PROTEIN STRUCTURE 



monofunctional chains with identical nonpolar Ri, Ro, ...R„ 

 residues. In a chain of this kind where side-chain interactions 

 are unhkely to be very strong, we might expect the chain to 

 adopt the most stable intrachain configurations. Interactions 

 between identical nonpolar side chains are not likely to be a 

 major factor in providing configurational stability. In the 

 Qj-keratin structure the situation is more complex. Because of 

 the different side-chain groups, the chain residues can no 

 longer be strictly equivalent. It has been necessary to invoke 

 side-chain interactions to account for distortion of the helix and 

 the formation of coiled coils. Some investigators have also 

 considered it necessary to consign the disulfide linkages to 

 amorphous (noncrystalline) regions along the fiber. Regions 

 of ordered "crystalline" intra- and inter-chain packing appear 

 to be separated by regions in which a regular chain configuration 

 is absent. 



When globular proteins are denatured by reagents and 

 under conditions which should not lead to the breaking of 

 covalent intrachain bonds they can be drawn out into fibers 

 with the /3 configuration (66). This observation provides 

 evidence for latent polypeptide chains within the native molecule. 

 It also relates the native state of the globular molecule to a 

 specific molecular configuration the loss of which is associated 

 with loss of biological specificity. A specific configuration is not 

 necessarily one which contains multiple repeats of some small 

 unit of packing. Any three-dimensional array which is uniquely 

 defined is specific. In globular proteins there is evidence that 

 the molecular configuration is an ordered or quasi-ordered array 

 of unit pattern repeats. This evidence comes from two different 

 sources. First there are fibrous proteins (a) which can exist in 

 the globular form. We may perhaps infer that there is a single 

 common mode of protein packing which consists of parallel or 

 quasi-parallel regions of peptide chain in an ordered chain 

 configuration. Studies of the vector structures (Patterson 

 diagrams) of soine proteins substantiate this inference. A 

 grossly oversimplified picture of a globular protein would 



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