76 PHYSIOLOGY 



THE STRUCTURE OF THE PROTEIN MOLECULE 



We can arrive at some idea of the manner in which the protein molecule 

 is built up only by breaking it down bit by bit, employing methods which, 

 while resolving the large molecule into its proximate constituents, will not 

 act too forcibly in changing the whole arrangements of these constituents. 

 The relation of the starches or polysaccharides to the sugars was found by 

 studying the hydrolysis of the former, and it is by the hydrolysis of the pro- 

 teins that we have arrived at most of our present knowledge of their con- 

 stitution. Contributory evidence may also be gained by the use of oxidising 

 agents or by employing the refined methods of analysis possessed by certain 

 living organisms bacteria, by which means we can effect limited oxidations 

 or reductions or can replace an NH 2 group by H, or a COOH group 

 byH. 



ACID HYDROLYSIS OF PROTEINS. For this purpose rather stronger 

 acids are used than for the hydrolysis of starch. The protein is heated for 

 twenty-four hours in a flask fitted with a reflux condenser either with con- 

 centrated hydrochloric acid or with a 25 per cent, sulphuric acid. Hydro- 

 chloric acid was first made use of by Hlasiwetz and Habermann, who added a 

 certain amount of stannous chloride to the mixture in order to prevent any 

 oxidation taking place. We obtain in this way an acid fluid containing an 

 extremely complex mixture of various substances, all of which belong to the 

 class of amino-acids, and must be regarded as the proximate constituents of 

 the protein molecule. 



A similar hydrolytic change may be effected by the use of digestive 

 ferments obtained either from the alimentary canal of higher vertebrates or 

 from certain plants. Thus we may use pepsin, the active constituent of the 

 gastric juice, trypsin, the proteolytic ferment secreted by the pancreas, 

 papaine, or other vegetable ferments obtained from papaya, from pineapple 

 juice, and so on. These ferments are all milder in their action than the 

 strong acids. Pepsin, for instance, only effects a partial decomposition of the 

 protein molecule. Its action results in the formation of substances which 

 still present all the protein reactions and are classified as hydrated proteins 

 or as proteoses and peptones. Trypsin carries the protein a stage further 

 and gives a mixture of amino-acids. Certain groups, however, of the protein 

 molecule present a considerable resistance to the action of trypsin, so that 

 when its action is complete these groups are still found not yet broken down 

 into their constituent amino-acids. 



The putrefactive processes determined by the process of bacteria in 

 solutions of proteins are somewhat too complicated in their results to throw 

 much illumination on the structure of the protein molecule itself. This 

 method is, however, of extreme value when it is applied to isolated con- 

 stituents of the proteins. Under the action of these bacteria we may have a 

 process of deamination which may be accompanied by simple hydrolysis or 

 by reduction. In the former case an amino-acid may be converted into 

 an oxyacid, in the latter case into a fatty acid. 



