PROTEINS 113 



and some associalcd calioiis ami anions. IIowc\ci, ollici piolcins 

 possess atlditional structures attached to the })cptide chains. Hiese 

 extra functional groups include certain metal cations, carbohydrates, 

 lipides, phosphate as an ester, one of a group of complex organic 

 compounds called porphyrins (see page 470), and the nucleic acids 

 discussed on [xiges 120 ct scq. 



Classifications and Nomenclature 



The study of proteins is an old one, and description of the work 

 recjuired that names be devised. As is customary in a new field, the 

 earliest names were hit or miss, but the use of the ending in was gen- 

 erally employed for proteins in combination with randomly chosen 

 roots, sometimes reflecting the source of the protein or some partic- 

 ular property of it. This system is still applied except that the suffix 

 ase is now used in naming the members of the very large catalytically 

 active group of proteins called enzymes. The names of some enzymes 

 known long ago, like pepsin and trypsin, still bear the ending more 

 characteristic of non-enzymatic proteins. 



As the number of known proteins increased, it was natural to com- 

 pare and contrast their properties, thus leading directly to classifica- 

 tion of proteins. Classifications are always based on properties, and 

 different systems developed emphasizing different groups of properties. 

 As more was learned of proteins, their great diversity became more 

 apparent; the class covers a great range of properties of many kinds. 

 Hence proteins alike in one respect differ in others and could be clas- 

 sified together or into separate categories, depending upon the focus of 

 attention. Thus many classifications have been developed, none satis- 

 factory for all purposes or even very many purposes. Some of the bases 

 used for systems of classification grouped proteins in various ways 

 according to their sources, solubilities, compositions, structures, or 

 biological fimctions. A scheme based on any one of these failed to im- 

 part the information conveyed by the others. It now seems impos- 

 sible to conceive of a classification sufficiently flexible to serve all com- 

 mon needs. Moreover, the study of proteins has progressed so far that 

 the field as a whole is mastered by no individual. Hence, each 

 worker tends to concern himself with a limited number of proteins 

 related in some way without worrying much about the classification of 

 the remaining vast array of different proteins. 



Nevertheless, a sort of hybrid classification should be set forth, not 

 so much because this is the best system, biu because it contains much 

 of the terminology of protein chemistry. 



I. Simple proteins. Contain only the ordinary a-amino acids without 



