PROTEINS 111 



Simple proteins 



These are naturally occurring proteins that on hydrolysis yield only 

 a-amino acids. This definition is not strictly correct because many of 

 the albumins contain small quantities (1-2 per cent) of carbohydrate, 

 e.g., mannose, galactose. Simple proteins are subdivided as follows: 



Albumins. Soluble in pure water and dilute salt solutions, coagulable 

 by heat, precipitated by saturation with ammonium sulfate. 



Globulins. Insoluble in pure water but soluble in neutral salt solutions 

 {e.g., 5 per cent NaCl) , coagulable by heat, precipitated by half-satura- 

 tion with ammonium sulfate. 



Glutelins. Insoluble in water or salt solution, but soluble in dilute 

 acids or alkalies (e.g., 0.1 per cent). 



Prolamins (Alcohol-soluble Proteins). Insoluble in water, dilute salt 

 solutions, or absolute alcohol, but soluble in 70-80 per cent alcohol. 



Albuminoids. Insoluble in the reagents given for the preceding pro- 

 teins. A heterogeneous group of simple proteins found usually in the 

 skeletal structures and protective coatings of animals; examples are kera- 

 tin from horn, hide, hoof, hair, feathers, and wool, elastin from ligaments, 

 collagen from hide and tendons, and fibroin and sericin from silk. Gela- 

 tin, although it does not fit into this group, is classed as an albuminoid 

 because it is obtained from collagen by boiling with water. It is more 

 properly a derived protein. 



Histones. Proteins (having basic properties) coagulable by heat, solu- 

 ble in water, dilute acids, or alkalis, but insoluble in dilute ammonia. 

 They form precipitates with other proteins and yield on hydrolysis large 

 quantities of the basic amino acids. Typical examples are globin from 

 hemoglobin and histones from the thymus gland and leucocytes. 



Protamines. These are the simplest natural proteins and contain only 

 a small number of amino acids, among which arginine predominates — 

 in some cases comprising 85 per cent or more of the protein. The pro- 

 tamines are strongly basic, soluble in water, and not coagulable by heat. 

 They form crystalline salts with mineral acids and precipitates with other 

 proteins, e.g., insulin (protamine-insulin). They are found in ripe sperm 

 cells. The most studied compounds have been obtained from fish sperm, 

 e.g., salmine from salmon, sturine from sturgeon, and clupeine from 

 herring. 



Conjugated proteins 



These proteins are combinations in which a simple jirotein is united 

 with a characteristic nonprotein group. The nonprotein group is called 

 a prosthetic group. The subdivisions are as follows: 



