4-hydroxytryptamine kinase; part of the gene cluster that mediates the biosynthesis of psilocybin, a psychotropic tryptamine-derived natural product (PubMed:28763571). The first step in the pathway is the decarboxylation of L-tryptophan to tryptamine by the decarboxylase psiD (PubMed:28763571). 4-hydroxy-L-tryptophan is accepted as substrate by psiD as well (PubMed:28763571). The cytochrome P450 monooxygenase psiH then converts tryptamine to 4-hydroxytryptamine (PubMed:28763571). The kinase psiK catalyzes the 4-O-phosphorylation step by converting 4-hydroxytryptamine into norbaeocystin (PubMed:28763571). The methyltransferase psiM then catalyzes iterative methyl transfer to the amino group of norbaeocystin to yield psilocybin via a monomethylated intermediate, baeocystin (PubMed:28763571). PsiK kinase can also turn psilocin into psilocybin (PubMed:28763571). This activity may represent a protective mechanism to rephosphorylate the unstable psilocin to the stable psilocybin in case of intracellular ester cleavage (PubMed:28763571). 4-hydroxytryptamine + ATP = 4-hydoxytryptamine 4-phosphate + ADP + H(+) Secondary metabolite biosynthesis. Belongs to the methylthioribose kinase family. When the mind bends - Issue 198 of December 2017