136 PROTEINS 



CONJUGATED PROTEINS AND THEIR 

 PROSTHETIC GROUPS 



The conjugated proteins (protein plus a prosthetic group) will, of 

 course, give other products than amino acids on hydrolysis. Some 

 important compounds that either constitute the prosthetic group or arise 

 from it on hydrolysis follow. 



Nucleo proteins 



Nucleic acids occur either in the free state or in combination with 

 proteins to form nucleoproteins. Because of their number, complexity, 

 and importance, the nucleoproteins and the nucleic acids will be discussed 

 in a separate chapter. 



Phospho proteins 



The phosphoproteins also give phosphoric acid on hydrolysis. Phos- 

 phoric acid is linked to serine through the hydroxy group of the amino 

 acid to form a phosphoric acid ester that can unite with bases, e.g., 

 calcium, to form salts. The serine phosphoric acid esters seem to account 

 for most of the phosphorus in the phosphoproteins. 



Glycoproteins 



These are ill-defined proteins containing carbohydrate complexes 

 (chondroitin sulfuric acid or mucoitin sulfuric acid) as prosthetic groups. 

 Typical examples of these proteins occur in salivary mucin, gastroin- 

 testinal mucus, and the vitreous humor of the eye. They are slippery 

 materials and, hence, serve as useful lubricants. They facilitate the 

 movement of the food through the intestinal tract and, since they are not 

 digestible, protect the tract against the proteolytic enzymes. 



On hydrolysis, chondroitin sulfuric acid gives one mole each of glu- 

 curonic acid, galactosamine, acetic acid, and sulfuric acid. Mucoitin 

 sulfuric acid gives glucosamine instead of galactosamine, but the other 

 hydrolysis products are the same as from chondroitin sulfuric acid. 

 There is still some uncertainty as to how the several products are bound 

 together. These carbohydrate complexes account for 25-35 per cent of 

 the glycoprotein. In consequence the nitrogen content is low, 9-13 

 per cent. 



