The structure of SpnF, a standalone enzyme that catalyzes [4+2] cycloaddition

In the biosynthetic pathway of the spinosyn insecticides, the tailoring enzyme SpnF performs a [4 + 2] cycloaddition on a 22-membered macrolactone to forge an embedded cyclohexene ring. To learn more about this reaction, which could potentially proceed through a Diels-Alder mechanism, we determined the 1.50-angstrom-resolution crystal structure of SpnF bound to S-adenosylhomocysteine. This sets the stage for advanced experimental and computational studies to determine the precise mechanism of SpnF-mediated cyclization.