CHAPTER II 



AMINO-ACID CATABOLISM 



Many heterotrophs can utilize organic nitrogen compounds, 

 in addition to carbohydrates, as primary sources of carbon 

 and energy. In general, the nitrogen is first removed from 

 the compound and the product is then fermented or oxidized 

 by the same terminal pathways that are operative in the 

 catabolism of carbohydrates and fatty acids. Certain hetero- 

 trophs, apparently lacking the ability to metabolize exogen- 

 ous sugars, are entirely dependent on organic nitrogen 

 compounds, such as amino-acids, purines or pyrimidines, 

 as sources of carbon and energy. Although the end-products 

 of the catabolism of these organisms have been studied, 

 little is yet known about the routes of their formation or the 

 enzymes responsible for the individual steps. 



The catabolism of amino-acids commences either with an 

 oxidative deamination or with the removal of a specific 

 group by a non-oxidative process. It is unlikely that the 

 latter is directly responsible for making energy available to 

 the organism, but in either case examples are known in 

 which the further metabolism of the products proceeds by 

 routes which result in the formation of energy-rich phos- 

 phate groups. Thus pyruvate may arise by the non-oxida- 

 tive deamination of serine (p. 23) or the oxidative deamina- 

 tion of alanine (p. 11), and its oxidation by the pyruvic 

 oxidase system is accompanied by the formation of energy- 

 rich phosphate groups [39]. The first part of this chapter 

 is concerned with mechanisms and enzymes which accom- 

 plish the oxidative catabolism of amino-acids, whilst the 

 second part is devoted to enzyme systems whose primary 

 mode of attack is non-oxidative. 



Amino-acid oxidases 



The amino-acid oxidases oxidize amino-acids to the 

 corresponding keto acids and are specific for either the L or 



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