232 



UNITY AND DIVERSITY IN BIOCHKMISTRY 



liver glycogen of animals contains carbon atoms derived from COj, 

 (those which occupy the 3 and 4 positions in the hexose molecule). 



This proves that glycolysis is partially reversible, a COg molecule being 

 introduced into a triosephosphate and passing through 3-phosphoglyceric 

 acid to glycogen. 



I 



CH2OPO3H2 



I 

 CHOH 



I 

 *COOH 



*COOH 



CHOH 



CH2OPO3H, 



3-phosphoglyceric acid 



CHaOPO.H, 



CHOH 

 '^CHO 

 " CH.OH 



CO 



CH20P03H2 



Triosephosphate 



CH2OPO3H, 



I 



CHOH 



1 

 ■■' CHOH 



I 

 '■' CHOH 



I 

 CO 



I 



CH20P03H2 



Fructose-l, 6-PP 



2. (x-carhoxylation 



This takes place by a reversal of the oxidative decarboxylation catalysed 

 by an a-ketodecarboxylase. a-Carboxylation (giving pyruvic acid or 

 a-ketoglutaric acid, for example), contrary to jS-carboxylation, requires the 

 intervention of ATP, the general reaction being 



R— C00-+ CO2 + 2H++ 2e + ATP ^ 



R— CO— COOH + P + ADP 



The reversibility of the oxidative decarboxylation catalysed by an 

 a-ketodecarboxylase can be considered as possible, but not as established. 



[b) Transmethylation 



This is the term describing the transfer of a — CH3 group (a labile methyl 

 group) from one molecule to another. In order to distinguish methyl 

 groups attached by transmethylation from methyl groups originating in 

 other ways, compounds containing such groups which can be added or 

 removed in the presence of transmethylases are described as possessing 

 "labile methyl groups". Such compounds are choline, methionine, betaine, 

 sarcosine, adrenaline, anserine, methyl-nicotinamide, creatine, dimethyl- 

 glycine, etc. A compound containing a labile methyl group is not necessarily 

 capable of giving it up to an acceptor molecule. But, those compounds 

 which can do this are given the name methyl donors. Examples are choline, 

 betaine and methionine. 



It would be wrong to believe that methyl donors, after activation by 



