CHAPTER III 



BIOSYNTHESES 



I. THE MATERIALS FOR BIOSYNTHESES 



The priming reactions described in the preceding chapter are the principal 

 sources of the coin which pays for the performance of cellular work : the 

 pyrophosphate bond of ATP. This coin also pays for the energy required 

 in biosyntheses, with the exception of assimilation phenomena whose 

 mechanism will be studied later (see Part Six). But the priming reactions do 

 not furnish only ATP, they also provide a series of construction materials 

 for these biosyntheses. For example two-carbon fragments in the form of 

 "active acetate" or acetyl-CoA are obtained during the priming reactions 

 from glucose, fatty acids, acetoacetic acid and amino acids. 



There are also one-carbon atom materials, together with CO2 which is 

 liberated at numerous points in the priming system. But the most important 

 material of this type is that referred to as "formate" (Ci) that is CHO- 

 or active formyl, into which can be converted not only formic acid, formal- 

 dehyde and methanol, but also the a-carbon of glycine, the a-carbon of 

 glycollic acid, and a- and ^-carbons of serine, the a-carbon of threonine, 

 the C — 2 of histidine, the C — 2 of tryptophan, the a-C and the C — 6 (or 

 C — 2) of phenylalanine and tyrosine, etc. Acetone can be split into an 

 acetyl fragment and a formyl fragment. 



CH9 



H— C==0 Formyl 



c=o -* + 



CH3 C=0 Acetyl 



Acetone | 



CH3 

 In certain bacteria (but not in mammals), pyruvic acid can undergo 

 fission into acetyl phosphate and formic acid (phosphoroclastic reaction), 

 providing Ci units. 



CH3 



I 

 C=0 CH3 



I I 



COOH +H3P04^ 0=C— O— PO,H, H- HCOOH 



pyruvic acid acetyl phosphate formic acid 



229 



