PsiK and PsiM finish the psilocybin pathway. A 2017 study characterized PsiK as the enzyme that adds the phosphate and PsiM as a methyltransferase that adds methyl groups one after another as the last step (Fricke et al., 2017). A 2022 bacterial production study confirmed that PsiK makes norbaeocystin from 4-hydroxytryptamine (Adams et al., 2022).
A 2024 structural study of PsiM sets out the order: PsiM adds one methyl to norbaeocystin to make baeocystin, then a second to make psilocybin. A third transfer, giving aeruginascin, has been proposed, but the authors say it lacks experimental evidence (Hudspeth et al., 2024).
These steps explain why mushrooms hold a ladder of related compounds rather than psilocybin alone; a 2022 survey measured five of them across 226 fruiting bodies (Gotvaldová et al., 2022). The names follow the same “Psi” convention as PsiP and PsiL, the enzymes behind blue bruising. Whether PsiM can methylate plain tryptamine inside a mushroom, outside the route described in 2017, is an open question.
Sources
- Fricke et al., 2017, Angewandte Chemie
- Hudspeth et al., 2024, Nature Communications: structure of PsiM
- Adams et al., 2022, Metabolic Engineering Communications
- Gotvaldová et al., 2022, International Journal of Molecular Sciences
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