PROTEINS 



133 



Chain I 



(1) 

 HiNPhe- 



Chain II HsNGly 



Chain III H2NGly 



Chain IV H^NPhe- 



(7) 



s 



s 



CO) I (7) 



•Cys-Cys- 



S 



I (6) (7) 



•Cys-Cys • 



S 

 I 



S 



1(7) 



■Cys- 



(11) 

 •Cys- 

 I 

 S 



I 



s 



I (11) 

 Cys- 



(19) 



■r 



s 



I 



s 



1(20) 



• Cys - 



(20) 



•Cys- 

 I 



S 

 I 



s 



1(19) 



•Cys- 



(30) 



•Ala-COOH 



(21) 



-AspNH2 



(21) 



-AspNHj 



(30) 



•Ala-COOH 



This diagram shows the four chains headed by phenylalanine and 

 glycine and ending in alanine and asparagine, AspNHo. The cystine 

 residues with one-half in one chain and the other half in the adjacent 

 chain form the — S— S— bridges that hold the chains together. The 

 numbers above the chains indicate the order of the amino acid residues 

 in the chains. The intervening amino acid residues given on p. 132 

 have been omitted because of limitations of space. The chains are prob- 

 ably not strung out in a long line, as shown in tlie diagram, but may be 

 coiled to form layers, as will be apparent from the discussion in the 

 next section. 



Even though the sequence of the amino acids in the chains is known 

 and the relation of the chains to one another may be as assumed in the 

 diagram, the question as to what there is about this arrangement that 

 gives insulin its hormone property still remains unansw^ered. In time 

 there may be an answer even to that question. 



Structure of the protein molecule 



A protein of moderate size sucli as egg albumin, having a molecular 

 weight of 40,000 and an average amino acid residue weight of 120, con- 

 tains about 350 amino acid residues. The residues are joined together 

 to form a number of chains of varying lengths. For example, egg 

 albumin and /^-lactoglobulin contain four chains, edestin — six, lactalbumin 

 — nine, and insulin — four. The average number of amino acid resi- 

 dues in the chains of these proteins is calculated to be 89 for egg albumin, 

 92 for /3-lactoglobulin, 16 for lactalbumin, and 25 for insulin. 



Since a protein has three-dimensional form, the chains must be ar- 

 ranged so as to provide such a structure. Many theories, based mainly 



