598 



METABOLISM 



this information not only to understand the history of protein in the 

 animal body, but also to follow intelligently the important work that 

 lias already been discussed concerning the relative value of different 

 proteins as food. A knowledge of protein chemistry has come to be 

 essential in practically all branches of medical science. 



Proteins, like starches, are composed of numerous smaller molecules 

 In the case of starch these molecules are the various monosaccharides — 

 glucose (dextrose), levulose and galactose; in the case of proteins they 

 are the amino acids. The breaking apart of the links that hold the mole- 

 cules together is effected in both cases by the process of hydrolysis, so 

 called because of the fact that the reaction consists in the taking up of a 

 molecule of water at each of the places where the chain falls apart. This 

 hydrolysis may be effected either by the action of mineral acids or alka- 

 lies, or by enzymes, the only difference in the action of these reagents 

 being that in the former case the breaking apart takes place more or 

 less indiscriminately, whereas in the latter it proceeds according to a 

 definite plan, which varies somewhat with the type of enzyme employed. 

 Just as a chemical knowledge of the structure of sugar or monosac- 

 charides is the basis of carbohydrate chemistry, so is that of the amino 

 acids the basis of protein chemistry. 



Amino Acids. — There are, so far as known, eighteen different amino 

 acids concerned in the constitution of protein, but they are all alike in 

 their characteristic structure. The most striking characteristic depends 

 on the presence in the molecule of: (1) an amino group with a basicity 

 comparable to that of ammonia, and (2) an acid group with an acidity 

 comparable to that of acetic acid. Let us take in illustration one of the 

 simplest fatty acids — namely, acetic. It has the formula CH 3 COOH. 

 The COOH group is called carboxyl, and on it depend the acid properties 

 of the compound. The CH 3 group is known as methyl, and the amino 

 group (NH 2 ) is attached to it in place of one of the hydrogen atoms, thus 

 giving the formula CH 2 NH,COOH, which is aminoacetic acid or gly- 

 cocoll. If we take the next higher acid of the fatty acid series, having 

 the name propionic and the formula CH 3 CH 2 COOH, its amino acid, called 

 alanine, has the formula CH 3 CHNH 2 COOH. Now let us place the formu- 

 las of these two acids side by side in the following manner: 



H 



I'll 



NH 2 - C - COOH 



NH 2 -C 



COOH 



(amino group) H (acid group) 

 Aminoacrtic acid 

 (glycocoll) 



(amino group) H (acid group) 

 Ainmopropionic acid 

 (alanine) 



