PROTEIN STRUCTURE 



proline and hydroxyproline content of this protein was respon- 

 sible for its unique configuration. Proline residues cannot be 

 written NH.GH(R)CO, since instead of a hydrogen atom there 

 is a = CHo group attached to the amino nitrogen, and a NH — O 

 hydrogen bond cannot therefore be formed. The proline 

 (hydroxyproline) ring is only slightly puckered (34,73), and this 

 imposes stereochemical restrictions on the orientation of adjacent 

 peptide chain residues. Models for the collagen structure are 

 therefore dependent upon an assumed residue sequence; they 

 are of the greatest interest at the present time, but we shall not 

 attempt to discuss them further here. 



Globular proteins appear most closely related in structure 

 to the a-type fibrous proteins. Fibers of denatured globular 

 proteins may be oriented in the stretched-out j8 configuration. 

 In the globular proteins studied the three-dimensional structure 

 is complex and the identification of the dominant peptide chain 

 configuration is only a part, though an important one, of the 

 whole problem. We shall discuss the recent advances in the 

 study of peptide chain configurations and their significance in 

 the a and /3 proteins. This will serve as an introduction to the 

 globular protein problem. 



In 1951 Pauling, Corey, and Branson specified the most 

 rigid and coherent set of chemical limitations on plausible 

 polypeptide chain configurations (82). First, on the basis of the 

 results from detailed crystal structure studies of amides, amino 

 acids, and some simple peptides, they defined precise dimensions 

 for the peptide chain residue. These are shown in Figure 3. 

 The amide group must be planar; there is nearly 50% double 

 bond character in the G' — N bond, which is consequently shorter 

 (1.32 A) than the normal single bond length of 1.47 A. The 

 planar requirement gives two possible configurations, either 

 trans as in Figure 3 or cis, as shown here. 



G. .G 



In the polypeptide chain configurations discussed below, which 



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