PROTEINS 



351 



are these: There are globular proteins, such as haemoglobin, which 

 are not attacked by polypeptidase until they are denatured (Hauro- 

 wiTZ, 1949), but since many proteolytic enzymes work under con- 

 ditions which cause denaturation (e.g. pepsin at p^ i), this fact is often 

 obscured. Further, it seems that the three amino acids of the pre- 

 formed groups in the molecular layer (Fig. 166) form tripeptides 

 when denatured (tripeptols of Jordan, 1947). 



On the other hand, there is no indication of how such an arrange- 

 ment leads to molecules with a definite weight. When 4 layers as seen 

 in Fig. 167 with 24 x 3 amino acids are superposed, a molecule of 

 288 amino acids is obtained, which would correspond, for instance, 

 to insulin. It is not clear, however, why piles of only four layers exist, 

 and why aggregations of such fourfold layers by 2, 4 etc. occur 

 according to the Svedberg series. It seems likely, therefore, that the 

 binding forces inside the molecules are stronger than those which 

 cause the aggregation of globular protein molecules to multiples and 

 crystal lattices (Fig. 84, p. 126). 



b. Si/k (Silk Fibroin) 



Microscopic and suhmicroscopic structure. A cross-section of the cocoon 

 thread of the silk-moth {Bombyx mori) reveals two halves in mirror 

 symmetry, which owe their existence to the paired silk-glands. These 

 ■produce two discrete fibroin threads which are covered with a layer 



Fig. 168. Fine-structure of silk, a) Microscopic cross-section through the cocoon filament 

 (after Ohara, 1933a); b) suhmicroscopic structure of the fibroin thread, i Skin and 2 

 cortex (fibroid texture with tangential scattering) of the sericin layer. 3 Skin, 4 cortex 

 (fibroid texture with radial scattering) and 5 central zone (fibrous texture) of the fibroin 



filament. 



of sericin (Fig. 168). The regular structure as seen in Fig. 168 a is 

 apparently disturbed where the threads cross in the cocoon, which 

 would go to show that the thread is still plastic when it leaves the 



