134 



FINE-STRUCTURE OF PROTOPLASM II 



polypeptide chain molecules. The proline ring can, however, also be 

 built into the peptide chain (see Fig. 173, p. 546). 



Considering the variety and the number of 20 amino acids (besides 

 some rare amino acids, Cohn and Edsall, 1943) which have thus far 

 been isolated from proteins, and in view of the fact that these can occur 

 as side chains at various points along the polypeptide chains, we realize 

 that the protein components of the cytoplasm represent a variegated 

 mosaic. It follows from Fig. 88, that the amino acid configuration 

 -CH-NHg-COOH does ;w/ contribute to the character of the mosaic, 

 since it is only responsible for the peptide interlinking. The chemical 

 behaviour of the polypeptides of protoplasm is determined by the 

 end and side groups of the amino acids, to which often little attention 

 is paid. 



The polypeptide chains show a number of properties which single 

 them out from the other substances of which protoplasm is built up. 



1. The principle of repetition which mhlology we know as segmenta- 

 tion or metamerism. Most high polymer substances are built according 

 to this principle. In the majority of these substances, however, iden- 

 tical monomer groups are repeated, whereas in the polypeptide chains 

 the side groups R, which occur at regular distances of 3.5 A, have 

 different constitutions. Probably the typical side chains also repeat 

 themselves regularly, but their period is much greater and is often not 

 accessible to experimental analysis. 



2. The principle of specificity. Owing to the numerous possible side 

 chains R and the unlimited variety in their arrangement along the 

 polypeptide chains, an almost infinite number of polypeptides is con- 

 ceivable, distinguished only by slight difference in construction. This 

 difference in construction may result in a different chemical behaviour 

 which becomes apparent in the specific properties of the proteins. 



3. The principle of contractility. The most striking property of poly- 

 peptide chains is their capacity to contract, as will be further discussed 

 on page 559. The origin of the mobihty of cells (protoplasmic flow, 

 cilia, contractible fibrils, etc.) must be sought in these molecular 

 structural units and for this reason they form undoubtedly the most 

 important structural elements in the fine-structure of protoplasm. 



The number N of amino acids in natural polypeptides seems to 

 obey the Bergmann-Niemann rule (1936/37) N = 2" -3™, which in- 

 dicates that there must be some threefold symmetry in protein mole- 



