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.