RESPIRATION 407 



In the course of this cycle all of the three carbon atoms of 

 the pyruvic acid are oxidised to cOg by means of the oxygen 

 of water while, at the same time, the hydrogen leaves the 

 cycle with the help of pyridine nucleotides and the corre- 

 sponding dehydrogenases. The splitting off of CO2 is brought 

 about directly by decarboxylases. 



Thus we see that the same types of enzymic mechanisms 

 act here as act in anaerobic metabolism, but the sequence 

 of reactions is substantially different. An important difference 

 is that the hydrogen which is liberated is not wasted but is 

 used to obtain a considerable extra supply of energy by its 

 oxidation by the oxygen of the air. The intermediate products 

 arising in the cycle carry it over into other metabolic systems 

 so that there is established a direct connection and mutual 

 dependence between the metabolism of carbohydrates, fats, 

 organic acids and proteins. A particular example is the trans- 

 formation of keto acids which leave the cycle, by reacting with 

 ammonia (i.e. by direct amination^") or by transamination,^** 

 into alanine, aspartic and glutamic acids and the formation 

 from these of various other amino acids which take part in 

 the synthesis of proteins, hormones, enzymes, etc. 



The incorporation of accessory respiratory transformations 

 in the chain of glycolytic reactions can take place not only 

 through the pyruvic acid at the end of the chain, but also 

 through its first links. 



As we noticed on p. 427, even among anaerobic alcohol 

 producers, e.g. Pseudomonas Undneri, the metabolism may 

 diverge somewhat from the general scheme of alcoholic 

 fermentation. In this case hexose-6-phosphate is not further 

 phosphorylated but immediately enters the path of anaerobic 

 dehydrogenation, being thereby transformed into 6-phospho- 

 gluconic acid. This is decarboxylated to a phosphorus deriva- 

 tive of pentose, which then breaks do^vn to give alcohol and 

 glyceraldehyde-3-phosphate. This glyceraldehyde-3-phosphate 

 then enters into the general scheme of alcoholic fermenta- 

 tion. Among many facultative anaerobes this is used as an 

 oxidative path. In these there takes place, alongside the 

 ordinary glycolytic breakdown of glucose according to the 

 scheme for alcoholic fermentation, the oxidation of glucose- 



