Diverging Mechanisms: Cytochrome-P450-Catalyzed Demethylation and γ-Lactone Formation in Bacterial Gibberellin Biosynthesis.

Biosynthesis of the gibberellin (GA) plant hormones evolved independently in plants and microbes, but the pathways proceed by similar transformations. The combined demethylation and γ-lactone ring forming transformation is of significant mechanistic interest, yet remains unclear. The relevant CYP112 from bacteria was probed by activity assays and 18 O2 -labeling experiments. Notably, the ability of tert-butyl hydroperoxide to drive this transformation indicates use of the ferryl-oxo (Compound I) from the CYP catalytic cycle for this reaction. Together with the confirmed loss of C20 as CO2 , this necessitates two catalytic cycles for carbon-carbon bond scission and γ-lactone formation. The ability of CYP112 to hydroxylate the δ-lactone form of GA15 , shown by the labeling studies, is consistent with the implied use of a further oxygenated heterocycle in the final conversion of GA24 into GA9 , with the partial labeling of GA9 , thus demonstrating that CYP112 partitions its reactants between two diverging mechanisms.