Further Insights on the Datura innoxia Hyoscyamine 6β-Hydroxylase (DiH6H) Based on Biochemical Characterization and Molecular Modeling

Hyoscyamine, anisodamine and scopolamine are tropane alkaloids present in some Solanaceae species and used in modern medicine. L-Hyoscyamine is hydroxylated to 6β-hydroxyhyoscyamine (anisodamine) and then epoxidated to scopolamine by the dual action of hyoscyamine 6β-hydroxylase (H6H), a 2-oxoglutarate dependent dioxygenase. A natural mutation in the Gly-220 residue to Cys was previously shown to be associated with the loss of function of H6H in Mandragora officinarum, preventing the accumulation of anisodamine and scopolamine in these plants. We show here that a deliberate Gly220Cys mutation in the Datura innoxia DiH6H protein caused a loss of both its enzymatic abilities and rendered it unable to hydroxylate L-hyoscyamine into anisodamine and to epoxidate anisodamine into scopolamine. By using protein modeling based on an available crystal structure of H6H from Datura metel, we show how the Cys220 residue causes a steric interference in the active site cavity impairing the interaction of both substrates, hyoscyamine and anisodamine with the active site of the protein. We also address the enantiomeric preference of DiH6H based on molecular modeling.