Z PROTEINS 569 



Knoch and Konig, 195 i) etc. In collagen fibres the striation period 

 of 640 A corresponds to the long-range spacing discovered by small- 

 angle X-ray diffraction. This method discloses even in keratin a long- 

 range periodicity of 200 A (McArthur, 1945). 



At first sight this widespread occurrence of a submicroscopic 

 striation in fibrous proteins seemed rather enigmatic. But its formation 

 can be studied nowadays, since there are soluble proteins which yield 

 striated fibrous proteins on precipitation. Such an example is blood 

 fibrin. Even more interesting is the fact that dissolved collagen can 

 be reconstituted to precipitated collagen fibrils with a striation period 

 of 635 A (Bahr, 1950). 



These experiments favour the view that the striated microfibrils are 

 formed by linear aggregation (Fig. 104a, p. 160) of globular particles. 

 In this way the submicroscopic striation is easily understood, but it 

 is difficult to explain how a chain lattice with polypeptide chains very 

 much longer than the diameter of the dissolved protein particles is 

 formed. In this dilemma a helpful suggestion may be that in globular 

 proteins the amino acids are only loosely bound and not yet tied 

 together by firm peptide bonds (see p. 329). Then, on denaturation 

 by precipitation, not only should peptide bonds be formed inside the 

 globular protein macromolecule, but should also bridge the amino 

 acids of the adjacent molecule, the result being polypeptide chains 

 running straight through numerous protein particles. It is more likely 

 that some such mechanism is involved than that preformed poly- 

 peptide chains curled up in the globular particle should unfold com- 

 pletely to form straight threads, which would be necessarily entangled 

 before a chain lattice can be formed. 



Chemical changes of the protein molecule due to the transformation 

 globular -> fibrillar of its shape have been recorded in fibrinogen 

 (Bailey, Bettelheim, Lorand and Middlebrook, 195 i). When 

 blood clots, fibrinogen (M.W. 500,000) is transformed into fibrillar 

 fibrin by the enzyme thrombin. This change is associated with the 

 appearance of amino-terminal residues of glycine by specific hy- 

 drolysis. Whereas fibrinogen has no such end groups, five terminal 

 glycine residues appear per mole of fibrinogen when converted into 

 fibrin. It should be emphasized that ordinary denaturation does not 

 cause this effect and that only thrombin is capable of inducing it. 



