Structural basis of noncanonical polyphenol oxidase activity in DLL-II: A lectin from Dolichos lablab.

Lectins known to possess an additional enzymatic function are called leczymes. Previous studies reported a unique polyphenol oxidase (PPO) activity in DLL-II-a leczyme from Dolichos lablab. DLL-II shares a high sequence and structural homology with DBL-another leczyme from Dolichos biflorus. Incidentally, DBL possesses lipoxygenase activity, but not the PPO activity. Legume lectins usually possess two metal-binding sites A and B. Although these sites are conserved in both DBL and DLL-II, site A in DLL-II is occupied by Mn and site B by Ca . In contrast, DLL-II binds Cu and Ca at sites A and B, respectively. Here, investigating the structural basis of PPO activity in DLL-II, we find that the PPO activity is only dependent on Cu , but not Ca ; and the lectin activity requires only Ca . Further, our analysis suggests that an alternative mechanism of PPO reaction may be operative in DLL-II, which involves a mononuclear Cu metal center; this is in contrast to the bi-nuclear Cu metal center commonly observed in all PPOs. Importantly, structural and computational approaches employed here, we hypothesize possible PPO binding sites and the corresponding migration channels for accessing the active site.