20 THE CHEMISTRY AND PHYSICS OF THE CELL 



molecule undergoes cleavage. It ms^y be illustratetl Ijy showing tlie formation of 

 the simplest polypeptid, (jlycy I glycine. 



N'H2 XH2 O 



i 



CH2 — C— I OH + H I HN — CH2— COOH = CH: — C — HN — CH>— COOH + HjO. 



(glycine) (glycine) (glycylglycine) 



For these reasons it is believed that tlie protein molec^ile consists of great numbers 

 of ami no-acid groups, combined with one another through their basic and acid radicals, 

 and that the various proteins are different from one another because they contain 

 different numbers or varieties or orders of combination of amino-acids. For 

 example, the globin of hemoglobin yields no glycine on hydrolysis, while gelatin 

 yields IG.o per cent. On the other hand, gelatin is free from tyrosine. Some of 

 the prolamins (proteins obtained chiefly from spermatozoa) yield as high as 58 

 to 84 per cent, of arginine, while the simpler amino-acids with but one N (mono- 

 amino-acids) are scanty, and most varieties are lacking. 



It will be noticed that when two amino-acids unite, as seen in the formation of 

 glycylglycine, an acid radical and a basic radical are still left free. In this may be 

 seen the e.xplanation of the peculiar amphoteric nature of proteins. As long a? 

 these two groups are free the proteins can combine with either acids or bases, a? 

 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 structiu-e of the complete molecule is simply 

 a long straight chain of amino-acids joined only in tlie same way as are the com- 

 ponents of glycylglycine. The existence of the diamiuo-acids. of the benzene 

 rings, of hydroxyl groups, (as in serine or t\'rosine), of ring compounds, (as pyrroli- 

 dine carboxylic acid), of substances with two acid groups, (as glutaminic and aspartic 

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

 how all the various building stones may be arranged. We must bear in mind the 

 size of the protein molecule, which Hofmeister has estimated (for serum albumin) 

 as having a molecular weight of 10,1GG, and for hemoglobin the molecular weight 

 has been estimated at 16,669. Within such a "giant molecule" there is room for 

 variety almost beyond comixitation. 



The Proteins of the Cell. — By physiological chemists proteins 

 are clas.sified 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-albiunin or 

 phospho-protein, and insoluble pi'oteins. 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 

 OcciH-s only as a building or intermediate cleavage product of the 

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

 slight importance in cell life, not participating in cliemical changes 

 except as a food-stuff. 



Albumins arc characterized chiefly by tiieir greater solubility in water, and in 

 being less easily j)rccipitated tluin most proteins. Th(>y seem to be a fundnmcntal 

 type of proteins. The tlircc forms of albumin that ha\(' ix'cii dc^i-rilu'd in animal 

 tissues or jjroducts arc egg-albumin, lactalbumiu of milk, aiul scrum albumin; pro- 

 hal>ly cell albumin 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 imper- 

 fect I v icmoved. 



