TYPES OF RESPIRATION 59 



According to the pyruvic acid theory, the following steps occur 

 sequentially: 



a. The hexose molecules become "activated"; that is, highly 

 reactive y-glucose or y-fructose comes into transitory existence. 

 These sugars are not straight carbon-chain complexes, being best 

 represented by a ring type of formula. 



b. The "activated" y-hexose is cleaved by glycolase into two 

 molecules of methylglyoxal and two of water, formally expressed 

 as: 



C 6 H 12 6 + Glycolase -* 2(CH 3 COCHO) + 2H 2 



Methylglyoxal 



c. As the next step, one molecule of methylglyoxal is reduced 

 to glycerol, and the other is oxidized, by a Cannizzaro reaction, to 

 pyruvic acid with the two molecules of water. A dehydrogenase 

 may catalyze this reaction: 



CH 3 • CO • CHO CH 2 OH ■ CHOH • CH 2 OH 



+ H 2 + HoO Glycerol 



+ II ^ + 



o 

 CH3COCHO • CH3COCOOH 



Pyruvic acid 



d. Immediately carboxylase splits the pyruvic acid into acetalde- 

 hyde and carbon dioxide, as follows: 



CH3COCOOH + Carboxylase -> CH 3 CHO + C0 2 



Pyruvic acid Acetaldehyde 



The course of events is identical up to this point, as has been 

 stated, whether the process is aerobic or anaerobic. 



e. If then anaerobic conditions prevail, the other molecule of 

 methylglyoxal produced in step b reacts with the acetaldehyde 

 molecule in step d, and by a Cannizzaro reaction a molecule of 

 pyruvic acid and one of alcohol are formed in this manner: 



CH3COCHO "> CH3COCOOH 



Methylglyoxal O Pyruvic acid 



+ + I! - + 



H 2 

 CH3CHO CH 3 CH 2 OH 



Acetaldehyde Ethyl alcohol 



It is of interest to note that no energy is released in the trans- 

 formations that result in the formation of methylglyoxal, glycerol, 

 and pyruvic acid. 



