170 SECTIONAL ADDRESSES. 



evidence at least suggests that some such conception of the process may not 

 be far from the truth. 



We may now pass to the consideration of the synthesis of proteins. 

 In the early part of this century, due largely to the elegant methods intro- 

 duced by Emil Fischer, rapid advances were made in our knowledge of the 

 structure of proteins. These advances led to a picture of protein structure 

 which has become generally accepted, namely, that the protein molecule 

 is formed by the union of amino-acids through an amide linkage. It is, 

 however, unlikely that this is the only bond of union between the amino- 

 acid units. If it were, the widely different physical and chemical proper- 

 ties which are met with in the proteins would have to be explained largely 

 on the basis of their amino-acid content. The differences in composition 

 which they exhibit do not seem to be sufficiently great to warrant this 

 assumption. The insolubility of the keratins, for instance, although 

 accounted for in part by the high proportion of cystine they contain, this 

 being a very insoluble amino-acid, is possibly also due to the presence of 

 other modes of union of the constituent amino-acids in addition to the 

 amide linkage. Similarly, the varying degree of liability, the phenomena 

 of denaturation, and the physicochemical behaviour of different proteins 

 are difficult to explain solely on a basis of differences in their components. 

 There is much therefore still to be learnt about the actual structure of the 

 protein molecule. 



The investigation of the structure of proteins which are closely allied 

 in origin, composition and general chemical behaviour by immunological 

 and in part by chemical methods, has taught us how intricate the 

 mechanism must be by which they are built up. The facts brought out 

 by the classical work of Dakin and Dale on the albumen of the duck's and 

 hen's egg serve to exemplify this. The only chemical difference that could 

 be shown between these two j)roteins was concerned with the disposition 

 in the molecule of some of the leucine, aspartic acid and histidine, which 

 resulted in different degrees of racemisation of these amino-acids when the 

 respective albumens were treated with caustic soda. 



But when used as antigens in the anaphylactic reaction they were 

 markedly specific. These results indicate that the stereochemical structure 

 of the molecule is different in proteins which are very similar both in 

 general chemical properties and in biological origin. They suggest that 

 the protein molecule produced by a particular type of cell is always built 

 up in a distinctive and, so far as we can determine, an unvarying pattern. 

 We may deduce from this that although the general method of protein 

 synthesis, that is to say, the mechanism by which amino-acids are joined 

 up, may be the same in all cells, yet there must be arrangements in the cell 

 which enable only one particular, final pattern, to result from the synthesis. 



What are the methods by which the amino-acids are caused to combine ? 

 Those used by the organic chemist in the synthesis of polypeptides can 

 have no place in the living cell although they have been of great assistance 

 in helping to reveal the general structure of proteins. The use of proteo- 

 lytic enzymes in an attempt to bring about synthesis under conditions 

 which have been partially successful with other substances has often been 

 tried and nearly as often has failed. Two examples of protein synthesis 

 by enzyme action have been described. The most acceptable of these is 



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