BIOSYNTHESIS 



241 



(j) Biosynthesis of Sugars 



The major process for the synthesis of sugars in Nature is the auto- 

 trophic synthesis which we shall be studying later (Part Six). 



However, the synthesis of sugar molecules can, in general, be brought 

 about in the cell by a reversal of the priming reactions. In this way, in 

 certain types of cell, a fatty acid may give acetyl-CoA which does not enter 

 the tricarboxylic acid cycle but instead follows the reversed glycolysis 

 chain whose non-reversible stages are replaced by synthetic reactions the 

 enzymes for which are shown in Fig. 40. 



Amino acids also can give molecules of hexoses and this "glucogenic" 

 property is particularly evident with those amino acids which enter the 

 cycle of priming reactions at the level of a-ketoglutaric acid and pyruvic 

 acid. Amino acids which enter at the level of acetoacetate can also, generally, 

 follow the same path, but they tend to bring about an accumulation of 

 acetoacetic acid and other "ketone bodies". They are therefore termed 

 "ketogenic". 



As for the biosynthesis of the pentoses, we have already shown that this 

 can take place during the operation of the hexosemonophosphate shunt 

 (p. 193) and we shall return to other aspects of this biosynthesis when we 

 consider the formation of pentose phosphates (p. 253). 



[k) Biosynthesis of Pyrimidines 



The origin of the carbon and nitrogen atoms contributing to the structure 

 of the pyrimidine ring is shown in Fig. 65. The biosynthesis of the pyri- 

 midine ring takes place in the course of pyrimidine nucleotide biosynthesis 

 (p. 255). 



NH, 



C 



6' 



-^Nl \ 3C 

 \ 



CO. 



aspartate 



->-C2 



4C 



N 



Fig. 65 — Origin of the carbon and nitrogen atoms in the pyrimidine ring. 



