RESPIRATION 465 



on the other hand, pyruvic acid is not decarboxylated but 

 combines with CO2 to form oxaloacetic acid. This then under- 

 goes anaerobic or oxidative transformations. 



In higher organisms, capable of respiration, the pyruvic 

 acid which arises in the ordinary ^vay, by glycolysis, undergoes 

 both oxidative decarboxylation and condensation with CO2 

 owing to the action of ^-carboxylase. As a result of this, 

 from every t^vo molecules of pyruvic acid there are formed 

 one of acetic acid (as in Strep, jaecalis) and one of oxaloacetic 

 acid (as in the propionic acid bacteria). 



In higher aerobes, however, by contrast to these bacteria, 

 this is not the end of the matter, and it is just at this point 

 in their metabolism that there is embodied the new closed 

 chain of transformations which has been called the Krebs 

 cycle, or the tricarboxylic acid cycle. ^*®' ^^^ A diagram of this 

 cycle is given below (Fig. 40). 



As we see from the diagram, the original sugar (glucose) is 

 first transformed into pyruvic acid (the route of this glycolytic 

 transformation, which is common to all organisms, is not 

 shown on the diagram). The pyruvic acid is then transformed 

 into acetic and oxaloacetic acids as was indicated above. 

 The oxaloacetic acid easily goes over to its enolic form 

 (hooc.ch:coh.cooh), which condenses with an activated 

 molecule of acetic acid to give citric acid. This acid is con- 

 verted first into m-aconitic acid and then into isocitx'ic acid, 

 which then undergoes dehydrogenation (in this reaction 

 ^.yocitric dehydrogenase and triphosphopyridine nucleotide 

 take part, the latter taking up the hydrogen). The oxalo- 

 succinic acid thus formed is decarboxylated and converted 

 to a-oxoglutaric acid. This acid again undergoes oxidative 

 decarboxylation to give succinic acid which loses hydrogen 

 owing to the action of succinic dehydrogenase and becomes 

 fumaric acid. The fumaric acid combines with a molecule 

 of water under the influence of fumarase to give malic acid. 

 Malic dehydrogenase acts on this, bringing about its trans- 

 formation mto oxaloacetic acid. This brings the cycle back 

 to the beginning again, as the oxaloacetic acid thus formed 

 can once more condense with a new molecule of activated 

 acetic acid so that the whole reaction of oxidative dissimila- 

 tion of pyruvic acid can be repeated. 



30 



