It is likely that additional enzymes are required to isomerize the major AmISY product so that it can be diverted into the antirrhinoside biosynthetic pathway. For example, the trans—trans iridodial could be converted into the more stable cis—cis iridodial by an epimerase that abstracts the labile C7a-H proton next to the C1-carbonyl (Fig. 1). Alternatively, iridoid synthase could utilize a helper protein to control the stereochemistry of cyclization. This question is not unique to the antirrhinoside pathway. In Nepeta species, a variety of nepetalactones with varying stereochemistry at the C4a and C7a carbons are observed. A species of Nepeta mussinii that exclusively produces the trans–cis iridoid isomer as a final product has an iridoid synthase that predominantly produces the cis–trans isomer. This mismatch also strongly suggests that additional enzymes are required to set the stereochemistry at the iridoid bridgehead carbons in Nepeta.5