Psilocybin, the hallucinogen from magic mushrooms, is being developed as a treatment for depression and other mental health conditions. Its biosynthesis from the amino acid L-tryptophan requires four sequential steps, the third of which is ATP-dependent phosphorylation of the intermediate 4-hydroxytryptamine, catalyzed by the enzyme PsiK. A crystallographic analysis and structure-based mutagenesis study of PsiK reveals how it recognizes its substrate. These findings will aid future bioengineering to create psilocybin variants with improved therapeutic properties.
The enzyme PsiM from the mushroom Psilocybe cubensis catalyzes the final step in psilocybin biosynthesis, adding two methyl groups to the substrate norbaeocystin. A single amino acid change, M247N, allowed this enzyme to evolve from ancestral monomethylating RNA methyltransferases into a dimethylating enzyme. Mutating this asparagine back to methionine (N247M) or alanine (N247A) eliminated the ability to perform the second methylation. High-resolution crystal structures and kinetic measurements show that Asn247 provides necessary space in the active site for multiple methylations and stabilizes nearby secondary structures through hydrogen bonds, enabling efficient substrate binding and catalysis.