CH. vin] THE PROTEINS AND PROTEASES 119 



but most of the globulins, unlike the corresponding animal products, 

 are only imperfectly coagulated on heating and some not at all. The 

 precipitate formed when coagulation is complete will not go into solution 

 again either in water, acid, alkali or salts. Alcohol precipitates the 

 proteins ; in the case of animal proteins, the precipitate becomes coagu- 

 lated and insoluble if allowed to remain in contact with the alcohol, but 

 this does not appear to be so with plant proteins. 



In addition, certain neutral salts, the chlorides and sulphates of 

 sodium, magnesium and ammonium, have the property of precipitating 

 proteins (except peptones) from solution when added in sufficient quan- 

 tity. The protein is quite unchanged in precipitation and can be made 

 to go into solution again. The various proteins are precipitated by 

 different concentrations of these salt solutions (see p. 124). 



The salts of calcium and barium and the heavy metals produce 

 insoluble precipitates with the proteins, and in this case the reaction is 

 irreversible. 



In regard to chemical composition, the proteins contain the elements 

 carbon, hydrogen, nitrogen, oxygen and sulphur. There is every reason 

 to believe that the protein molecule is constituted of amino-acids con- 

 densed, with elimination of water, on the plan which may be depicted 

 as follows : 



Ri Rii Riii 



i I , I 



NH 2 CH COIOH HiNH CH CO|OH HiNH CH COiOH HiNH 



COiOH H-NH CH^-COOH 



. 



Conversely, when the proteins are acted upon by hydrolyzing en- 

 zymes, a series of hydrolytic products are formed which have smaller 

 molecules than the original proteins. They may be enumerated as : 



1. Albumoses. 



2. Peptones. 



3. Amino-acids. 



In the same way when proteins are boiled with acids, a number of 

 the amino-acids are obtained as an end-product. 



The above amino-acids may be either aliphatic or aromatic, and they 

 are characterized by having one or more hydrogen atoms, other than 

 those in the carboxyl groups, replaced by the group NH 2 . Thus they 

 are acids by virtue of the carboxyl groups, and bases by virtue of the 

 NH 2 groups: towards strong acids they act as bases, and towards 



