Synthetic biologists have engineered brewer's yeast to produce marijuana's main ingredients--mind-altering THC and non-psychoactive CBD--as well as novel cannabinoids not found in the plant itself.
Feeding only on sugar, the yeast are an easy and cheap way to produce pure cannabinoids that today are costly to extract from the buds of the marijuana plant, Cannabis sativa.
"For the consumer, the benefits are high-quality, low-cost CBD and THC: you get exactly what you want from yeast," said the author. "It is a safer, more environmentally friendly way to produce cannabinoids."
Cannabis and its extracts, including the high-inducing THC, or tetrahydrocannabinol, are now legal in 10 states and the District of Columbia, and recreational marijuana--smoked, vaped or consumed as edibles--is a multibillion-dollar business nationwide. Medications containing THC have been approved by the Food and Drug Administration to reduce nausea after chemotherapy and to improve appetite in AIDS patients.
CBD, or cannabidiol, is used increasingly in cosmetics--so-called cosmeceuticals--and has been approved as a treatment for childhood epileptic seizures. It is being investigated as a therapy for numerous conditions, including anxiety, Parkinson's disease and chronic pain.
But medical research on the more than 100 other chemicals in marijuana has been difficult, because the chemicals occur in tiny quantities, making them hard to extract from the plant. Inexpensive, purer sources--like yeast--could make such studies easier.
Cannabinoids join many other chemicals and drugs now being produced in yeast, including human growth hormone, insulin, blood clotting factors and recently, but not yet on the market, morphine and other opiates.
Researchers started assembling in yeast a series of chemical steps to produce, initially, the mother of all cannabinoids, CBGA (cannabigerolic acid). In both marijuana and yeast, the chemical reactions involve the acid form of the compounds: CBGA and its derivatives, THCA and CBDA. They readily convert to CBG, THC and CBD when exposed to light and heat.
Turning yeast into chemical factories involves co-opting their metabolism so that, instead of turning sugar into alcohol, for example, yeast convert sugar into other chemicals that are then modified by added enzymes to produce a new product, such as THC, that the yeast secrete into the liquid surrounding them. The researchers ended up inserting more than a dozen genes into yeast, many of them copies of genes used by the marijuana plant to synthesize cannabinoids.
One step, however, proved to be a roadblock for the group and competing groups: an enzyme that performs a key chemical step in making CBGA in the marijuana plant didn't work in yeast.
Rather than engineer a different synthetic pathway, researchers isolated a second enzyme, prenyl transferase, that does the same thing, and stuck it in the yeast. "It worked like gangbusters," the author said.
Once they had yeast-producing CBGA, they added another enzyme to convert CBGA to THCA and a different enzyme to create a pathway to CBDA. Though the products the yeast produce are predominantly THC or CBD, each must still be separated from other chemicals present in tiny quantities.
They also added enzymes that made the yeast produce two other natural cannabinoids, CBDV (cannabidivarin) and THCV (tetrahydrocannabivarin), whose effects are not well understood.
Surprisingly the authors discovered that the enzymatic steps involved in making CBGA in yeast are flexible enough to accept a variety of starter chemicals--different fatty acids in place of the one used by the marijuana plant, hexanoic acid--that generate cannabinoids that do not exist in the plant itself. They also got the yeast to incorporate chemicals into cannabinoids that could later be chemically altered in the lab, creating another avenue for producing never-before-seen, but potentially medically useful, cannabinoids.
Complete biosynthesis of cannabinoids in yeast
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