Biosynthesis
Complex thiamine biosynthetic pathways occur in bacteria, some protozoans, plants and fungi. The thiazole and pyrimidine moieties are synthesized separately and then assembled to form ThMP by thiamine-phosphate synthase (EC 2.5.1.3). The exact biosynthetic pathways may differ among organisms. In E. coli and other enterobacteriaceae ThMP may be phosphorylated to the cofactor ThDP by a thiamine-phosphate kinase (ThMP + ATP → ThDP + ADP, EC 2.7.4.16). In most bacteria and in eukaryotes, ThMP is hydrolyzed to thiamine, that may then be pyrophosphorylated to ThDP by thiamine diphosphokinase (thiamine + ATP → ThDP + AMP, EC 2.7.6.2).
The biosynthetic pathways are regulated by riboswitches in all organisms that synthesise thiamine. If there is sufficient thiamine present in the cell then the thiamine binds to the mRNA encoding genes required in the pathway, preventing the translation of the enzymes. If there is no thiamine present then there is no inhibition, and the enzymes required for the biosynthesis are produced. The specific riboswitch, the TPP riboswitch, is the only riboswitch identified in both eukaryotic and prokaryotic organisms.
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