PROTEIN STRUCTURE 



this linkage by pyrophosphatase involves no release of inorganic 

 phosphate into the solution, but such release can be produced 

 by subsequent addition of the prostate phosphatase which splits 

 primary phosphate esters. Thus it appears that this pyrophos- 

 phate linkage actually serves as a cross link between peptide 

 chains. Breakage of these cross links results in a separation of 

 the protein molecule into smaller units, indicating that these 

 phosphorus linkages actually hold different peptide chains 

 together. In /3-casein (83) likewise, diester cross linkages are 

 found, but here the specificity of the enzyme involved indicates 

 that the linkages are of the type — O — P — O — , illustrated above. 

 Here again it is found that after removal of the phosphorus, 

 material soluble in trichloroacetic acid is present in the solution, 

 so that a splitting into smaller molecules has occurred, as a 

 result of the breakage of the — O — P — O linkage. Finally 

 Perlmann has pointed out that the single atom of phosphorus 

 present in the pepsin molecule is probably also present in an 

 ester linkage of the O — P — O type (83). Pepsin appears to be 

 a protein consisting of only one peptide chain; in this case, 

 therefore, the phosphorus must cross link a portion of the chain 

 into a cyclic loop. It is still uncertain how general the signifi- 

 cance of these phosphate cross linkages in proteins may prove to 

 be. They are certainly much less common than the disulfide 

 cross linkages, but their discovery is an important event 

 in enlarging our general picture of the possibilities of protein 

 structure. 



OTHER CROSS LINKAGES 



As yet we have spoken only of cross linkages involving 

 covalent bonds. Cross links involving hydrogen bonding 

 between amino acid side chains, or perhaps between groups in 

 the side chains and CO or NH groups of the peptide linkages, 

 may also be of great importance in fixing the protein molecule 

 in its native state. A protein molecule may hold together in 

 solution as a functional unit wiih a high degree of stability, 

 even when it consists of several different peptide chains with no 



389 



