PROTEIN STRUCTURE 



coiled in the form of an a-helix, the /3-carbon atoms of successive 

 residues emerge from the core of the hehx at intervals of 

 approximately 1.5 A, along the helical axis, and they are also 

 rotated with respect to each other through an angle of approxi- 

 mately 100° about this axis. In general the longer side chains 

 have appreciable freedom of rotation around the j8-carbon atom, 

 so that the exact position of the terminal groups of a side chain 

 is adjustable within certain limits, even if the helical core is 

 rigid. A given side chain may, and probably will, approach as 

 closely to another side chain three or four residues away from it 

 in the main peptide chain, as to its nearest neighbor in the 

 sequence. The end of the side chain may also, of course, 

 approach most closely to the end of another side chain arising 

 from a peptide chain which is geometrically adjacent, although 

 perhaps far away in the sequence, or belonging to a different 

 subunit of the molecule. If the chain structure is not that of 

 an a-helix, but something different, then another quite different 

 set of possibilities arises. Thus any speculations based on ob- 

 served sequences of amino acid residues in a single chain are 

 necessarily tentative; the riddle cannot be read without a 

 knowledge of the stereochemistry involved, but hints may be 

 obtained from the sequences nevertheless. 



GOVALENT CROSS LINKAGES: DISULFIDE AND PHOSPHATE 



The commonest and most important covalent cross links 

 are, of course, the disulfide bonds of cystine residues. They may 

 serve to link two different chains together, or they may link two 

 half cystine residues within the same chain. Both types of 

 linkage are exemplified in the insulins, and the intrachain links 

 by such proteins as serum albumin or ribonuclease, which are 

 discussed later. 



Cross linkages through phosphate groups have been re- 

 vealed as significant in several proteins, by the recent work of 

 Perlmann (85). Phosphate groups may be attached in mono- 

 ester linkage to one of the hydroxyamino acid residues in a 

 peptide chain : 



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