Simple experiment opens new route to sulfur chemistry

It is remarkably easy to enzymatically oxidize thiols, the sulfurous equivalent of alcohols. That is what biochemists from the University of Groningen and Wageningen University discovered, independently of each other. They published their findings together in Angewandte Chemie, one of the world’s leading chemistry journals. The discovery may point to a way to reduce bad smells.

‘We were assessing the activity of a new oxidizing enzyme we recently described , HMFO’, explains University of Groningen biochemistry professor Marco Fraaije. HMFO can oxidize alcohol groups in a range of compounds. ‘Now, a thiol group is in some respects similar to an alcohol, with a sulfur atom replacing the oxygen. So we decided to test whether HMFO could oxidize thiols.’

Willem Dijkman, a PhD student in the Fraaije group, did the experiments. ‘It worked right away’, he says. ‘When we added the enzyme to a potential substrate with a thiol group, we immediately saw oxygen consumption. So the enzyme oxidized the substrate. It was a very simple experiment. The hard part was proving what sort of reaction had occurred.’

Meanwhile, Fraaije and Dijkman’s colleagues at Wageningen University were on the same trail, but in their case by accident. They added a thiol compound to an alcohol-oxidizing enzyme in order to protect the enzyme from oxidative stress. The enzyme is yellow when resting, but red when active. The Wageningen group noticed that the solution had changed colour, and soon discovered that the enzyme had converted the thiol compound.

‘We cooperate a lot with the Wageningen group, so we heard of each other’s results’, says Fraaije. ‘So we decided to join forces and publish together.’ The paper, which appeared in Angewandte Chemie on 3 Octobober, describes the enzymatic oxidation of thiols.

What is special about an enzyme that can oxidize a thiol group? Fraaije: ‘The first thing is that nobody knew enzymes could perform this reaction. We tested three other oxidizing enzymes alongside HMFO, and they all performed the same trick.’ It also confirms that enzymes are not as specific as we once thought. ‘These oxidases don’t discriminate between alcohols and thiols.’

The third special thing is that thiols are notorious, as they are often part of nasty-smelling molecules. ‘The foul-smelling fluid skunks use to repel attackers is based on thiols’, says Fraaije. On the other hand, the fragrant aroma of coffee is also caused by thiols. So, whether for reducing stench or modulating aromas, a thiol-oxidizing enzyme might prove useful.

Fraaije: ‘We have to wait and see how the biochemistry community responds and what ideas they generate. I know that a similar finding with an amine-oxidizing enzyme sparked new chemistry and the creation of a company that now employs dozens of chemists. So who knows!’

Reference: The Oxidation of Thiols by Flavoprotein Oxidases: a Biocatalytic Route to Reactive Thiocarbonyls. Tom A. Ewing, Willem P. Dijkman, Prof. Dr. Jacques M. Vervoort, Marco W. Fraaije and Prof. Dr. Willem J. H. van Berkel. Angewandte Chemie DOI: 10.1002/ange.201407520