UCLA and Dartmouth scientists have identified a crucial enzyme in plant vitamin C synthesis, which could lead to enhanced crops. The discovery now makes clear the entire 10-step process by which plants convert glucose into vitamin C, an important antioxidant in nature.
"If we can find ways to enhance the activity of this enzyme, it may be possible to engineer plants to make more vitamin C and produce better crops," said Steven Clarke, UCLA professor of chemistry and biochemistry, director of UCLA's Molecular Biology Institute and co-author of the research study, to be published as a 'Paper of the Week' in the Journal of Biological Chemistry and currently available online.
"We hit on gold," Clarke said, "because we now have a chance to improve human nutrition and to increase the resistance of plants to oxidative stress. Plants may grow better with more vitamin C, especially with more ozone in the atmosphere due to pollution."
Carole Linster, a UCLA postdoctoral fellow in chemistry and biochemistry and lead author of the study, discovered the controlling enzyme, GDP-L-galactose phosphorylase, which serves as the biosynthetic pathway by which plants manufacture vitamin C.
"Our finding leads to attractive approaches for increasing the vitamin C content in plants," Linster said. "We now have two strategies to provide enhanced protection against oxidative damage: Stimulate the endogenous activity of the identified enzyme or engineer transgenic plants which overexpress the gene that encodes the enzyme."
When life on Earth began, there was almost no oxygen, Clarke noted.
"Two billion years ago, plants devised an efficient way to get sunlight to make sugar from carbon dioxide that produced oxygen as a waste product; that waste product probably killed off most of all living species at that time," Clarke said. "The only organisms that survived developed defenses against it, and one of the best defenses is vitamin C. Plants learned how to make vitamin C to protect themselves."
Prior to the new research, vitamin C may have been the most important small molecule whose biosynthetic pathway remained a mystery.
An essential vitamin for humans, vitamin C is also an important antioxidant for animals and plants. Humans do not have the ability to make vitamin C and get it from dietary sources, especially from plants. It was not until 1998 that a biosynthetic pathway was proposed to explain how plants make this compound. Research confirmed much of the pathway, although one crucial missing link continued to baffle scientists and remained unknown until this new research.
Clarke, who studies the biochemistry of aging, said the finding is an example of serendipity in science.
The research started as an effort to understand the role of a gene in Caenorhabditis elegans, a tiny worm used as a model for aging studies by Tara Gomez, a former UCLA undergraduate in Clarke's laboratory and now a graduate student at the California Institute of Technology. The gene's sequence suggested that it was related to a family of genes altered in cancer, known as HIT genes; these genes are studied in the laboratory of Charles Brenner at the Norris Cotton Cancer Center at Dartmouth Medical School.
Collaboration between Clarke's and Brenner's laboratories revealed a similarity between the worm gene and the product of the VTC2 gene of Arabidopsis thaliana, a small roadside plant. Mutations in this gene had been previously linked to low levels of vitamin C. Linster and Gomez were able to express and to purify the plant VTC2 enzyme from bacteria. The research team, led by Linster, produced the GDP-L-galactose substrate and reconstituted in test tubes the mysterious seventh step in vitamin C synthesis.
Clarke and Brenner liken the first six steps in vitamin C synthesis to a roadmap in which there are multiple possible routes from glucose to a variety of cellular compounds. Once the GDP-L-galactose compound takes the exit marked "VTC2," however, the atoms are reconfigured to make vitamin C. The remaining three steps, like a curving driveway, "require some turns but no real choices and no backing up," Brenner said.
The researchers are still studying what VTC2-related genes do in animals and how these genes may relate to aging and cancer.
The scientific team included UCLA researcher Lital Adler; Princeton undergraduate and former UCLA research assistant Brian D. Young; and Dartmouth researcher Kathryn Christensen.