PROTEINS 103 



isomers. It is interesting to note that certain componnds containing 

 D-amino acids are toxic. Such substances, often peptides, apparently 

 cannot be split into the individual components and metabolized but 

 as intact units block normal cell reactions and impair cell function. 

 Such compounds are toxic to microorganisms as well as to higher 

 forms, and some have been used as antibiotics when the toxicity for 

 bacteria exceeds that for animals. 



Metabolism 



Owing to the variety and complexity of the naturally occurring 

 amino acids, the transformations they undergo in the cells of differ- 

 ent species are both numerous and diverse. Advances in the knowl- 

 edge of these reactions are being made very rapidly, and the present 

 information is so extensive that it cannot even be outlined here. How- 

 ever, it is desirable to illustrate some of the known reactions in the 

 metabolism of one set of amino acids. The system chosen (see Figure 

 20-1) includes six of the amino acids listed earlier as commonly oc- 

 curring in proteins and four others that function as metabolic intei- 

 mediates. 



When the arrows signifying reaction are double pointed, the partic- 

 ular step is known to be reversible. Single points signify lack of 

 known reversibility. Each reaction stage shown requires at least one 

 enzyme as a catalyst. Some reactions actually are known to be com- 

 plex, involving additional factors like metals, special phosphate deriva- 

 tives, coenzymes, and even more than one enzyme. Nearly every com- 

 pound shown participates in other metabolic sequences not given 

 here, either leading to or coming from other products. Moreover, not 

 all species utilize these same pathways for these compounds, but this 

 scheme does appear to be rather widespread. 



The circular sequence of compounds appearing at the top of this 

 metabolic scheme is called the Krebs, citric acid, or tricarboxylic acid 

 cycle. This part of the system is examined in more detail on page 

 171. The lower cycle of four amino acids is sometimes referred to as 

 the ornithine cycle. It apparently is the principal source of the urea 

 excreted by most mammals in eliminating waste nitrogen. 



Sir.'ce many cellular processes are either reversible or cyclic, they may 

 serve either in the synthesis or the degradation of the compounds con- 

 cerned. The mechanisms controlling the directions of such pathways 

 are largely unknown but are thought to depend on the available sup- 

 ply of and demand for the intermediates in the cell. Details of this 

 and other pathways for the metabolism of amino acids are available 

 in any one of a number of reviews devoted to this subject. 



