Terpenoids can act as components in molecular signaling pathways, e.g., as toxins, fragrances, or hormones, in larvae of the horseradish leaf beetle Phaedon cochleariae. The basic unit of all terpenes is a simple molecule containing five carbon atoms that can be joined to chains of different length. Enzymes involved in chain elongation belong to the group of isoprenyl diphosphate synthases.

Scientists at the Max Planck Institute for Chemical Ecology, Jena, and the Leibniz Institute for Plant Biochemistry, Halle, both Germany, studied how chain elongation is regulated.

Experiments with larvae in which the enzyme encoding gene was silenced showed that the protein was involved in the formation of the C10 monoterpene chrysomelidial. It is produced to defend the larvae against enemies such as ants.
In vitro analysis of the protein, including measurements of product formation in the presence of different metal ions as co-factors, revealed that geranyl diphosphate (C10), a precursor for the defensive substance chrysomelidial, was produced after addition of cobalt and manganese ions. Adding magnesium ions resulted in the formation of farnesyl diphosphate (C15). It is a potential precursor for juvenile hormones, which is 5 carbon atoms longer. All three metals were found in larval tissue, leading to the assumption that enzyme catalysis is directed by the concentration of the different metal co-factors in the larvae.
How the different metal ions modify the product range of the enzyme is still unclear. It is very likely that the varying atomic radii of the metal ions effect changes in the spatial structure of the enzyme, which prevent or allow the admission of a third C5 unit.

The directing influence of metal ions on the product formation of isoprenyl diphosphate synthases is a novel “control element” in the regulation of the terpene metabolism.

^{© Sindy Frick, MPI for Chemical Ecology}

• Metal ions control product specificity of isoprenyl diphosphate synthases in the insect terpenoid pathway,
  Sindy Frick, Raimund Nagel, Axel Schmidt, René R. Bodemann, Peter Rahfeld, Gerhard Pauls, Wolfgang Brandt Jonathan Gershenzon, Wilhelm Boland, Antje Burse,
  Proc.Nat. Acad. Sci. USA 2013, Early Edition, February 25.
  DOI: 10.1073/pnas.1221489110