ACETIC ACID OXIDATION 153 



" oxidative phosphorylation " and it is clear that it has 

 resulted in the production of energy-rich ATP. This reaction 

 demonstrates how oxidation energy is made available for 

 synthetic purposes by the cell; the energy released by the 

 oxidation accumulates in ATP just as it did in the case of 

 the oxidation of glyceraldehyde phosphate in the fermentation 

 cycle. Both fermentation and oxidation therefore have the 

 same end result in the synthesis of energy-rich ATP bonds 

 within the cell. 



Under aerobic conditions, acetic acid itself is oxidised by 

 Esch. coli: 



CH3.COOH + 2O2 = 2CO2 + 2H2O (7) 



The enzymic processes involved in this reaction in bacteria are 

 not yet clear. In animal tissues a similar type of oxidation 

 takes place through a complex cycle known as the " Krebs " 

 or " citric acid cycle." This cycle involves a complex of many 

 enzymes intervening between acetate (or acetyl phosphate) and 

 the cytochrome system. It has two main results: (1) it splits 

 up the liberation of oxidative energy into small steps instead of 

 a single large outburst, and (2) since a number of steps involve 

 uptake of inorganic phosphate (not shown in Fig. 12) followed 

 by synthesis of ATP, it makes energy available to the cell in 

 the form of energy-rich phosphate bonds. The steps and 

 enzymes involved in the cycle are set out in Fig. 12. Although 

 this cycle has now been well estabUshed for animal tissues, 

 there is considerable doubt whether it exists in bacteria. That 

 part of the cycle connecting pyruvic acid through oxalacetic 

 acid to succinic acid is the same as that which has been 

 discussed in Chap. VII in the section on CO2 fixation; it has 

 recently been shown that acetyl phosphate will condense with 

 oxalacetate in the presence of coenzyme A to yield citric acid 

 in Esch. coli. On the other hand, Esch. coli is differentiated 

 from Aerobacter in that it cannot attack citric acid, while in 

 Azotobacter it is found that the rate of oxidation of acetate is 

 greater than that of any of the intermediate substances in 

 the postulated cycle. For the present it must suffice to 



