Living Cells And What They Are Made Of 21 



to utilize sugar, which then accumulates in the blood — is due to a 

 deficiency of insulin. It is well known that this deficiency in human 

 beings can be made good by injecting insuHn of animal origin; and 

 for this reason large quantities of animal insulins are prepared. 



Sanger's experiments showed that the arrangement of amino 

 acids in beef insulin is quite unique. Every molecule has the same 

 amino acids in precisely the same order. Experiments have also 

 shown that a very sUght modification of insulin, e.g. the removal or 

 modification of only one of the amino acids, may spoil its activity 

 as an anti-diabetic agent. When a comparison was made of insulins 

 prepared from different species, the ox, pig and sheep, they were 

 found to be very similar, differing only in a small section of three 

 amino acids only, in which some variation of the insulins from 

 different sources was found. ^ 



We meet here one of the most characteristic features of living 

 organisms — their ability to construct long protein chains, in which 

 the order of the amino acids is precisely maintained. We shall see 

 that this ability is the basic characteristic of living things, which 

 enables them to make the large numbers of highly specific and 

 exactly constructed proteins which they need for their purposes. 



Plate 1 shows a model of a much more complicated protein, myo- 

 globin, which contains several peptide chains, as worked out by 

 Dr J. C. Kendrew. 



' The variations are in the section of chain marked B which is bridged by -S-S- 

 (Fig. 3), i.e. in the positions 8, 9 and 10 from the top. 



