THE METABOLISM OF PROTEIN 635 



cal valency bond of the central carbon atom, which therefore must be 

 considered as the center of the entire molecule. The various amino acids 

 entering into the structure of protein differ from one another solely with 

 regard to the chemical nature of the group that is attached to this ver- 

 tical valency bond. Evidently, then, the reactions that amino acids pos- 

 sess in common must depend on the end groups containing the carboxyl 

 and amino radicles, whereas the characteristic reaction of each of the eight- 

 een amino acids must depend upon the differences in the radicles attached 

 to the upper vertical bond. This may be represented thus : 



Any radicle 

 NH 2 -C-COOH 



H 



(Any amino acid) 



The end groups endow the amino acids with the power to combine with both acids 

 and bases. With acids they behave like substituted ammonias to form salts, which can 

 here ionize into the amino acid, as the cation, and the acid group, as the anion. With 

 bases the carboxyl group reacts to form' salts, which yield amino acid as the anion. A 

 most important reaction consists in the condensation of aldehydes with the amino group. 

 This occurs particularly readily with formaldehyde, water being eliminated in the re- 

 action, and the basic nature of the amino acid thus destroyed. Upon this reaction de- 

 pends the method of Sorensen for determining the amount of amino acid in a mixture 

 (see page 641). The titration is performed by rendering the solution of amino acids 

 neutral, then adding formaldehyde and titrating with standardized acid, using phe- 

 nolphthalein as the indicator. This tells us to what degree the acidity of the mixture 

 has become increased as a result of adding the formaldehyde, and since this increase 

 in acidity must depend upon the number of amino groups, we are furnished with an 

 indirect estimate of the concentration of the amino acids. The reaction is illustrated 

 by the equation : 



radicle H radicle 



NH 2 -C-COOH + H-Cz=O = CH 2 = N - - COOH + H 2 O 



(amino acid) (formaldehyde) 



Another reaction of amino acid of physiological interest is that known as the car- 

 b amino reaction, consisting in a union of the amino acid with calcium and carbonic acid. 



Finally, it is important to note that the amino group is very firmly attached; it 

 remains intact in acids and alkalies and is removable only by a process of oxidation. 

 This can be accomplished by treating the amino acid with such reagents as hydrogen 

 peroxide or with potassium permanganate, when the amino group is displaced and a 

 so-called ketonic acid formed. The reaction will be evident from the accompanying 

 equation : 



CH 3 CH 3 



I I 



O + NH,-C-COOH ^ O = C-COOH+ NH 3 



H 



(alanine) (pyruvic acid) 



