The Chemical 



R 



I 

 H-C— NH 2 



COOH 



R 

 I 

 H— C— NH 2 



COO - + H + 



acidic behavior 



R 

 I 

 H— C— NH 2 

 I 

 COOH 



+ HCI 



HCI 



h-c-nhi + cr 



I 



COOH 



basic behavior 



Fig. 4-14. Amino acids (and proteins) are amphoteric. 

 Each may dissociate either as an acid or as a base, 

 successively or simultaneously. 



carboxyl radical tends to neutralize the ex- 

 cess of hydroxyl ions, by liberating hydrogen 

 ions. This dual action is called an ampho- 

 teric behavior, and the amino acids and pro- 

 teins are spoken of as amphoteric compounds, 

 or ampholytes. Moreover, proteins and amino 

 acids are said to exert a buffer action, since 

 they tend to stabilize the reaction of a solu- 

 tion, preventing it from shifting very much 

 in either an acid or a basic direction, when 

 acids or bases are added to the solution. 



All the amino acids derived by the hydrol- 

 ysis of natural proteins are crystalline solids 

 that dissolve very freely in water. Amino 



and Physical Structure of Protoplasm - 85 



acid molecules are all relatively small, and 

 all can enter the cell through the boundary 

 membranes, albeit rather slowly. 



Peptide Bonding in Relation to Protein 

 Structure. Given a full assortment of amino 

 acids, a cell can synthesize all of its protein 

 components. Protein synthesis, essentially, is 

 a multiple dehydration synthesis. The amino 

 acids become linked into an exceedingly 

 elongate chain, as is indicated in Figures 

 4-15 and 4-16. The principal linkages in 

 such a chain are bonds that extend from 

 the carbon atom of the carboxyl group of 

 one amino acid to the nitrogen atom of the 

 amino group of the next amino acid (Fig. 

 4-15). This type of linkage is called a peptide 

 bond, and the elongate skeleton of the pro- 

 tein molecule, which may be more than a 

 thousand units in length, is referred to as a 

 peptide chain. 



Specificity of Proteins. A full comple- 

 ment of the various amino acids is present 

 in most proteins, but the proportion and 

 serial arrangement of the amino acids varies 

 widely. And since each different arrangement 

 represents a different protein substance (Fig. 

 4-16), it follows that an almost infinite va- 

 riety of proteins can exist. Just as an almost 

 infinite variety of words can be formed from 

 the 26 letters of our alphabet, so a large 

 number of specifically different proteins can 

 be synthesized from the 25 kinds of amino 

 acids. 



The proteins of each different kind of cell 

 — its genes, enzymes, ribosomal and mem- 

 brane proteins, etc. — appear to be uniquely 

 and individually self-characteristic. Similar 

 proteins may fulfill similar functions in dif- 

 ferent cells, but perfect identity is probably 

 never found. For example, the hormone insu- 

 lin, which is a relatively simple protein, con- 

 tains almost but not quite the same comple- 

 ment and arrangement of amino acids in the 

 three different animals (beef cattle, swine, and 

 horse) for which a detailed analysis has been 

 accomplished (Fig. 4-16). Furthermore, it is 

 known that a small change, involving just a 

 single protein of a cell, may induce a large 



