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The crystal structure of an enzyme that helps give the herb basil its particular zing, could lead to new insights into how this, and other plants, produce fragrant natural products. It might even help plant breeders develop stronger-smelling crops that attract pollinating insects or ward off pests more effectively. Joseph Noel of the Salk Institute for Biological Studies and the University of Michigan have determined the structure of eugenol synthase, frozen in mid-action as it makes its natural product, eugenol. This molecule is the key aromatic component of fresh basil leaves that gives them their characteristic warm and sweet aroma. Eugenol synthase catalyses the reductive displacement of acetate from the propenyl side chain of the substrate coniferyl acetate to produce allyl-phenylpropene eugenol, explain the researchers, an example of the phenylpropenes, a large group of plant volatile compounds with various roles in defense and pollinator attraction, containing a propenyl side chain. "This particular enzyme is very interesting since it belongs to a large family of enzymes that perform what we call household reactions but, through evolutionary selection, acquired an additional and completely new function," says Noel, "Eugenol Synthase takes a basic building block that is usually employed to make wood and turns it into something that is almost the complete opposite of wood - a volatile molecule that easily becomes airborne, is highly aromatic and possesses antimicrobial and pain-dulling properties," Noel adds. Plant genetics has recently uncovered a lot of genes with important functions but understanding the protein products of these genes at the molecular level is the ultimate aim of the research. Previously, Noel and University of Michigan plant biochemistry professor Eran Pichersky, had investigated the enzymes that synthesize eugenol and its chemical cousin isoeugenol from the woody polymer lignin. Now, with the help of a novel substrate molecule designed and synthesised by team member Thomas Baiga, the researchers have caught eugenol synthase in the act, mid-reaction as the enzyme locks on to its substrate and its vitamin co-factor is brought into play. It turns out that eugenol synthase is rather unusual in that it acts on its substrate in two distinct steps. "A subtle change in the substrate binding site after natural selection during the course of plant evolution added the capability to form a reactive intermediate through a concerted push-pull mechanism, which facilitates the second chemical step, a classic reduction reaction carried out by many evolutionary relatives of eugenol synthase," explains team member Gordon Louie. This is not the end for basil though, the researchers are now comparing eugenol synthases and other closely related synthases to try and determine the evolutionary roots of these enzymes. Related links:
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Noel and colleagues, figuring out what makes basil's zing
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