POLYPEPTIDES AND PROTEINS 235 



varies somewhat. Certainly there are a number of facts which 

 suggest that several pure individual protein-like substances 

 may form, as it were, a family of proteins, being composed 

 of the same amino acids but differing from one another in 

 the amounts of some of the amino acid residues in the peptide 

 chain. This may be demonstrated particularly clearly as 

 regards haemoglobin.^* 



In this connection we must emphasise the fact that proteins 

 having the same biological function may differ markedly 

 from one another chemically. Insulin serves as a good 

 example of this. The hormone was isolated from the pancreas 

 as an individual protein of comparatively low molecular 

 weight, the structure of w^hich is now very well worked out. 

 However, it has been shown that the insulins obtained from 

 oxen, pigs and sheep, though they have the same physio- 

 logical activity, nevertheless differ from one another chemi- 

 cally. In particular, pig insulin contains threonine at a 

 position in its peptide chain where it is not present in ox 

 insulin. Thus it is evident that the physiological properties 

 of hormonal proteins do not require absolute uniformity of 

 structure. ^^ The same may also be said of enzymes. It now 

 seems quite clear that we include under the same name 

 (pepsin, invertase, phosphomonoesterase, etc.) proteins which 

 have the same enzymic activity though they sometimes differ 

 markedly among themselves in respect of molecular size, 

 isoelectric point and other physico-chemical properties and 

 even in respect of their amino acid compositions.^® 



It follows that the catalytic properties of a given protein 

 are not associated with the whole of its molecule and that 

 this may contain parts which are completely inactive and can 

 easily be altered without destroying the enzymic properties. 

 It follows that some variations in amino acid composition do 

 not necessarily cause noticeable alterations in their biological 

 properties. 



It is now well known that different forms of organisms 

 can contain proteins which are identical in their biological 

 functions but which differ in their amino acid composition. 

 It has also been established that changes in the living condi- 

 tions of organisms bring about variations in the composition 

 and properties of their proteins. 



