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Scientists Solve Long-Standing Plant Biochemistry Mystery

Single amino acid exerts “remote control” over double bond placement by desaturase enzymes

UPTON, NY — Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and collaborators at the Karolinska Institute in Sweden have discovered how an enzyme “knows” where to insert a double bond when desaturating plant fatty acids. Understanding the mechanism — which relies on a single amino acid far from the enzyme’s active site — solves a 40-year mystery of how these enzymes exert such location-specific control. The work, published in the Proceedings of the National Academy of Sciences the week of September 19, 2011, may lead to new ways to engineer plant oils as a renewable replacement for petrochemicals.

“Plant fatty acids are an approximately $150-billion-dollar-a-year market,” said Brookhaven biochemist John Shanklin, lead author on the paper. “Their properties, and therefore their potential uses and values, are determined by the position of double bonds in the hydrocarbon chains that make up their backbones. Thus the ability to control double bond positions would enable us to make new designer fatty acids that would be useful as industrial raw materials.”

The enzymes responsible for double-bond placement, called desaturases, remove hydrogen atoms and insert double bonds between adjacent carbon atoms at specific locations on the hydrocarbon chains. But how one enzyme knows to insert the double bond at one location while a different but closely related enzyme inserts a double bond at a different site has been a mystery.

“Most enzymes recognize features in the molecules they act on that are very close to the site where the enzyme’s action takes place. But all the carbon-hydrogen groups that make up fatty-acid backbones are very similar with no distinguishing features — it’s like a greasy rope with nothing to hold onto,” said Shanklin.

In describing his group’s long-standing quest to solve the desaturation puzzle, Shanklin quotes Nobel laureate Konrad Bloch, who observed more than 40 years ago that such site-specific removal of hydrogen “would seem to approach the limits of the discriminatory power of enzymes.”