Formate dehydrogenase is used in biocatalytic processes to make electrons available as fuel for further reactions by removing them from a substrate. In current applications, the substrate is NAD+ and the product is NADH. The waste product is CO2. "This is a good thing in this case, as it escapes as a gas and therefore prevents the desired reaction from taking place in reverse," explains Dirk Tischler. As the NADPH variant – with one more phosphate group – is often required as a product, his team created mutants of the biocatalyst that converts NADP+ into the desired product.
What if the catalyst also breaks the C-N bond?
While working on the formate dehydrogenase, the researchers realized that little was known about alternative starting materials for the biocatalyst. "We looked at various possible substrates (formate derivatives) and found that a C-O bond is always broken when they are converted by the biocatalyst," explains Dirk Tischler. "This gave us an idea: What if the enzyme could also cleave C-N bonds – traditionally a difficult task to solve?"
Further tests showed that the biocatalyst is indeed capable of this: It can also convert formamides, derivatives of the formate with an additional compound to form a nitrogen, as a starting material. Since formamides are common and inexpensive solvents, they serve as both solvent and substrate in this reaction. It's also possible to provide NADH and NADPH in this constellation. CO2 is produced as a waste product in these reactions, too, and, by escaping, prevents the reactions from running backwards.
"This reaction has never been described before," says Dirk Tischler. His team showed that using formamides as an electron source for NADPH formation can achieve equivalent or even slightly better results compared to the conventional system with formate. "This opens up completely new possibilities, as our more stable mutants are still active in up to 40 percent by volume of formamides," says the researcher.
