2 PROTEINS 567 



When the side bridges of these chains are detached, the primary- 

 valence chains crumple and contract with great force. Cystine-sulphur 

 bridges, which only relatively drastic treatment can rupture, are sup- 

 posed to be active in a-keratin. The side chain bonds in actomyosin, 

 on the other hand, are far more labile, with the result that only a 

 slight change in the reaction of the surrounding medium is needed 

 for contraction. On account of its lability, actomyosin has been com- 

 pared with raw rubber and keratin with vulcanized rubber and, as 

 already stated, the tonofibrils have been described as "vulcanized" 

 myofibrils (Mark and Philipp, 1937). Convenient as such com- 

 parisons undoubtedly may be, they should be applied only with the 

 utmost discretion, particularly as long as our knowledge of acto- 

 myosin is no fuller than it is at the present time. 



Collagen group. The proteins of the collagen type are wide-spread 

 in the animal kingdom. They occur as collagen proper in the meso- 

 dermal tissues of Vertebrates (connective tissue, tendons, bones), as 

 elastoidin in scales and fins of fishes, as ovokeratin in the egg capsule 

 of rays, as ichthyocol in the swimbladder of fishes, as bysso-keratin 

 in the byssus threads of Pinna nohilis (Mollusca), as ascaro-collagen 

 in the cuticle of Ascaris (Nematoda), as connective tissue in the 

 peristome of the sea urchin Arbacia (Echinodermata), in the axial 

 stalk of the sea pens (Coelenterata), as spongin in the Porifera etc. 

 (IVIarks, Bear and Blake, 1949). All these fibrous proteins have a 

 main chain spacing of 2.8 A, whilst the backbone and side chain 

 spacings are similar to those of the k-m-e-f group with the main chain 

 spacing of 3.5 A (Table XXXII). The shortening of the collagen 

 fibre spacing by 20% is due to the cis-position of the chain member 

 >CHR related to the peptide bond -NH - CO- or -N = COH- 

 (Fig. 173, p. 346). Thus the fibre proteins of the collagen group are 

 composed of polypeptide chains in the cis-form, whilst those of the 

 k-m-e-f group assume the trans-form. The latter are capable of re- 

 versible contractions, whereas those of the collagen group have a 

 strong tendency towards an irreversible supercontraction when the 

 lateral bonds of the chain lattice are destroyed; as an extreme result, 

 globular proteins can be formed (e.g. gelatin). 



Long-range spacings. Besides the short main chain spacings which 

 characterize the keratin and the collagen group, there are long-range 

 spacings in the fibre proteins which are disclosed by low angle X-ray 



