Pyramine biosynthesis

Thiamine is present in cells as the free form 1, as the diphosphate 2, and as the diphosphate of the hydroxyethyl derivative 3 (Scheme 1) in variable ratio. The component heterocyclic moieties, 4-amino-5-hydroxymethyl-2-methylpyrimidine (4) and 4-methyl-5-(2-hydroxyethyl)thiazole (5) are also presented in Scheme 1, with the atom numbering. This numbering follows the rules of nomenclature of heterocyclic compounds for the ring atoms, and is arbitrary for the substituents. To avoid the use of acronyms, compound 5 is termed as the thiazole of thiamine or more simply the thiazole. This does not raise any ambiguity because unsubstituted thiazole is encountered in this chapter. Other thiazoles are named after the rules of heterocyclic nomenclature. Pyrimidine 4 is called pyramine, a well established name in the field. A detailed account of the present status of knowledge on the biosynthesis of thiamine diphosphate from its heterocyclic moieties can be found in a review by the authors.1 This report provides only the minimal information necessary for understanding the main part of this chapter (Scheme 2). 

Scheme 23.—Some important steps in the biosynthesis of adenosine 5 -phosphate, and the branching of the sequence at the AIR level, leading ultimately to pyramine, in S. ryphimurium cells.

Scheme 30.—Correspondence between ribose and pyramine carbon atoms in the biosynthesis by S. typhimurium.

It has been shown already that C-2 of ribose is the precursor of the methyl group, and C-l is eliminated in the biosynthesis. The following observation can be pertinent to the point. Pyrimidine (58) is very unstable and quickly decar-boxylates in aqueous solution at room temperature to give pyramine (Scheme 32).67 Thus, if a C-l -C-2 fragment of the ribose part of AIRs became attached by C-2 to C-2 of a pyrimidine, oxidation of C-l to produce a carboxylic acid function could result in its smooth elimination. 

Much less is known about the participation of sugars in the biosynthesis of pyramine in yeasts, and although it has been proven that sugars can provide some carbon atoms, the exact nature of the more advanced intermediates of sugar origin is not yet clear. Some features of the biosynthesis in S. cerevisiae are summarized in Scheme 33. Two l5N atoms from DL-(l,3-,5N2)histidine were incorporated into the N-3 and amino nitrogen atoms of pyramine. The nitrogen atom of (,5N)aspartate, a known precursor of N-l of histidine, was incorporated into pyramine without dilution.58-70 It was also found that N-l and C-2 of pyramine came respectively from N-l and C-2 of pyridoxol.71-73... 

Scheme 33.—The participation of L-histidine and pyridoxol to the biosynthesis of pyramine yeast.

Scheme 34.—The participation of D-glucose to the biosynthesis of pyramine by C. urilis.

VII, The Distribution of the Four Biosynthetic Routes in Nature 1. Biosynthesis of Pyramine... 

Escherichia coli Adenine and adenosine are inhibitory74 and the synthesis of thiamine can be derepressed by culture in their presence.13,75 adth- Mutants are known.76 [l4C]Formate incorporates at C-2 of pyramine without dilution of molar activity. Glycine labeled with stable isotopes was fed to E. coli and the pyramine was analyzed by mass spectrometry. The two carbon atoms of glycine separated during the biosynthesis. The carboxyl was found12 at C-4, and the C-N fragment was the precursor of C-6-N-1. In conclusion, it is beyond doubt that pyramine synthesis follows the AIR pathway in E. coli. 

Although the gross features of the biosyntheses of thiazole and pyramine have been elucidated, nothing is known about the nature and order of the individual steps. The relevant enzymes have not yet been found, although it might be hoped that the knowledge accumulated on the precursors and the paths of atoms will help in this respect. An attempt has been made recently to find the genes involved in the biosynthesis of thiazole.82... 

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