PROTEINS 97 



where the two functional groujDs are written in their ionic forms for 

 reasons presented on page 98. R represents any one of a great 

 variety of structures. In the amino acids Avith imino instead of amino 

 groups, one of the hydrogen atoms of the latter is replaced, usually 

 either by a methyl group or by connection to the R group to form 

 a pyrrolidine ring as in proline. In a few cases other substituents are 

 involved or additional hydrogens are replaced. 



Dated lists of the known naturally occurring amino acids show 

 continuous changes. Some compounds have been isolated that are 

 now believed not to occur in living systems but apparently arose 

 during chemical treatments of isolation and identification. On the 

 other hand, the number of known natural amino acids has increased 

 steadily, especially in the last 10 years. New examples will undoubt- 

 edly be discovered but probably at a decreasing rate. 



A partial list of the present day includes 88 amino acids identified 

 from at least one biological source. A number of these compounds are 

 thought to have limited distribution, occurring in but few organisms 

 as components of alkaloids, antibiotics, or hormones, for example. 

 Many others participate in only the complex metabolic processes of 

 cells. These latter amino acids are widely distributed, of course, 

 usually in free form and in rather small amounts. Still other examples 

 are widely distributed, functioning in special roles like the first group, 

 in metabolism like the second, and in addition as components of 

 proteins. The members of this last group are among the most abun- 

 dant organic substances, though often in combined form. 



The names and structures of the amino acids combined to form 

 most proteins are listed in Table 5-1. It must be emphasized that 

 many other amino acids have been observed in other materials and 

 studied, but space does not permit their tabulation or discussion here. 

 Hydroxyproline is included as is customary, although this amino acid 

 has been demonstrated only in the proteins of the widespread gelatin- 

 collagen group, where it is quite abundant. 



Electrochemical Properties 



The electrical charges on amino acids influence the properties 

 markedly. These charges arise from the fact that amino acids possess 

 both basic and acidic groups and are thus amphoteric. In addition, 

 other ionizable groups are present in certain cases and all play impor- 

 tant roles. 



All molecules with lioth acidic and basic groups might exist as 

 uncharged molecules, as dij)olar ions with ionic charges of opposite 

 sign, or as a mixture of these. Many different lines of evidence indicate 



