Crystal structure and enantioselectivity of terpene cyclization in SAM-dependent methyltransferase TleD

TleD is a SAM (S-adenosyl-L-methionine)-dependent methyltransferase and acts as one of the key enzymes in the teleocidin B biosynthesis pathway. Besides methyl transferring, TleD also rearranges the geranyl and indole moieties of the precursor to form a six-membered ring. Moreover, it does not show homologies with any known terpenoid cyclases. In order to elucidate how such a remarkable reaction could be achieved, we determined the complex crystal structures of TleD and the cofactor analogue S-adenosyl-L-homocysteine with or without the substrate teleocidin A1. A domain-swapped pattern via an additional N-terminal alpha-helix is observed in TleD hexamers. Structural comparison and alignment shows that this additional N-terminal alpha-helix is the common feature of SAM methyltransferase-like cyclases TleD and SpnF. The residue Tyr(21) anchors the additional N-terminal alpha-helix to a 'core SAM-MT fold' and is a key residue for catalytic activity. Molecular dynamics simulation results suggest that the dihedral angle C23-C24-C25C26 of teleocidin A1 is preferred to 60-90 degrees in the TleD and substrate complex structure, which tend to adopt a Re-face stereocenter at C25 position after reaction and is according to in vitro enzyme reaction experiments. Our results also demonstrate that methyl transfer can be a new chemical strategy for carbocation formation in the terpene cyclization, which is the key initial step.