230 UNITY AND DIVERSITY IN BIOCHEMISTRY 



One-carbon materials also exist which are not in the oxidized form as in 

 CO2 and Ci, but are in the reduced form of labile methyl groups (CH3-). 

 These labile methyl groups can give Ci units and they originate from Ci 

 units. 



By oxidation it is possible to pass from Ci to COg, but the reverse reaction 

 is impossible, at least in mammals. 



Glycine is another material which is used. These molecules of glycine 

 come from the proteins of the diet, from the degradation of serine or 

 threonine, or are provided by synthesis from pyruvic acid. 



In addition, biosyntheses draw materials of a more complex structure 

 from different points in the priming reactions, and in particular from the 

 tricarboxylic acid cycle. 



II. BIOSYNTHESIS 



A. Biosynthesis of Simple Chemical Structures 



(a) The Utilization of CO 2 to Form — C — C — Bonds {excepting 



Assimilation by Autotrophes) 



This is essentially the opposite of the reversible decarboxylations (p. 212) 

 of ketonic acids. We shall therefore consider jS-carboxylation, the inverse 

 of the decarboxylation catalysed by a j8-ketodecarboxylase, and a-carboxy- 

 lation, the inverse of the decarboxylation catalysed by an a-ketodecarboxy- 

 lase (see pp. 212 and 213). 



1. fi-carboxylation 



This process for the fixation of CO2 is very important. Two examples 

 are, fixation by pyruvic acid to form oxaloacetic acid, and fixation of CO2 

 by a-ketoglutaric acid. The former is the mechanism for the synthesis of 

 dicarboxylic acids and the latter for the synthesis of tricarboxylic acids. 



{a) Fixation of CO2 by oc-ketoglutaric acid. — The conversion of isocitric 

 acid into a-ketoglutaric acid and COg in the tricarboxylic acid cycle has 

 been shown by Ochoa to take place in two stages : 



isocitric acid + TPN+ ^ oxalosuccinic acid + TPNH (1) 



Mn++ 



oxalosuccinic acid ^ a-ketoglutaric acid + CO2 (2) 



The sum of (1) and (2) is 



Mn++ 

 isocitric acid + TPN+ ^ a-ketoglutaric acid + COg + TPNH (3) 



