Tetrahydrocannabinolic acid (THCA) synthase (full name Δ1-tetrahydrocannabinolic acid synthase) is an enzyme responsible for catalyzing the formation of THCA from cannabigerolic acid (CBGA). THCA is the direct precursor of tetrahydrocannabinol (THC), the principal psychoactive component of cannabis, which is produced from various strains of Cannabis sativa. Therefore, THCA synthase is considered to be a key enzyme controlling cannabis psychoactivity.[1] Polymorphisms of THCA synthase result in varying levels of THC in Cannabis plants, resulting in "drug-type" and "fiber-type"C. sativa varieties.[2][3]
The FAD moiety is the location of enzymatic activity and is covalently bound to His114 and Cys176. FAD is also bound by hydrogen bonds with neighboring amino acid main chains and side chains. The co-crystallization of THCA synthase with substrate or product has not been accomplished yet.[1]
FAD cofactor (in green) is located between domain I and domain II of THCA synthase. Alpha helices are cyan and beta sheets are magenta.
FAD (in green) is covalently bound to histidine 114 and cysteine 176 (in pink).
Reaction mechanism
Chemical structure of cannabigerolic acid (CBGA), the substrate for THCA synthase.
A hydride is transferred from CBGA to reduce FAD, concerted by deprotonation of a hydroxyl group by a tyrosine residue. The monoterpene moiety in CBGA is then positioned to complete cyclization into THCA. Oxidation of reduced FAD by O2 produces hydrogen peroxide (H2O2).[6][1]
Reaction mechanism of THCA synthase. Modified from Shoyama et al. J. Mol. Bio. 2012.
Biological function
THCA synthase is expressed in the glandular trichomes of Cannabis sativa. THCA synthase may contribute to the self-defense of Cannabis plants by producing THCA and hydrogen peroxide, which are both cytotoxic. Because these products are toxic to the plant, THCA synthase is secreted into the trichome storage cavity.[7] THCA also acts as a necrosis-inducing factor by opening mitochondrial permeability transition pores, inhibiting mitochondrial viability and resulting in senescence in leaf tissues.[8]
Non-enzymatic decarboxylation of THCA during storage or smoking forms THC, the principal psychoactive component of cannabis.[9] Further degradation by temperature, auto-oxidation, and light forms cannabinol.[10] THC and other cannabinoids are well-known to reduce nausea and vomiting and stimulate hunger, particularly in patients undergoing cancer chemotherapy.[11]
Similar enzymes to THCA synthase catalyze the formation of other cannabinoids. For example, cannabidiolic acid (CBDA) synthase is a flavoprotein that catalyzes a similar oxidative cyclization of CPGA into CBDA, the dominant cannabinoid constituent of fiber-type C. sativa. CBDA undergoes a similar decarboxylation to form cannabidiol.[12]
Significance
Demand is high for pharmaceutical grade THC and other cannabinoids due to interest in their potential therapeutic use, but is stymied by legal regulations of C. sativa cultivation in many countries.[13] Direct chemical synthesis of THC is difficult due to high costs and low yields.[14] Therefore, the use of THCA synthase for the production of THC has been explored, as CBGA is easy to synthesize and THCA readily decarboxylates to form THC.[10] Biosynthesis of THCA by expressing THCA synthase in organisms has been attempted in bacteria, insects, and tobacco plants with limited success. Production of THCA on a milligram scale has been demonstrated in Pichia pastoris yeast cells in two independent studies.[15][13]
References
^ abcdeShoyama Y, Tamada T, Kurihara K, Takeuchi A, Taura F, Arai S, Blaber M, Shoyama Y, Morimoto S, Kuroki R (October 2012). "Structure and function of ∆1-tetrahydrocannabinolic acid (THCA) synthase, the enzyme controlling the psychoactivity of Cannabis sativa". Journal of Molecular Biology. 423 (1): 96–105. doi:10.1016/j.jmb.2012.06.030. PMID22766313.
^Staginnus C, Zörntlein S, de Meijer E (July 2014). "A PCR marker linked to a THCA synthase polymorphism is a reliable tool to discriminate potentially THC-rich plants of Cannabis sativa L". Journal of Forensic Sciences. 59 (4): 919–26. doi:10.1111/1556-4029.12448. PMID24579739. S2CID29683549.
^Kojoma M, Seki H, Yoshida S, Muranaka T (June 2006). "DNA polymorphisms in the tetrahydrocannabinolic acid (THCA) synthase gene in "drug-type" and "fiber-type" Cannabis sativa L". Forensic Science International. 159 (2–3): 132–40. doi:10.1016/j.forsciint.2005.07.005. PMID16143478. S2CID38866142.
^Taura F, Morimoto S, Shoyama Y, Mechoulam R (1995). "First direct evidence for the mechanism of Δ1-tetrahydrocannabinolic acid biosynthesis". J. Am. Chem. Soc. 117 (38): 9766–9767. doi:10.1021/ja00143a024.
^Lange K, Schmid A, Julsing MK (October 2015). "Δ(9)-Tetrahydrocannabinolic acid synthase production in Pichia pastoris enables chemical synthesis of cannabinoids". Journal of Biotechnology. 211: 68–76. doi:10.1016/j.jbiotec.2015.06.425. PMID26197418.