Living cells are masters of compartmentalization. Whether it’s stuffing DNA inside the nucleus or cramming energy-generating machinery into the rod-shape organelles called mitochondria, biology separates tasks into specialized compartments within cells to make it easier to do specific jobs without interference.
Researchers in Indiana are following that same strategy to isolate enzymes inside hollow virusmade nanoparticles in order to make hydrogen for fuel. The process does not yet produce enough hydrogen to compete with commercial alternatives. But the researchers, whose results appear today in Nature Chemistry, already have their sights set on improvements aimed at generating large amounts of fuel cheaply.
Many different bacteria generate molecular hydrogen (H2) as a fuel source for their own metabolism. They do so using enzymes called hydrogenases, proteins that catalyze the combination of protons (H+) and electrons (e-) into molecules of H2. Hydrogenases can also run in reverse, converting H2 into protons and electrons that can be used to power cell metabolism.
Researchers have long looked for simple systems to combine protons and electrons to form H2, a carbon-free fuel that can be burned directly to power cars or run through a fuel cell to generate electricity. Commercial systems that make molecular hydrogen typically require high temperatures, expensive catalysts, or both, limiting their market.
Hydrogenases instead work at ambient temperatures with simple, cheap starting materials. But researchers have faced two main problems in using them. One is that the enzymes work by pairing up to create the active form that generates hydrogen. Yet when hydrogenases are removed from bacteria and placed in solution, they drift around and rarely find one another, and thus remain inactive. Hydrogenases are also easily poisoned by the presence of oxygen, which is abundant outside the protective environs of the cells where they normally work.
In hopes of addressing both issues, researchers led by Trevor Douglas, a chemist at Indiana University, Bloomington, took a page from biology’s use of compartments. They started with P22 bacteriophages, a type of virus that infects salmonella bacteria. These viruses coax the bacteria they infect to produce tiny hollow spheres, made from 420 copies of an outer “coat” protein. The viruses also induce the bacteria to produce a second set of proteins that helps guide the assembly of P22 spheres, from the coat proteins, on the outer surface of the cell membrane.
Douglas and his colleagues kept coat proteins largely intact. But they altered the DNA for making the guide proteins, clipping off the portion that wedges into the cell membrane. In its place they inserted a linker designed to grab on to a hydrogenase enzyme.
When they put it all together and infected salmonella bacteria with the remade viruses, the new guide proteins helped assemble the spheres and inserted roughly 100 copies of the hydrogenase into the interior of each one.
The tight packing of the hydrogenases caused them to pair up into the catalytically active complex, and the closely packed coat proteins prevented oxygen from diffusing in. The researchers then spiked their solution with protons and electron-ferrying molecules, which were able to diffuse into the spheres, where the hydrogenase pairs readily converted them into H2. In fact, as the team reports, the setup increased the efficiency of H2 production 100-fold over previous attempts with hydrogenases.
“It’s exciting, pioneering work,” says Seung-Wuk Lee, a bioengineer at the University of California, Berkeley, who was not involved with the project. Today the cheapest way to make H2 is in a chemical plant that breaks down natural gas. And the new process can’t beat that yet. But Douglas says he and his colleagues already have ideas for increasing the efficiency of their process. He also is beginning to work on more complex designs that would corral multiple types of enzymes inside his capsules. That could allow the group to convert chemical starting materials into more complex fuels, such as methane or methanol, that can be used to power vehicles with conventional engines.