Characterization of CyrI, the hydroxylase involved in the last step of cylindrospermopsin biosynthesis: Binding studies, site-directed mutagenesis and stereoselectivity.

Cylindrospermopsin, a cytotoxin from cyanobacteria, is biosynthesized by a complex pathway, which involves CyrI, an iron and 2-oxoglutarate dependent hydroxylase that transforms 7-deoxy-cylindrospermopsin into cylindrospermopsin and its epimer, 7-epi-cylindrospermopsin, in the last step. The activity of CyrI from Oscillatoria sp. PCC 7926 depends on Fe(II) (Km  = 2.1 μM), and 2-oxoglutarate (Km  = 3.2 μM), and is strongly inhibited by 7-deoxy-cylindrospermopsin at concentration higher than 1 μM. Using tryptophan fluorescence, we measured the binding to CyrI of Fe(II) (KD  = 0.02 μM) and 2-oxoglutarate (KD  = 53 μM and KD  = 1.1 μM in the absence or presence of 10 μM Fe(II), respectively). The Oscillatoria sp. PCC 6506 CyrI mutants H157A, D159A, H247A, and R257A were all inactive, and impaired in the binding of Fe(II) or 2-oxoglutarate, confirming the identity of the iron ligands and the role of R257 in the binding of 2-oxoglutarate. We constructed several chimeric enzymes using the Oscillatoria sp. PCC 7926 CyrI protein (stereoselective) and that from Oscillatoria sp. PCC 6506 (not stereoselective) to help understanding the structural factors that influence the stereoselectivity of the hydroxylation. Our data suggest that a predicted α-helix in CyrI (positions 87-108) seems to modulate the stereoselectivity of the reaction.