Phytocannabinoids: Origins and Biosynthesis

Abstract

Phytocannabinoids are bioactive natural products found in some flowering plants, liverworts, and fungi that can be beneficial for the treatment of human ailments such as pain, anxiety, and cachexia. Targeted biosynthesis of cannabinoids with desirable properties requires identification of the underlying genes and their expression in a suitable heterologous host. We provide an overview of the structural classification of phytocannabinoids based on their decorated resorcinol core and the bioactivities of naturally occurring cannabinoids, and we review current knowledge of phytocannabinoid biosynthesis in Cannabis, Rhododendron, and Radula species. We also highlight the potential in planta roles of phytocannabinoids and the opportunity for synthetic biology approaches based on combinatorial biochemistry and protein engineering to produce cannabinoid derivatives with improved properties. Phytocannabinoids The term phytocannabinoid (see Glossary, also cannabinoid) defines meroterpenoids with a resorcinyl core typically decorated with a para-positioned isoprenyl, alkyl, or aralkyl side chain [1]. The alkyl side chain typically contains an odd number of carbon atoms, where orcinoids contain one carbon, varinoids three, and olivetoids five. Cannabinoids with an even number of carbon atoms in the side chain are known but rare. The term cannabinoid generally refers to molecules with a characteristic chemical structure; however, the term may also refer to pharmacological ligands of human endocannabinoid receptors [2]. In this review we use the chemical definition of cannabinoids. Phytocannabinoids occur in flowering plants, liverworts, and fungi (Figure 1, Key Figure). They were first isolated from Cannabis sativa L. (Cannabaceae), a plant with a long and controversial history of use and abuse [3]. The mammalian brain has receptors that respond to compounds found in C. sativa. Accordingly, these receptors were named cannabinoid receptors (CB x) and are the basis of the endocannabinoid system. Studies in human and animals demonstrated that the endocannabinoid system regulates a broad range of biological functions, including memory, mood, brain reward systems, and drug addiction, as well as metabolic processes such as lipolysis, glucose metabolism, and energy balance [2]. More than 113 different cannabinoids have been isolated from C. sativa and these are classified into distinct types: cannabigerols (CBGs), cannabichromenes (CBCs), cannabidiols (CBDs), (−)-Δ 9-trans-tetrahydrocannabinols (Δ 9-THCs), (−)-Δ 8-trans-tetrahydrocannabinols (Δ 8-THCs), cannabicyclols (CBLs), cannabielsoins (CBEs), cannabinols (CBNs), cannabinodiols (CBNDs), cannabitriols (CBTs), and the miscellaneous cannabinoids (Figure 2) [4]. C. sativa predominantly produces alkyl type cannabinoids that carry a monoterpene isoprenyl moiety (C10) and a pentyl side chain (C5) [1]. The most abundant constituents are trans-Δ 9-THC, CBD, CBC, and CBG, together with their respective acid forms (Δ 9-THCA, CBDA, CBCA, and CBGA) [5]. Cannabinoid biosynthetic pathways ty...

Origin and evolution of the cannabinoid oxidocyclase gene family

“…In accordance with this domain structure, THCAS and CBDAS have been found to be catalytically active in the extracellular storage cavity of the glandular trichome and rely on covalently bound FAD and O2 for their activity (Sirikantaramas et al 2005;Rodziewicz et al 2019). CBCAS is less extensively studied, but considering its high sequence similarity with THCAS, probably shares these biochemical activities (Morimoto et al 1997;Gülck and Møller 2020). However, the latest phylogenetic classification of plant BBE-like genes was based on Arabidopsis sequences only (Brassicaceae) (Daniel et al 2016) and consequently lacks genes from Cannabis and related genera.…”

“…The two most abundant and well-known cannabinoids are ∆ 9 -tetrahydrocannabinol (THC) and cannabidiol (CBD), but more than 120 others have been identified in Cannabis (ElSohly et al 2017). Some other plant genera such as Rhododendron and Radula have also been found to make cannabinoids (Iijima et al 2017;Gülck and Møller 2020). THC is responsible for the psychoactive effect of Cannabis through its partial agonist activity at endocannabinoid receptors (Gaoni and Mechoulam 1964;Mechoulam and Parker 2013).…”

The Pharmacological Case for Cannabigerol

“…Cannabinoid synthesis begins with the precursor molecules olivetolic acid and geranylpyrophosphate, which combine to form cannabigerolic acid (CBGA) (Shoyama et al, 1975;Fellermeier and Zenk, 1998;Fellermeier et al, 2001;Gülck and Møller, 2020). CBGA serves as the precursor to most other cannabinoids and is converted to Δ9-THCA (tetrahydrocannabinolic acid), CBDA (cannabidiolic acid), and CBCA (cannabichromenic acid) ( Figure 1).…”

Metabolic Engineering Strategies of Industrial Hemp (Cannabis sativa L.): A Brief Review of the Advances and Challenges

“…The quantity of CBGA will also be increased by simultaneous silencing of THCAS and CBDAS. Besides, silencing of both THCAS and CBDAS will expedite the production of minor cannabinoid, cannabichromenic acid (CBCA) which is difficult and expensive for pharmacological studies in clinical trials because of low abundance (Gülck and Møller, 2020).…”

“…In cannabinoid synthesis, toxicity effects must be considered, as several cannabinoid pathway metabolites such as CBGA and THCA cause cell death via apoptosis in host plant (Sirikantaramas et al, 2005). In hemp, olivetolic acid synthesized in cytosol is transferred to plastid, where olivetolic acid and geranyl-PP are converted into CBGA, which is finally released to apoplast (Gülck and Møller, 2020). It will be critical to elucidate the mechanism underlying transport and accumulation of metabolites and apply it to better hemp phytochemical production in other plant species commercially ( Table 1).…”