DECOMPOSITION OF PROTEINS BY MICROORGANISMS 127 



These various amino acids combine to form simple and complex 

 peptides. The greater the number of amino acids which are com- 

 bined together, the more nearly does the complex approach the 

 characteristics of pure proteins. The protein molecule itself is 

 built up in a similar manner out of a large number of amino acids, 

 and has the following general formula, where R may be any of a 

 large number of radicals: 



NHo ■ CHR • CO • (NH • CHR • CO) • NH • CHR • COOH 



When a protein is treated with hot alkaline or acid solutions or 

 with certain proteolytic enzymes, it is hydrolyzed to the individual 

 amino acids of which it is constituted. This may be illustrated 

 by the hydrolysis of a dipeptide as follows: 



NHo • CH2 • CO • NH • CH2 • COOH 



+ 



HoO 



i 



NH2 • CHo • COOH + NHo . CH2 • COOH 



The amino acids themselves are readily used by most of the soil 

 microbes as sources of energy, while the nitrogen is liberated as 

 ammonia. In the process of decomposition of amino acids, 

 carbon dioxide, ammonia, organic acids, alcohols, and other com- 

 pounds are formed. In addition to these substances, amino acids 

 may also give rise, under anaerobic conditions, to amines, hydrogen 

 sulfide, mercaptans, and a considerable variety of incompletely 

 oxidized substances. Some amino acids, such as tyrosine, which 

 contains a benzene group in the molecule, give rise to such com- 

 pounds as cresol and phenol as intermediary products of decom- 

 position. Although some of the amino acids are decomposed very 

 readily, others are resistant to decomposition. 



The nitrogen of the amino acid molecule finally appears as 

 ammonia. A certain part of the nitrogen may be reassimilated 

 by the organisms, either as the amino acid directly or in the form 

 of ammonia, and changed into the protein constituents of the 

 microbial cells. Some of the nitrogen may thus be removed again 

 from circulation and a part of it may become quite resistant to 

 decomposition. 



