psilocybin

Psilocybin is a psychoactive tryptamine alkaloid naturally produced in Psilocybe mushrooms. Once ingested, it functions as a prodrug that is rapidly converted into the active compound psilocin, which structurally resembles serotonin and acts as an agonist on 5-HT2A serotonin receptors. In clinical and therapeutic applications, it exhibits efficacy in treating severe mental disorders, including major depressive disorder, post-traumatic stress disorder, and treatment-resistant depression [339].

In nature, the biosynthetic pathway of psilocybin converts L-tryptophan to psilocybin via four consecutive enzymes: L-tryptophan decarboxylase (PsiD), cytochrome P450 monooxygenase (PsiH), kinase (PsiK), and N-methyltransferase (PsiM). Reconstructing this natural pathway in heterologous hosts presents a significant bottleneck due to the poor activity and complex requirements of the eukaryotic membrane-bound PsiH monooxygenase [339].

To address these challenges, metabolic engineering strategies employ artificial biosynthetic routes that bypass the PsiH-catalyzed step. One strategy involves initiating the pathway with a bacterial tryptophan 4-hydroxylase to produce 4-hydroxytryptophan (4-HTP), followed by decarboxylation using a substrate-specific dopa decarboxylase to yield 4-hydroxytryptamine (4-HT), which is subsequently phosphorylated by PsiK and methylated by PsiM. Another bioconversion strategy utilizes 4-hydroxyindole and L-serine as precursors, converting them to 4-HTP via a tryptophan synthase, which is then converted to psilocybin through PsiD, PsiK, and PsiM. To optimize de novo production from simple carbon sources, engineering approaches focus on deregulating precursor amino acid pathways, overexpressing rate-limiting enzymes of the shikimate pathway, enhancing intracellular S-adenosyl-L-methionine (SAM) availability, and optimizing fermentation parameters such as temperature and promoter induction [339].