GENERAL STRUCTURE AND COMPOSITION 11 



and evidently have some important structural difference as com- 

 pared with the proteins of the cellular tissues since the Body can- 

 not make use of them in building up its cell proteins in the way it 

 uses other protein foods. 



4. Protamins. These are the simplest proteins known. They 

 have thus far been found only in the spermatozoa of fishes. Their 

 molecules consist of a relatively small number of amino acid group- 

 ings and contain no sulphur. 



5. Histons are intermediate in complexity between protamins 

 and proteins of the albumin class. The one of chief importance 

 in the body is globin, which is combined with a pigment to form 

 hemoglobin, the red coloring matter of the blood. 



Conjugated Proteins. In addition to the simple proteins de- 

 scribed above there are present in the Body certain groups of com- 

 pounds consisting of proteins combined with non-protein sub- 

 stances. The most important of these are: 



Nucleo proteins, consisting of protein combined with nucleic 

 acid. These are of great interest physiologically since they form 

 the chief constituents of cell nuclei, to which structures are as- 

 signed the function of exercising special control over the activities 

 of living cells. 



Glycoproteins, consisting of protein combined with a carbo- 

 hydrate (see p. 14). Mudn, the substance which gives the secre- 

 tions of the mouth, nose, and throat their peculiar viscous charac- 

 ter, is an example of this group. 



Phospho proteins, consisting of protein combined with a phos- 

 phorous-containing substance. The casein of milk, which forms 

 the curd, is the most familiar member of this group. 



Hemoglobins, compounds of protein with a pigment. These are 

 of great physiological importance on account of the property, 

 common to all of them, of acting as transporters of oxygen. The 

 type member of the group, the hemoglobin of Mammalian blood, is 

 of interest chemically on account of the great size of its molecules, 

 which are estimated to contain not less than 2,300 atoms each and 

 to have molecular weight exceeding 16,000. 



Derived Proteins. The members of this group are derived, as 

 their name indicates, from the simple proteins. In the process of 

 protein digestion, by which the protein portions of the food are 

 made available for the needs of the Body by being split into simpler 



