Structures of Iridoid Synthase from Cantharanthus roseus with Bound NAD⁺, NADPH, or NAD⁺/10-Oxogeranial: Reaction Mechanisms

Structures of the iridoid synthase nepetalactol synthase in the presence of NAD⁺, NADPH or NAD⁺/10-oxogeranial were solved. The 10-oxogeranial substrate binds in a transoid-O1-C3 conformation and can be reduced by hydride addition to form the byproduct S-10-oxo-citronellal. Tyr178 Oζ is positioned 2.5 Å from the substrate O1 and provides the second proton required for reaction. Nepetalactol product formation requires rotation about C1-C2 to form the cisoid isomer, leading to formation of the cis-enolate, together with rotation about C4-C5, which enables cyclization and lactol production. The structure is similar to that of progesterone-5β-reductase, with almost identical positioning of NADP, Lys146(147), Tyr178(179), and F342(343), but only Tyr178 and Phe342 appear to be essential for activity. The transoid 10-oxogeranial structure also serves as a model for β-face hydride attack in progesterone 5β-reductases and is of general interest in the context of asymmetric synthesis.