L-Tryptophan decarboxylase

The enzyme commission number for L-tryptophan decarboxylase is EC 4.1.1.105.[2] Other common names include psilocybin biosynthesis decarboxylase and psiD.[4] The first digit in the enzyme number is representative of the class of enzymes known as lyases, which catalyze elimination reactions.[2][4] The second and third digits are representative of the subclass of lyases known as decarboxylases that cleave carbon-carbon bonds.[2][4] The last digit is representative of the enzyme’s specific substrate, L-tryptophan.[2]

This enzyme is a part of the PLP-independent phosphatidylserine decarboxylase family and most compared to hypothetical proteins of other basidiomycetes fungi.[5] These include Fibulorhizoctonia sp with 60% identical amino acids and Moniliophthora roreri with 52% identical amino acids.[5] A similar enzyme that is not related to L-tryptophan decarboxylase is called aromatic-L-amino-acid decarboxylase with an enzyme number of EC 4.1.1.28.[2]

The first step in the reaction is the substrate binding of L-tryptophan, which reacts with a coenzyme hydrogen.[2] The decarboxylase enzyme is able to transform L-tryptophan to tryptamine in the second step by cleaving off two oxygens and a carbon to form tryptamine and carbon dioxide as the products.[2] Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, which makes the molecule polar, while tryptamines have an indole ring structure, a fused double ring consisting of a pyrrole ring, and a benzene ring, which is joined to an amino group by two carbon side chains.[7]

This is the chemical reaction that takes place:

H⁺ + C₁₁H₁₂N₂O₂ = CO₂ + C₁₀H₁₃N₂

L-Tryptophan decarboxylase has been characterized in bacteria, plants, and fungi.[2] Fungi that produce psilocybin and psilocin express incredible diversity, as they are a part of at least eight genera with hundreds of species belonging to them.[3] The specific reaction pathway for L-tryptophan decarboxylase has been described in twelve species, including Psilocybe cubensis, commonly known as magic mushrooms.[2] All fungi in the genus Psilocybe have a well-defined, umbrella-like cap with gills underneath and a stipe.[3] Other main characteristics of Psilocybe species include purple-brown reproductive spores, the presence of an annulus, and blue bruising with contact.[3] All Psilocybe species are described to feed on microscopic detritus and are found on a variety of surfaces, such as herbivore dung, grasses, roots, wood, and soil.[3] Humans have a documented history of ingesting this psilocybin producing fungi.[3] There are 57 species that are found in Mexico, and out of these there have been reports of 35 species and nine varieties being used by ethnic groups.[3]

In Psilocybe cubensis, L-tryptophan decarboxylase has been described with two other enzymes to biosynthesize psilocybin in a one pot reaction.[4][5] These other two enzymes in this process are psiK, an enzyme that catalyzes the phosphotransfer step, and psiM, an enzyme that catalyzes the iterative N-methyl transfer step.[4] The biosynthesis of psilocybin takes place as follows:

L-Tryptophan is decarboxylated to 4-hydroxytryptamine by psiD → psiK phosphorylates 4-hydroxytryptamine to create norbaeocystin → psiM then processively N,N-dimethylates the compound to yield psilocybin.[4]

Serotonin 2A receptors (5-HT2ARs) stimulation by the active metabolite, psilocin, disrupts serotonergic neurotransmission and produces the characteristic psychedelic effects of this species of fungus.[6] The product formed by L-tryptophan decarboxylase, tryptamine, is relevant to humans because the mammalian brain contains very low concentrations of tryptamine; and serotonin is a tryptamine natural derivative involved in regulating central nervous system processes like sleep, cognition, memory, temperature regulation and behavior.[7] In contrast, the naturally occurring derivatives of tryptamine are found in magic mushrooms (Psilocybe cubensis).[7]

L-Tryptophan decarboxylase is 439 amino acid residues long in its native form and a calculated pI 5.3.[4] The crystal structure of L-tryptophan decarboxylase has been modeled and predicted by AlphaFold with an average confidence of 91.17% and SWISS-MODEL with an average confidence of 25.37% as an oligo-state monomer, but the crystal structure remains to be described.[1][8]

Due to the multi-step process of psilocybin biosynthesis and its restricted phylogenetic distribution, the pathway involving L-tryptophan decarboxylase has been suggested to evolve via horizontal gene cluster transfer.[9][10] The phylogenies of the genes involved in psilocybin biosynthesis (including L-tryptophan decarboxylase) suggests that the process first evolved in wood-decaying fungi, and then evolved in dung-decaying fungi through vertical and horizontal gene transfer due to shared environmental pressures.[9][10] Neurological effects from psilocybin in wood and dung decaying fungi posit that psilocybin could be an ecological modulator acting on insect behavior.[9] This would advantage the fungi by disrupting and inhibiting the behavior of competitors, such as termites and fruit fly larvae, for wood and dung resources.[9][11] Invertebrate competitors that would be especially impacted by this are social insects, where neurotransmitter mimics would disrupt coordination.[12]