Considerations on activity determinants of fungal polyphenol oxidases based on mutational and structural studies

Polyphenol oxidases (PPOs) are an industrially relevant family of enzymes, being involved in the post-harvest browning of fruits and vegetables, as well as in human melanogenesis. Their involvement lies in their ability to oxidize phenolic or polyphenolic compounds, that subsequently form pigments. PPO family includes tyrosinases and catechol oxidases, which in spite of their high structural similarity, exhibit different catalytic activities. Long-standing research efforts have not yet managed to decipher the structural determinants responsible for this differentiation, as every new theory is disproved by a more recent study. In the present work, we combined biochemical along with structural data, in order to rationalize the function of a previously characterized PPO from Thermothelomyces thermophila (TtPPO). The crystal structure of a TtPPO variant, determined at 1.55 [A] resolution, represents the second known structure of an ascomycete PPO. Kinetic data of structure-guided mutants prove the implication of "gate" residue L306, residue HB1+1 (G292) and HB2+1 (Y296) in TtPPO function against various substrates. Our findings demonstrate the role of L306 in the accommodation of bulky substrates and that residue HB1+1 is unlikely to determine monophenolase activity as suggested from previous studies. IMPORTANCEPPOs are enzymes of biotechnological interest. They have been extensively studied both biochemically and structurally, with a special focus on the plant-derived counterparts. Even so, explicit description of the molecular determinants of their substrate specificity is still pending. Especially for ascomycete PPOs, only one crystal structure has been determined so far, thus limiting our knowledge on this tree branch of the family. In the present study, we report the second crystal structure of an ascomycete PPO. Combined with site-directed mutagenesis and biochemical studies, we depict the amino acids in the vicinity of the active site that affect enzyme activity, and perform a detailed analysis on a variety of substrates. Our findings improve current understanding of structure-function relations of microbial PPOs, which is a prerequisite for the engineering of biocatalysts of desired properties.