Similar enzymes, different smells

Petunia flowers and basil leaves use similar enzymes to give the plants their fragrance, say biologists in the US. The enzymes also give spices, such as cloves and cinnamon, their spice.

Enzymes from petunia petals and basil-leaf glands both react with coniferyl acetate to make eugenol (in basil) and isoeugenol (in petunias), according to Eran Pichersky, from the University of Michigan. Pichersky identified the enzymes, their genes and the reaction pathway they trigger. 

The eugenols are phenylpropenes, many of which have antimicrobial properties and attractive aromas and fragrances. ’These compounds have been used for millennia by humans, so finally finding out how they are made is satisfying,’ Pichersky told Chemistry World.

The biosynthesis of eugenol and isoeugenol was always a puzzle, said Pichersky. ’The envisioned reduction of a primary alcohol (coniferyl alcohol) to an alkene would have been highly unusual and not likely to occur on several grounds. We showed that indeed coniferyl alcohol is not directly converted to eugenol or isoeugenol, and that instead the reduction occurs on an acyl ester intermediate, whose existence had not been previously known,’ he explained. 

Pichersky also found that many plants have genes encoding proteins similar to basil eugenol synthase and petunia isoeugenol synthase.

Pichersky’s group had already predicted that eugenol synthase and isoeugenol synthase were similar enzymes. ’This is how evolution works - by small changes in protein sequence, you get small changes in substrate specificity or product specificity,’ Pichersky said. 

His team has just worked out the last stage in the enzymatic pathway - the synthesis of the coniferyl ester that is used as a substrate for eugenol synthase or isoeugenol synthase. But the finer details of the enzymatic mechanism still need to be unravelled, he said.

The practical possibilities that this information leads to include: genetically engineered plants to make spices; predator-resistant plants; and engineered floral fragrances, both for florists and for farmers who want to improve pollination of their crops, said Pichersky. Katharine Sanderson