# Biotechnological Fungal Platforms for the Production of Biosynthetic Cannabinoids
## 2 Chemical Structure of Cannabinoids, Types, and Metabolic Routes for Biosynthesis  
Phytocannabinoids are bioactive natural meroterpenoids with a resorcinol core bearing a para-positioned isoprenyl, alkyl, or aalkyl side chain [14]. The side chain usually contains an odd number of carbon atoms (cannabinoids containing an even number of carbon atoms are rare). Phytocannabinoids are found in angiosperms, fungi, and liverworts and are produced in several plant organs, such as the flower and glandular trichome of _C. sativa_, the scales of _Rhododendon_, and the oil bodies of _Radula_ species. Moreover, it has been found that the mammalian brain has receptors that respond to compounds found in _C. sativa_. These cannabinoid receptors form the endocannabinoid system and regulate several biological functions and metabolic processes. Therefore, these bioactive compounds can be beneficial for the treatment of pain, anxiety, and cachexia in humans [15; 16]. The different cannabinoids produced by _C. sativa_[17] are classified into structural families [18; 19], such as cannabidiols (CBDs), cannabigerols (CBGs), cannabicyclols (CBLs), cannabinoids (CBNDs), cannabitriols (CBNs), cannabichromenes (CBCs), \(\Delta^{9}\)-THC, and miscellaneous cannabinoids [20]. The acid forms are the final product of the cannabinoid biosynthetic pathways, although several spontaneous modifications such as oxidation, decarboxylation, and cyclization frequently take place because of the poor oxidative stability of phytocannabinoids, in particular \(\Delta^{9}\)-THC [14]. Phytocannabinoids are accumulated in glandular thricomes all over the aerial parts of the plant, especially in the female flowers [21; 22]. The phytocannabinoid biosynthetic pathway is split between different cellular compartments and organelles: the cytosol of gland cells (polyketide pathway), the plastids (methylerythnitol 4-phosphate (MEP) pathway for prenylation), and the extracellular storage cavity (oxidocyclization and storage) (Figure 1). It is still unclear how intermediates and precursors are transported between compartments, although it is most likely that vesicle trafficking and transport proteins are involved in the movements of these intermediates across the interface between the gland cells and the storage cavity. Inside the cytosol, the biosynthesis of cannabinoids involves the integration of several steps in polyketide and isoprenoid metabolism. C18 fatty acids are desaturated, preoxygenated, and cleaved into hexanoic acid, which is transformed into thioester hexanoyl-CoA in a reaction catalyzed by acyl-activated enzyme 1 (AAE1). Later, the hexanoyl-CoA is elongated with malonyl-CoA in a reaction catalyzed by olivetol synthase (OLS) and cyclized by olivetolic acid cyclase (OAC) to produce olivetolic acid (OA) [23; 24; 25]. Inside plastids, geranyl pyrophosphate (GPP) is synthesized through the MEP pathway. OA is prenylated using GPP by cannabigerolic acid synthase (CBGAS), thereby forming cannabigerolic acid (CBGA), the first cannabinoid. CBGA is an essential cannabinoid because it is the precursor of several other cannabinoids. Then, the CBGA is converted into \(\Delta^{9}\)-tetrahydrocannabinolic acid (\(\Delta^{9}\)-THCA) and cannabidiolic acid (CBDA) in the apoplastic space by the action of two enzymes, \(\Delta^{9}\)-THCA synthase (THCAS) and CBDA synthase (CBDAS), respectively. This conversion causes the reduction of O\({}_{2}\) into hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) via oxidative cyclization reactions. Another important enzyme, cannabichromenic acid synthase (CBCAS), participates in the synthesis of cannabichromenic acid (CBCA) from CBGA using FAD and O\({}_{2}\). THCAS, CBGAS, and CBDAS are also flavoproteins, which are strictly dependent on the presence of O\({}_{2}\) as an electron acceptor. All these oxidocylases carry a secretion signal peptide and are exported to the extracellular resin space. THCAS and CBDAS are active in the resin space, but it is still unknown whether their activity is exclusive to the extracellular space [23; 26; 27].