The biosynthesis of vitamin B1 (Thiamine). This complicated reaction begins from 4 (or 5, depending how you look at it) different starting points: either tyrosine or glycine becoming dehydroglycine; pyruvate and glyceraldehyde 3 phosphate to make DXP (1-deoxy-D-xylulose-5-phosphate); taking a sulfur from a cysteine for the thiazole ring; and using an intermediate from purine (DNA base) biosynthesis called AIR (5-aminoimidazole ribotide) to make HMP-PP (4-Amino-5-hydroxymethyl-2-methylpyrimidine pyrophosphate). Finally, all these pieces are brought together to make thiamine monophosphate, which can then be phosphorylated once more to make the biologically active molecule of thiamine pyrophosphate (or diphosphate, whichever you prefer).
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