KERATIN AND MOLECULAR BIOLOGY 3 



The special suitability of the proteins for these various roles is attributed 

 to their being high molecular weight, long chain polypeptides: 



R R R 



I I I 

 CH.CO.NH.CH.CO.NH.CH.CO.NH 



.... CHR . CO . NH . . . . = amino acid residue 

 R = side chain 

 with a large number of reactive side chains which, as a result of the folding 

 of the main chain, may form a variety of surface patterns permitting 

 specific interactions with other molecules, large and small, and with other 

 proteins (Frey-Wyssling, 1953). The fibre-forming habit, common among 

 the structural proteins, is seen as a special example of protein-protein 

 interaction which can lead by association to macromolecular aggregates. 

 Here we are concerned more with the structural proteins, so named 

 because they are major constituents of the large-scale structures of cells 

 and tissues; in their mass they are readily visible in the light microscope 

 and obviously are related to the function of the organ in which they occur. 

 Resistant structural proteins are produced by introducing various cross- 

 linkages between the polypeptide chains. 



It is a basic assumption that structure and function are interrelated and 

 for this reason the determination of structure is the primary concern of 

 macromolecular biology. Two other problems, which relate macro- 

 molecular structures immediately to main themes of biology, morpho- 

 genesis and differentiation, are the biosynthesis and the appearance of the 

 molecules in the course of individual development. Further, since 

 biological macromolecules were developed in the course of a long 

 biochemical evolution, there are phylogenetic as well as ontogenic aspects 

 to the problem of their adaptation to a biological role. In these terms then, 

 keratinization may be looked upon as a particular phenomenon character- 

 ized by the appearance in epidermal tissues of certain macromolecules 

 giving rise to gross structures capable of a protective function. 



Orders of magnitude 



The objects which are the concern of molecular biology range in size 

 from small molecules of diameter a few Angstrom units to massive materials 

 which can be examined with the unaided eye. It is helpful when attempting 

 to form a conception of the vast range of size involved to have some 

 visual aid to hand and for this purpose reference may be made to Fig. 1. 



Orders of magnitude are indicated on the right by a logarithmic scale 

 and various levels of organization : molecular, macromolecular, cytological 

 and histological are distinguished. The formations at the higher levels 

 of organization are constructed from the smaller macromolecular and 



