Comparative analysis of polyphenol oxidase from plant and fungal species

Polyphenol oxidase from plants and fungi is a metalloenzyme containing a type-3 copper center and is homologous to oxygen-carrying hemocyanin of molluscs. Molluscan hemocyanin consists of two domains, an N-terminal domain containing the copper center and a smaller C-terminal domain, connected by an alpha-helical linker. It is presumed that the same is true of polyphenol oxidase from plants and fungi although the structure of a polyphenol oxidase containing the C-terminal domain has not been determined. We show that a number of important structural features are conserved in the N-terminal domains of polyphenol oxidases from various plants and fungi, including a tyrosine motif which can be considered a landmark indicating the beginning of the linker region connecting the N- and C-terminal domains. Our sequence alignments and secondary structure predictions indicate that the C-terminal domains of polyphenol oxidases are likely to be similar in tertiary structure to that of hemocyanin. Detailed bioinformatics analyses of the linker regions predict that this section of the polypeptide chain is intrinsically disordered (lacking fixed tertiary structure) and contains a site of proteolytic processing as well as a potential phosphorylation site.