Classification of polyphenol oxidases shows ancient gene duplication leading to two distinct enzyme types.

Polyphenol oxidases (PPOs) are coupled binuclear copper proteins that catalyze the oxidation of phenols. New functions of PPOs are continuously being discovered, latest with several fungal o-methoxy phenolases, which are active on lignin-derived compounds. Here, we perform a comprehensive phylogenetic analysis of PPOs from a wide taxonomic origin and define 12 PPO groups. We find that a deep gene duplication has led to two distinct PPO types. Type 1 includes PPOs from chordates and molluscs, as well as the fungal o-methoxy phenolases. Type 2 includes plant PPOs, molluscan hemocyanins, and fungal tyrosinases. Most of the type 2 proteins have a C-terminal shielding domain and a thioether bond in the copper-binding site. We also find that most ascomycetes contain high numbers of the PPO type 1 that includes the o-methoxy phenolases, which may indicate a role in the lignin conversion strategy of these fungi.