Methionine methyl sulphonium

Biological methylation of arsenic is effected via the transfer of a carbocation, a process known as oxidative methylation. The methyl transfer agent can vary from simple sulphonium salts, for example, 5 -adenosyl-.S -methionine (67)... 

The biological importance of the second thiol amino add, homocysteine, is as the thioether and sulphonium ion derivatives. The free thiol occurs only as a metabolic intermediate. Methionine, the methyl thioether, is one of the twenty amino adds utilized for protein synthesis. Our concepts of the special significance of methionine in protein structure and function are only beginning to be developed, and will not be considered here. N-Formyl methionine also has the distinctive role of being a chain initiator in protein synthesis. The most extensively studied form of this thiol is S-adenosyl methionine or SAM, the sulphonium ion cofactor. This is the principal methylating reagent of biological systems and other alkyl transfers from the sulphonium ion are also known. 

The S-adenosyl homocysteine produced in the transmethylation reactions is generally cleaved to adenosine and homocysteine. The latter can be degraded as previously discussed or be remethylated to methionine and eventually regenerate S-adenosyl methionine. Thus the operation of a methionine cycle provides a route whereby one-carbon metabolites reduced through the tetrahydrofolic acid sequence provide methyl groups for biosynthetic pathways. Certain other sulphonium compounds such as... 

Methyl transfer is not the only kind of alkylation that can be effected by the sulphonium centre. The best studied example is the synthe s of the polyamines spermine and spermidine, important counter ions for nucleic acids. S-Adenosyl methionine undergoes a decarboxylation of the homocysteine side chain producing a thiopropyl amine derivative. The propyl amine residue then is transferred, first to one and then to the other amino group of putrescine yielding in turn spermine and spermidine . 

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