Paper accepted: "Catechol Oxidase versus Tyrosinase Classification Revisited by Site-Directed Mutagenesis Studies"

Today, Dr. Sarah Prexler’s manuscript, co-authored by her Master student Martin Frassek and her supervisors Prof. Bruno Moerschbacher and Dr. Mareike Dirks-Hofmeister, on mutational studies concerning the monophenolase activity of dandelion polyphenol oxidases was finally accepted for publication in the highly reputed journal “Angewandte Chemie”! The paper describes a thorough study on why plant PPOs - unlike fungal and human tyrosinases to which they are very closely related - tend to be inactive on monophenolic substrates, only oxidising diphenolics. Plant PPOs, also called catechol oxidases, are responsible for browning of cut fruits such as apple or banana, a reaction that forms part of the wound closure and immune system of plants. In contrast, human and animal tyrosinases are responsible for melanin production and are, thus, involved in skin and hair pigmentation. Two alternative hypotheses trying to explain the difference between catechol oxidases and tyrosinases had been based on crystal structure analyses of the enzymes, mostly without being backed by functional studies. Sarah’s and Martin’s systematic functional analysis of different dandelion PPOs surprisingly showed that some of them (“group 1” PPOs) are actually tyrosinases rather than catechol oxidases (“group 2” PPOs), and this appears to hold true for other plant species, too. This in itself already proved both of the current hypotheses wrong, and that conclusion was further supported by site directed mutagenesis studies followed by detailed kinetic analyses of both enzymes. While our study disproved the current hypotheses, it allowed suggesting new ones, to be tested in future. The king is dead, long live the king!