Max Planck scientists in Jena, Germany, have discovered an unusual regulation of enzymes that catalyze chain elongation in an important secondary metabolism, the terpenoid pathway. In the horseradish leaf beetle Phaedon cochleariae a single enzyme can trigger the production of two completely different substances depending on whether it is regulated by cobalt, manganese or magnesium ions: iridoids, which are defensive substances the larvae use to repel predators, or juvenile hormones, which control insect's development. Insects unlike plants do not have a large arsenal of the proteins called isoprenyl diphosphate synthases. Therefore they may have developed another efficient option to channel metabolites into the different directions of terpenoid metabolism by using metal ions for control. (PNAS, Early Edition, February 25, 2013, DOI:10.1073/pnas.1221489110)
Natural products: 40,000 terpenes
Apart from the primary metabolism which produces substances that ensure the survival of the cells, there are additional biosynthetic pathways in all organisms. Their products may be less important for a single cell, but they can nevertheless be essential for the whole organism. These pathways are summarized as secondary metabolism. One of them is the terpenoid pathway: with more than 40,000 different known structures it generates one of the largest classes of natural products. Terpenoid molecules have diverse functions and can act as components in molecular signaling pathways, as toxins, fragrances or hormones.
The basic unit of all terpenes is a simple molecule containing five carbon atoms that can be joined to chains of different length. There are monoterpenes (C10 units, 2 x C5), sesquiterpenes (C15, 3 x C5), and even polymers, such as natural rubber, which comprises several hundred C5 units. Special enzymes mediate chain elongation. These enzymes have attracted the curiosity of scientists at the Max Planck Institute for Chemical Ecology, Jena, and the Leibniz Institute for Plant Biochemistry in Halle. They studied mechanistic alternatives of how chain elongation is regulated.
Metal ions instead of specialized enzymes
Enzymes involved in chain elongation belong to the group of isoprenyl diphosphate synthases. Such an enzyme was isolated from larvae of the horseradish leaf beetle Phaedon cochleariae. It raised the interest of Antje Burse, project group leader in the Department of Bioorganic Chemistry at the Max Planck Institute for Chemical Ecology.
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 that larvae produce to defend themselves against predators. The larvae accumulate this monoterpene in special glands and release it as a defensive secretion when they are attacked by their enemies, such as ants.