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 Mn²⁺ and site B by Ca²⁺ . In contrast, DLL-II binds Cu²⁺ and Ca²⁺ 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 Cu²⁺ , but not Ca²⁺ ; and the lectin activity requires only Ca²⁺ . Further, our analysis suggests that an alternative mechanism of PPO reaction may be operative in DLL-II, which involves a mononuclear Cu²⁺ metal center; this is in contrast to the bi-nuclear Cu²⁺ 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.