234 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



transmethylases, form a common pool of methyl groups. On the contrary 

 each donor lies on a definite metabolic pathway : 



choline -> betaine -> methionine ->■ acceptor 



demethylation 



gTycine-betainc 



methionine <-f 



I 



choline s-adenosyimethionine 



t 



dimethylglycine 



Precursors of Cj 



\ COi 



serine 



— —— — ——— ^^ — — — — — — ->^ purin; 



etc... 



compourids containing 

 "^ lobile methyls 



excretion 



> = transmethylation 

 s z=^ =: activation 

 > = other reactions 



Fig. 59 (Verly) — The methyl-group cycle. At each cycle, two out of three methyl groups 

 are lost, since the dimethylglycine formed by demethylation of betaine is not a methyl 



donor. 



In the last analysis, the labile methyl group always passes to the acceptor 

 from methionine. This is an operation requiring ATP. In fact, the true 

 donor is methionine activated by ATP, or S-adenosyl-methionine. 



N- 



:NHa 



NHs C-N 



\ 

 CH 



/ + 



N=C— N— CH— CHOH— CHOH-CH— CHj— S— CH2— CHz— CH— COOH 



I o I I I 



CH3 NH2 



S-adenosylmethionine 



o 



These labile methyl groups cannot originate from COg although their 

 oxidation can yield this substance. The precursors of labile methyl groups 

 are "formates" (Ci) and as in other cases where Ci fragments are required 

 the presence of folic acid and cyanocobalamine is necessary. 



