Enzymology to Understand the Biosynthesis of Psychoactive ​​Compounds

Title: Monoamine Biosynthesis via a Noncanonical Calcium-Activatable Aromatic Amino Acid Decarboxylase in Psilocybin Mushroom

Authors: Jing-Ke Weng, et al. 

Journal: ACS Chemical Biology

Year: 2019 

https://pubs.acs.org/doi/10.1021/acschembio.8b00821

Microorganisms, plants, and fungi possess incredible assemblies of biomolecular machinery that allow them to fabricate metabolites of immense chemical diversity. Historically, many of these molecules have proven exceptionally valuable for us. For instance, penicillins – the square-looking small molecules synthesized by the Penicillium fungi – serve as antibiotics up to this day, and taxol – a bewilderingly complex molecule made by the bark of the Pacific yew tree – is used to treat a variety of cancers (Figure 1A, 1B). 

Figure 1: Examples of natural products derived from plants and fungi. While Penicillins (A) and Taxol (B) have found use in the medical community, psilocibin (C) is famed for its use as a psychoactive drug.

In other instances, these metabolites can find more… controversial roles in our society. Such is the case of psilocybin (Figure 1C), an alkaloid molecule made by the psilocybin fungi, also infamously known as magic mushrooms. Psilocybin is a Schedule I-controlled recreational drug with hallucinogenic and mind-altering effects similar to those of LSD, mescaline (peyote), or DMT (ayahuasca). However, a wave of current research is suggesting that psilocybin could find therapeutic used in the treatment of psychiatric disorders like addiction, depression, or OCD. This notion, in turn, has fortified research interest in the biosynthesis of this alkaloid. Not only is it fundamentally interesting, from a discovery point of view, to elucidate the chemistry with which these mushrooms build psilocybin, but it is also important to characterize which genes and enzymes are involved in the process in order to potentially engineer them to arrive at psilocybin-like products with more desirable therapeutic properties. 

In this research, the lab of Jing-Ke Kang at MIT identified and characterized one of the enzymes in this biosynthesis: an enigmatic decarboxylase called that kickstarts the making of psilocybin.

To give the reader some context, the biosynthesis of the compound is presumed to follow as in Figure 2. First, the amino acid tryptophan (1) is decarboxylated by this elusive enzyme to generate tryptamine (2). From here, the enzyme PsiH adds a hydroxyl group to the indole backbone to yield the oxidized product 3,which is phosphorylated by PsiK to get to compound 4. The final step towards psilocybin involves PsiM, an enzyme that adds two methyl groups to the appending amine, and voilà. 

Figure 2: Proposed biosynthesis of Psilocybin in the psilocybin mushrooms

Using computational tools to analyze the genome of magic mushroom P. cubensis, the group was able to identify a candidate decarboxylase enzyme that they named ncAAAD. They then rely on established molecular biology techniques to purify the protein and test its putative activity. They indeed find that ncAAAD can convert tryptophan (1) into tryptamine (2) in vitro, as well as mediate the decarboxylation of other aromatic amino acids like tyrosine and phenylalanine. In vivo, they observe that yeast cells engineered to have large amounts of ncAAAD produce a lot of these decarboxylated products, confirming the activity of the enzyme in vivo as well. The bulk of the paper is then devoted to study and probe the structure of ncAAAD. The authors were able to obtain crystals of the enzyme and solve its high-resolution x-ray structure (Figure 3). Some of the details get very technical from here on, and I encourage you to look at the original research if it’s of your interest. One of the most revealing findings is the existence of a regulatory element in the C-terminal region of the protein. The group is able to verify that calcium binding to this domain increases enzymatic activity, thus establishing an unusual modulation mode not observed generally in enzymes of this type. 

Figure 3: Structure of the elusive PsiD enzyme responsible for the decarboxylation of tryptophan en route to psilocybin. Image taken directly from ACS Chem. Biol., 201813 (12), pp 3343–3353.

This research is yet another marvelous example of how enzymology and biochemistry can be put to use to uncover long-sought fragments of prticular biosintheses and a reminder that even relatively notorious natural products can open the door to new knowledge.

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