Martin Frassek successfully defended his MSc thesis

Today, Martin Frassek successfully defended his MSc thesis on the structural basis of monophenolase versus diphenolase activities of plant polyphenol oxidases, supported by Bruno Moerschbacher and the second referee, Prof. Joachim Jose from the Faculty of Chemistry and Pharmacy. Martin was part of our small PPO group externally guided by our former doctoral student Mareike Dirks-Hofmeister and supervised in the lab by Sarah Prexler. Sarah had already started to investigate the role of single amino acid residues close to the active site of dandelion PPOs on their ability to oxidize monophenolic substrates, an unusual trait in plant PPOs. We had been shocked earlier by a surprise publication in Angewandte Chemie from the group of Prof. Heinz Decker at the University of Mainz, a former PhD committee member of Sarah, who claimed that a single asparagine residue was responsible for this activity. We were immediately convinced that this hypothesis was by far too simple, and Martin helped us to experimentally verify our suspicion. He generated an impressive range of mutants of two dandelion PPOs, one with and one without monophenolase activity, which allowed him to falsify both this and also an alternative explanation recently put forward in the literature. With the support of Dr. Michael Liss from the company Thermo Fisher, he designed a site saturation mutagenesis library, permutating 21 amino acids in the vicinity of the active site of one of these PPOs. He developed an assay to screen for monophenolase and diphenolase activities and tentatively identified a few candidate residues involved in substrate specificity. We will now investigate these in more detail - without the help of Martin who opted for further studies in Computational Science at the University of Amsterdam.