PROTEINS 



135 



Sulfide Linkage. One-half of a cystine molecule may be part of one 

 chain; the other half may be located in a second chain to form an 

 — S— S— linkage. 



Chain 1 



•NH-CHCO" 



I 

 CH2 



Chain 2 



I 



S 



I 

 CH2 



I 

 CO-CHNH 



Hydrogen Bonding. A third, and more important, type of binding is 

 the hydrogen bond, in which electrons are shared between the hydrogen 

 of an imino group ( — NH— ) located in one chain and the oxygen of a 

 carboxyl group ( — CO—) in another chain. 



Chain 1 



Chain 2 



Shared electrons forming a 

 hydrogen bond or bridge 



There are approximately as many — NH — and — CO — groups in a 

 protein molecule as there are amino acid residues. Hence the number 

 of hydrogen bonds set up would be in the hundreds. Although a single 

 hydrogen bond constitutes only a weak chemical linkage, a dozen to- 

 gether provide about as much strength as a covalent (e.g., — C — C — ) 

 bond. 



Proteins are classified on the basis of their shape as fibrous or cor- 

 puscular. ("Globular" was formerly used as the descriptive term, but 

 "corpuscular" seems now to be the preferred designation.) As the term 

 implies, the fibrous proteins are long and slender, or unsymmetrical. In 

 some cases the length is 30 times the cross section. The corpuscular 

 proteins are more nearly symmetrical. Many of these have a long axis 

 (length) only twice as great as the short axis (thickness). Methemo- 

 globin is reported to be 64 A long, 48 A wide, and 36 A thick.^ 



^ One millimeter equals 10,000,000 A (angstrom units). 



