The Protein Text 



87 



unusual composition. This occurs mainly in cases where the mechanical pro- 

 perties of protein fibers are important, as in keratin, collagen and silk. These 

 have been omitted from Tabic V. The most extreme case known to me is the 

 silk of the Congolese moth Anaphe nialoneyi, where glycine and alanine together 

 constitute 94 per cent of the entire protein (1 1 1). 



Fox and Homeyer (1 12) have also noted the general similarity of composition 

 of various proteins, but have interpreted it in a quite novel manner. Their 

 suggestion is that proteins are similar because the time that has elapsed since 

 the origin of life has been too short to allow more differences to develop between 

 the various proteins, all of which are presumed to be descendants of a single 

 molecule. I believe the composition of silk tends to indicate that there has been 

 ample time for any conceivable differentiation. 



V. LENGTH OF PEPTIDE CHAINS 



I have previously called attention to the apparent fact that the number of 

 residues in naturally occurring peptide chains is an exact multiple of three (113). 

 Since then, a more exact determination of the composition of ribonuclease (79) 

 and the elucidation of the structure of glucagon (63) have shown that this 

 statement is incorrect (Table VI). In view of the predominance of chain lengths 



Table VI. Length of Protein and Peptide Chains in Number of Residues 

 (Note: Cystine counted as two cysteine residues.) 



that are multiples of three, it might perhaps be suspected that the exceptions 

 are due to secondary removal of residues, as occurs, for example, in the activa- 

 tion of pepsinogen, trypsinogen, chymotrypsinogen and fibrinogen. The tenta- 

 tive finding of Akabori (quoted in (41)), that the B chain of fish insulin has 

 twenty-nine residues, rather than the thirty found in cattle insulin, makes it 

 doubtful that secondary removal of residues is the explanation. Since twenty- 

 nine (the number of residues in glucagon) is a prime number, and not a factor 

 in the chain lengths of other peptides, it seems reasonable to conclude that 

 peptide chains are not multiples of some fixed number of residues. 



VI. THE CODING PROBLEM 



Having examined the protein text, we can now discuss what conclusions we 

 may draw as to the storage, transfer and replication of the information contained 

 in the protein molecule. 



