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The road to green hydrogen runs through mazes in algal proteins

Hydrogen fuel cells produce electricity, we are increasingly thinking about hydrogen as the successor of crude oil. But where will the hydrogen come from? Its ecologically cleanest source could be industrial - or even domestic! - bioreactors with green algae. Their future construction will be possible thanks to an international team of researchers, who have for the first time precisely described the chemical reactions responsible for the stability of hydrogen generation in an aerobic environment by algal enzymes.

Hydrogen, that is, the fuel of the ever-nearer future. Its ecologically cleanest source is water that can be photosynthetically split by algae operating in suitable conditions. However, there is a serious obstacle standing in the way of constructing efficient and at the same time truly green (figuratively and literally) bioreactors: hydrogenases, the enzymes that are directly responsible for the production of hydrogen, are inactivated in the presence of oxygen. Would it be possible to make them resistant to the effect of the atmosphere? Until now this was not possible, because the mechanism of their degradation by oxygen was not sufficiently well understood. Only now have these complex processes been fully described for the first time and published in the journal Nature Chemistry in an article prepared by a team of scientists from Poland, France, Great Britain, Spain and the United States.

Since the late 1990's it has been known that when there is no sulfur in the algae's environment, instead of oxygen they start to produce... hydrogen. What's more, it was quickly realized that in natural conditions algal enzymes are one of the best hydrogen catalysts! Unfortunately, hydrogenases have a serious drawback: when exposed to oxygen they undergo degeneration and stop working. Meanwhile, providing them with an anaerobic environment in industrial or domestic conditions would be very difficult.

The authors of the publication in Nature Chemistry investigated reactions occurring in the iron-iron (Fe-Fe) hydrogenases. The researchers were particularly interested in identifying the migration paths of the oxygen molecules from the surroundings to the active site within the protein structure and understanding the course of the reactions they were involved in there.

The research team proposed and verified the full catalytic cycle associated with the reactions of oxygen in the active site of the hydrogenases. Two pathways along which oxygen molecules penetrated into the site were identified in the structure of the proteins, wherein one of them proved to be frequented particularly often. The availability of electrons in the protective mechanism was also found to play a key role.

In order to confirm the correctness of the new catalytic model the researchers proposed making some small modifications to the structure of the tested hydrogenases. The substitution of one of the small amino acids with a bulky one on the path of transport of oxygen was expected to hinder the migration of gas molecules to the active site. Careful experiments with appropriately mutated hydrogenase confirmed these assumptions and validated the model.