Thiamine from 5-amino-1- imidazole

From this observation of the inhibition by adenosine, and other observations, Newell and Tucker suspected the existence of a common synthetic pathway for adenosine and thiamine, and proved (with the help of a collection of mutants) that the bifurcation occurred after the 5-amino- l-(P-D-ribofura-nosyl)imidazole 5 -phosphate (46) step (Scheme 23). Finally, they found that 5-amino-l-(0-D-ribofuranosyl)imidazole (47), labeled with l4C in the imidazole ring, was incorporated into pyramine without significant loss of molar radioactivity by a mutant that is able to use this nucleoside (presumably after phosphorylation).53,54... 

The thiamin biosynthetic pathway is outlined in Fig. 22 [88]. Overall the pathway involves the separate synthesis of the thiazole 111 and the pyrimidine 114 which are then coupled. l-Deoxy-D-xylulose-5-phosphate (see sections 11 and 12 of this review) is the precursor to the five carbon unit of the thiazole [89], cysteine is the sulfur source [90, 91], and the C2-N3 atoms of the thiazole are derived from the a-carbon and the amino group of tyrosine [92-94]. The pyrimidine is derived from 5-aminoimidazole riboside (AIRs), an intermediate on the purine biosynthetic pathway. This reaction involves a complex rearrangement in which the C4 carbon of AIRs is inserted into the C4-C5 imidazole double bond, converting the imidazole to a pyrimidine, and the C2 carbon of AIRs is used to methylate the C2 position of the imidazole [95-98]. 

Only oxazole, of the trio, does not play any part in normal biochemical processes, though there are secondary metabolites (especially from marine organisms) which incorporate thiazole (and oxazole) units - the antibiotic cystothiazole A, from the myxobacterium Cyctobacter fuscus is an example. Imidazole occurs in the essential amino acid histidine histidines within enzymes are intimately involved in catalysis requiring proton transfers. The structurally related hormone, histamine, is a vasodilator and a major factor in allergic reactions such as hay fever. The thiazolium ring is the chemically active centre in the coenzyme derived from thiamin (vitamin B,). 

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