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PsiK - 4-hydroxytryptamine kinase from Psilocybe cubensis

PsiK is a 4-hydroxytryptamine kinase, which catalyses the conversion of 4-hydroxytryptamine to norbaeocystin.

• NCBI: ASU62237.1
• UniProt: P0DPA8
• EC number: 2.7.1.222

Usage and Biology

The mechanism of psilocybin biosynthesis in Psilocybe sp. was recently elucidated by Fricke et al.^[1], demonstrating that L-tryptophan proceeds through decarboxylation (mediated by PsiD), hydroxylation (mediated by PsiH), phosphorylation (mediated by PsiK), and finally N,N-dimethylation (mediated by PsiM) to yield psilocybin.

PsiK is a native enzyme obtained from Psilocybe cubensis, which is involved in the metabolic biosynthesis of psilocybin from tryptophan. It accepts both 4-hydroxytryptamine and psilocin as substrates to yield norbaeocystin (Fig. 1) and psilocybin (Fig. 2), respectively. In a native state, PsiK is a 362 amino acid protein (40.4 kDa) with a theoretical pI of 5.26 calculated with the ExPASy ProtParam tool^[2].

Fig. 1: Phosphorylation of 4-hydroxytryptamine to norbaeocystin, mediated by PsiK. The reaction is dependant upon ATP, and ADP is released as a by-product. Source: KEGG

Fig. 2: Phosphorylation of psilocin to psilocybin, mediated by PsiK. The reaction is dependant upon ATP, and ADP is released as a by-product. Source: KEGG

Heterologous expression of PsiK has been achieved in a T7 induction system using pET-28c(+) transformed into Escherichia coli BL21(DE3), co-transformed with chaperone plasmid pGro7 (Fig. 3), resulting in a 397 amino acid polypeptide, with a computed molecular weight of 44.2 kDa.

Fig. 3: pET-28c(+):PsiK plasmid map, showing C-terminal histidine tag, and T7 promoter under the control of the lac operator. Translated peptide is shown as the thin lime green arrow.

A band consistent with expression of PsiK in cells induced with IPTG was observed on polyacrylamide gel electrophoresis (Fig. 4).

Fig. 4: Polyacrylamide gel electrophoresis image of soluble protein extract from uninduced and IPTG induced E. coli BL21(DE3)::pGro7 cells containing pET-28c(+):PsiD, pET-28c(+):PsiK, and pET-28c(+):PsiM, run on an Mini-PROTEAN® TGX Stain-Free™ precast gel (Bio-Rad) at 120V for 60 minutes.

Activity of PsiD was confirmed with LC/MS. Protein extract from E. coli BL21(DE3) cells co-transformed with pET-28c(+):PsiD and pGro7 was subject to LC/MS, both with and without the addition of the PsiD substrate, tryptophan. The PsiD product, tryptamine, was only identified in the sample to which tryptophan was added, confirming the activity of PsiD in vitro (Fig. 5).

Fig. 5: Mass spectra obtained LC/MS of protein extract of E. coli BL21(DE3) co-transformed with pET-28c(+):PsiD and pGro7, with the addition of tryptophan. A peak at m/z = 161.10732 with chemical formula C10H13N2 was identified, matching PsiD product tryptamine.

The presence of tryptophan in sample to which tryptophan was added may indicate autoxidation of tryptamine, as this compound was also observed in the mass spectrum of a 1 mM tryptamine standard (Table 1). However, neither hydroxytryptamine nor tryptamine was not observed in the sample to which tryptophan was not added (Table 2).

In vivo expression of PsiD was also confirmed. PsiD, PsiK, and PsiM were cloned into a pUS250 backbone as a gene cluster using Golden Gate cloning, yielding pUS387 (Fig. 6). Expression in pUS387 is driven by a class 1 integron gene cassette Pc promoter controlled by a cumate induction system. E. coli DH5α cells co-transformed with pUS387 and pGro7 were cultured in terrific broth (TB) supplemented with 4-hydroxytryptamine. Whole cell culture was subject to LC/MS.

Fig. 6: pUS387 plasmid map, showing PsiD, PsiK, and PsiM genes organised in a cluster, driven by a class 1 integron gene cassette Pc promoter, flanked by CuO, a cumate repressor binding operator.

The presence of tryptophan in LC/MS confirms the presence of endogenous tryptophan in E. coli. As tryptophan was not added to these cultures, in vivo activity of PsiD is confirmed by observation of PsiD product tryptamine (Fig. 7).

Fig. 7: Mass spectra obtained LC/MS of protein extract of E. coli DH5α co-transformed with pUS387 and pGro7, with the addition of 4-hydroxytryptamine. A peak at m/z = 161.10732 with chemical formula C10H13N2 was identified, matching PsiD product tryptamine.

Given these results, we conclude that we have successfully expressed the tryptophan decarboxylase PsiD from Psilocybe cubensis in Escherichia coli both in vivo and in vitro.

Sequence and Features

References

1. ↑ Fricke, J., Blei, F. & Hoffmeister, D. Enzymatic Synthesis of Psilocybin. Angew Chem Int Ed Engl 56, 12352-12355 (2017).
2. ↑ Artimo, P. et al. ExPASy: SIB bioinformatics resource portal. Nucleic Acids Res 40, W597-603 (2012).