Building Better Hydrogenases

A breakthrough in artificial enzymes research could help realize one long-standing goal of hydrogen researchers: synthesizing catalysts that can produce H₂ to power fuel cells, make liquid fuels and chemicals, and serve other applications. A newly made synthetic version works as well as the natural type.

When it comes to making hydrogen, hydrogenase enzymes are the champs. The catch is, only bacteria and algae that metabolize hydrogen know how to make the enzymes in exactly the right way. Scientists have tried to synthesize iron-based cluster complexes that mimic the catalytic site in hydrogenases. But the activity of the synthetic analogs acting on their own has fallen far short.

In an effort to understand those shortcomings, researchers led by Marc Fontecave of the University of Grenoble, in France, and Thomas Happe of Ruhr University, in Germany, tried to insert one of the synthetic analogs, a diiron dithiolate complex bearing carbon monoxide and cyanide ligands, into the natural hydrogenase. To their surprise, they learned that it can be incorporated into the enzyme with unexpected ease, and it skipped a significant chunk of the biosynthetic pathway.

In natural hydrogenase biosynthesis, three maturase enzymes are needed to “mature” the hydrogenase—that is, to build, transport, and attach the diiron complex to a preassembled iron-sulfur cluster on the hydrogenase skeleton. In work published in June, the researchers fed their synthetic diiron complex to one of the maturases and found that it took care of the job of transporting and installing the diiron complex in the active site all by itself—the other two maturases weren’t needed (Nature 2013, DOI: 10.1038/nature12239). The resulting hydrogenase is as active as natural hydrogenases.

With further study, the team has found that not even the single maturase scaffold protein is needed (Nat. Chem. Biol. 2013, DOI:10.1038/nchembio.1311). After adding the synthetic diiron cluster to the hydrogenase skeleton in a test tube, the diiron cluster was able to find its way unassisted to the right spot in the active site, hook itself up, and wiggle into the right shape ready for H₂ production, which starts up in just a few minutes.

“This is big news for the hydrogenase community,” says Marcetta Y. Darensbourg of Texas A&M University, whose group studies the synthetic analogs. “Hydrogenase enzymes were not expected to do this. It’s a great coup for both synthetic chemists and biologists.”

Cutting out the maturase middlemen works with both bacterial and algal hydrogenases, thus the researchers believe that their discovery of the spontaneous cluster assembly could be a general approach to producing artificial enzymes for H₂ production.

“This report opens the door to a number of new research directions,” write Souvik Roy and Anne K. Jones of Arizona State University in a commentary in Nature Chemical Biology. “It’s exciting to think that a model might prove to have better properties than the native cluster.”