PROTEINS 21 



the formation of glycylglycine, an acid radical and a basic radical are 

 still left free. In this may be seen the explanation of the peculiar 

 amphoteric nature of proteins. As long as these two groups are free 

 the proteins can combine with either acids or bases, as they are well 

 known to do, and hence they react as either acids or bases under dif- 

 ferent conditions. 



It must not be imagined that the structure of the complete mole- 

 cule is simply a long straight cliain of amino-acids joined only in the 

 same way as are the components of glj^cylglycine. The existence of 

 the diamino-acids, of the benzene rings, of hydroxyl groups, (as in 

 serine or tyrosine), of ring compounds, (as pyrrolidine carboxylic 

 acid), of substances with two acid groups, (as glutaminic and aspar- 

 tic acid), adds complications to the formation until it is impossible 

 to estimate just liow all the various building stones may be arranged. 

 "We must bear in mind the size of the protein molecule, which Hof- 

 meister has estimated (for serum albvimin) as having a molecular 

 weight of 10,166, and for hemoglobin the molecular weight has been 

 estimated at 16,669.^ Within such a "giant molecule" there is room 

 for variety almost beyond computation. 



The Proteins of the Cell. — By physiological chemists proteins are 

 classified into simple proteins, of which egg and serum albumin are 

 types; and compound proteins, which are characterized by having 

 some special non-protein group which can be split off, leaving behind 

 a characteristic protein residue, e. g., nucleo-proteins, glyco-proteins. 

 As primary cell constituents the following varieties of proteins may 

 be mentioned ; albumin, globulin, nucleo-protein, nucleo-albumin or 

 phospho-protein, and insoluble proteins. At one time it was thought 

 that cytoplasm consisted chiefly of albumin, like white of egg, but 

 we now know that this forms but a small part of the cell proteins, 

 often occurring only as traces. It is held by some that true albumin 

 occurs only as a building or intermediate cleavage product of the 

 more complicated forms of cellular proteins, and is itself of relatively 

 sliglit importance in cell life, not participating in chemical changes 

 except as a food-stuff. 



Albumins are characterized chiefly by their greater solubility in 

 water, and in being less easily precipitated than most proteins. They 

 seem to be a fundamental type of proteins. The three forms of al- 

 bumin that have been described in animal tissues or products are egg- 

 albumin, lactalbumin of milk, and serum albumin ; probably cell al- 

 bumin is most closely related to the last, and what has been described 

 as cell albumin is perhaps in many cases but serum albumin that has 

 been imperfectly removed. 



4 Robertson (Jour. Biol. Chem., 1909 (6), 105) susrgests that these high molec- 

 \ilar weights represent polymerization, the single protein molecules being much 

 smaller; casein salts in neutral solutions have a molecular weight of but 2000, 

 according to his investigations. 



