The Second Methylation in Psilocybin Biosynthesis Is Enabled by a Hydrogen Bonding Network Extending into the Secondary Sphere Surrounding the Methyltransferase Active Site

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.

Study at a glance

Abstract

Abstract The Psilocybe cubensis SAM‐dependent methyltransferase, PsiM, catalyzes the last step in the biosynthesis of psilocybin. Likely evolved from monomethylating RNA methyltransferases, PsiM acquired a key amino acid exchange in the secondary sphere of the active site, M247 N, which is responsible for its capacity to dimethylate. Two variants, PsiM N247M and PsiM N247A , were generated to further examine the role of Asn247 for mono‐ and dimethylation in PsiM. Herein, we present the kinetic profiles of both variants and crystal structures at resolutions between 0.9 and 1.0 Å. Each variant was crystallized as a ternary complex with the non‐methylated acceptor substrate, norbaeocystin and S ‐adenosyl‐ l ‐homocysteine, and in a second complex with the cofactor analog, sinefungin, and the monomethylated substrate, baeocystin. Consistent with the inability of the variants to catalyze a second methyl transfer, these structures reveal catalytically non‐productive conformations and a high level of disorder of the methylamine group of baeocystin. Additionally, both variants exhibit destabilization in the β5‐β7 sheets and a conserved β‐turn of the core Rossmann fold, causing 20‐fold reduced substrate binding and 2‐fold lower catalytic efficiency even with norbaeocystin. Our structural and kinetic analyses of the variants suggest that Asn247 is essential to allow enough space in the active site for multiple methylations while also participating in a network of hydrogen bonds that stabilizes secondary structure elements in the immediate vicinity of the active site for optimal methylation of norbaeocystin.