110 



PROTEINS 



Table 5-3 

 Examples of enzyme proteins and nonfood proteins 



I*R0TEIN Classification Source 



Keratin Albuminoid Hair, wool, feathers, 



hide, nails, etc. 



Fibroin Albuminoid Silk 



Sericin Albuminoid Silk 



Spongin Albuminoid Sponge 



Amylopsin Albumin Pancreas 



Pepsin Albumin Stomach 



Trypsin Albumin Pancreas 



Papain Albumin Latex of papaya tree 



Urease Globulin? Jack bean 



Catalase Chromoprotein Liver, etc. 



Flavoprotein Chromoprotein Yeast, heart, etc. 



Insulin Globulin? Pancreas 



Mosaic virus Nucleoprotein Diseased tobacco plants 



Tuberculin Unclassified Tubercle bacillus 



Avidin Albumin? Egg white 



Crotoxin Albumin Rattlesnake venom 



Bacitracin Polypeptide Antibiotic from 



Bacillus licheniformis 



Botulinum toxin A . Globulin Toxin from CI. botulinum 



The elementary composition of proteins varies within wide limits, 

 but the average figures show C, 53 per cent; H, 7 per cent; 0, 23 per 

 cent; N, 16 per cent and S, 1 per cent. Nitrogen shows the greatest varia- 

 tion, ranging from about 10 per cent for the glycoproteins to 30 per cent 

 for the protamines. In the common food proteins it varies within much 

 narrower limits, 15.5-18.7 per cent, and, hence, an average figure of 16 

 per cent is taken. On the other hand, proteins from entirely different 

 sources, and different in character, may have the same elementary com- 

 position. Except in a very general way elementary composition is of 

 but little value in the differentiation of proteins. 



CLASSIFICATION 



Because of their number and complexity, proteins are difficult to 

 classify. The basis for classification is mainly (1) products on hydrolysis, 

 (2) solubility, (3) coagulability, and (4) precipitability. It is probable 

 that many compounds quite unlike in structure fall into the same group, 

 and it is certain that many so-called proteins are not chemical entities. 

 Even when crystallized, the proteins are not always homogeneous. How- 

 ever, even though imperfect, a classification is indispensable for study 

 and discussion. The official classification, with slight modifications, 

 follows. For examples of many of the classes see Tables 5-2 and 5-3. 



