Vitamin B1 (thiamin) biosynthesis and salvage pathway (Zea mays)

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Thiamin, in its active form thiamin diphosphate (ThDP), is the cofactor for transketolases, decarboxylases, and other enzymes that make or break C-C bonds. Thiamin contains pyrimidine and thiazole moieties, which are synthesized separately in plastids and then coupled together in plastids to yield thiamin monophosphate (ThMP) (Goyer, 2010; Rapala-Kozik, 2011). The next steps are (in unknown order) export from the plastid and hydrolysis to thiamin (Goyer, 2010). Thiamin then is converted to ThDP in the cytosol. A remarkable feature of the pathway is that the thiazole synthesis protein Thi4 uses itself as sulphur donor for thiazole formation and undergoes only a single turnover (Chatterjee et al., 2011). This makes thiazole energetically very costly to produce, and puts a high premium on salvaging thiazole moieties from ThDP breakdown. Biosynthesis and metabolism of thiamin and its various derivatives (mono-, di- , tri- phosphates) and degradation products in plants (or other organisms) are far from being elucidated. This diagram is an attempt to capture the current state of the field.