that was part of idea of Scialog—we need to give people the opportunity to start exploring these out-there ideas, ideas that are high-risk, but that could potentially produce some real benefit.”