CARBOHYDRATE METABOLIC PATHWAYS 65 



be emphasized by the simple diagram of the Krebs cycle 

 shown in Fig. 3.1. This sequence shows how compound 

 A (pyruvate) can be converted to B (acetyl CoA) which, in 

 turn, combines with a molecule of C (oxalacetate) to pro- 

 duce D (citric acid) and continue stepwise to the re-formation 

 of compound C. 



Viewed in this light the Krebs cycle is revealed for what 

 it really is— a catalytic cycle. As has been pointed out 

 previously (2), the removal of a few molecules of any of 

 the intermediates pictured here for synthesis stops the 

 cycle, unless there is some provision for the independent 

 formation of di- or tricarboxylic acids. Thus we find in 

 most organisms a functioning malic enzyme, or oxalacetic 

 decarboxylase, or malate synthetase system; if one or more 

 of these is operating there is an additional source of Krebs 

 cycle intermediates, so that removal of one or more of the 

 intermediates for synthesis of amino acids, for example, 

 may be possible. Without such independent mechanisms, 

 the Krebs cycle as wt usually picture it could not operate, 

 and energy metabolism therefrom would soon also grind to 

 a halt. 



Another reason for making initial mention of the citric 

 acid cycle is that it helps to emphasize the importance 

 that this mechanism has held, not only as a pathway of 

 metabolism, but also perhaps even more importantly as a 



H 



Fig. 3.1 The Krebs tricarboxylic acid cycle— a schematic diagram. 



