Type-3 copper proteins: recent advances on polyphenol oxidases.

Recent investigations in the study of plant, fungal, and bacterial type-3 copper proteins are reviewed. Focus is given to three enzymes: catechol oxidases (CO), tyrosinases, and aureusidin synthase. CO were mostly found in plants, however, in 2010 the first fungal CO was published. The first plant-originated tyrosinase was published in 2014, before tyrosinases were only reported in fungi, bacteria, and human. Aureusidin synthase from yellow snapdragon (Antirrhinum majus) was first published in 2000, as part of yellow flower coloration pathway. In the last years, many important results on type-3 copper enzymes originated from X-ray crystallographic investigations. In addition, studies on site-directed mutagenesis of amino acids around the active site were performed to identify the regions determining monophenolase and/or diphenolase activity. Although X-ray crystallographic structures of CO and tyrosinases are available, many questions like the response for the activation via proteases, sequence-based or structural-based differences between CO, as well as the physiological roles of many polyphenol oxidases still remain to be addressed.