clean energy production
Visualization of the active site (colored sticks) that is impaired by oxygen molecules within the core of the Fe-Fe hydrogenase with SOMO orbitals (green/pink colored). Image credit: CIC nanoGUNE

Next Step Taken in Using Enzymes for Clean Energy Production

The search for alternative energy sources has been ongoing since the energy crises in the 1970’s. When concerns regarding greenhouse effects surfaced in the 90’s, the search was intensified. As hydrogen’s combustion only produces water, is has been one of the most popular candidates. One field that is exceptionally poised for providing methods for using hydrogen as a source of clean energy is Biotechnology.

Hydrogenases are enzymes that naturally occur in various microorganisms that live in anaerobic ecosystems such as unicellular algae, the intestinal tract of animals and some bacteria living in soil. Using hydrogenases as a source for hydrogen is one possibility that is being explored, as they are able to produce and split hydrogen.

Biotechnological devices containing fuel cells can utilize combustion energy released when hydrogenases catalyze the conversion of protons in hydrogen molecules. The active site where this reaction takes place contains metallic ions (Iron or both iron and nickel). The most active variety of hydrogenases is the one containing only iron. The H-cluster of hydrogenases is buried within the core of a large protein and this is where the complex activities take place.

When hydrogenases are brought to the aerobic conditions of a bioreactor under normal oxygen pressures, the oxygen degrades their active site. Thus far, this has been the downfall of research done on exploiting hydrogenases for biotechnological applications. The mechanism of the degradation process of the H-cluster are not yet fully understood, and without this knowledge, designing a hydrogen-based fuel cell is not possible.

International teams of researchers has joined forces and are sharing theoretical calculations, experiments and molecular simulations to try to solve this problem. The team includes an Ikerbasque Research Fellow from CIC nanoGUNE. Researchers from UCL (UK) and CNRS (France) are taking the lead on the study.

As a first step, the rates of the different reaction steps involved in the degradation of the enzyme by oxygen have been measured by means of electrochemical methods. The dependency of these rates on experimental parameters such as pH, H2O/D2O exchange, the electrode potential and mutation of specific amino acids in the protein, are being studied.

To clarify the reaction products and appraise the rate constants for the individual reaction steps, density functional theory has been applied. NanoGUNE’s Ikerbasque Fellow David de Sancho has conducted molecular dynamics simulations that show the tunnels oxygen follows to reach the active site of the protein. The results of these simulations could be used to identify potential spots for blocking the access tunnels.

The study has allowed researchers to characterize the complex reactions that occur in these large biological macromolecules. An innovative combination of experimental and computational approaches was used to achieve the results.

De Sancho feels that the study has opened new opportunities to exploit enzymes from living systems efficiently for clean energy production, but admits that major challenges need to be overcome before industrial applications will become viable.

The full study was published in the journal Nature Chemistry.