CHAPTER V 



PROTEINS: THEIR CLASSIFICATION AND 

 PROPERTIES 



FROM what has already been said in Chapter IV regarding the 

 protein substances it will be recognized that the grouping of the diverse 

 forms of this class of substances in a logical manner is not an easy 

 task. The fats and carbohydrates may be classified upon the funda- 

 mental principles of their stereo-chemical relationships, whereas such a 

 system of classification in the case of the proteins is absolutely im- 

 possible since, as we have already stated, the molecular structure of 

 these complex substances is unknown. Because of the diversity of 

 standpoint from which the proteins may be viewed, relative to their 

 grouping in the form of a logically classified series, it is obvious that 

 there is an opportunity for the presentation of classifications of a widely 

 divergent character. The fact that there were until recent years at 

 least a dozen different classifications which were recognized by various 

 groups of English-speaking investigators emphasizes the difficulties in 

 the way of the individual or individuals who would offer a classification 

 which should merit universal adoption. Realizing the great handi- 

 cap and disadvantage which the great diversity of the protein classifi- 

 cations was forcing upon the workers in this field, the Chemical and 

 Physiological Societies of England drafted a classification which ap- 

 pealed to these groups of scientists as fulfilling all requirements and 

 presented it for the consideration of the American Physiological Society 

 and the American Society of Biological Chemists. The outcome of 

 this has been that there are now only two protein classifications which 

 are recognized by English-speaking scientists, one the British Classi- 

 fication, the other the American Classification. These classifications 

 are very similar and doubtless will ultimately be merged into a single 

 classification: In our consideration of the proteins we shall conform 

 in all details to the American Classification. In this connection we 

 will say, however, that we feel that the English Societies have strong 

 grounds for preferring the use of the term sclero proteins for albu- 

 minoids and chromo proteins for hemoglobins. The two classifications 

 are as follows: 



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