The biosynthesis of thiamin (vitamin B1) occurs in microorganisms and plants. In the initial stages of the synthesis produces two precursors: 5 - (2-hydroxyethyl)-4-methyl and 4-amino-5-hydroxymethyl-2-methyl-pyrimidine. They are phosphorylated, after which a key enzyme, thiamin phosphate pyrophosphorylase, combines them into thiamine phosphate molecule. It is the free thiamine dephosphorylation, and this is converted pirofosfokinazy involving thiamine, metabolically active coenzyme, thiamin diphosphate.