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630 



R\ifus Lumry 



THE STRUCTURE -FUNCTION RELATIONSHIP IN CHYMQTRYPSIN 



Space prevents more than the briefest mention of studies from our labora- 

 tory^^'' and that of Hess(^°' at Cornell on the possibility of significant confor- _ 

 mation changes in chymotrypsin during reaction. It is sufficient to say that ■ 

 although rather large changes in optical rotation and fluorescence yield, possi- 

 bly indicating conformation change, can be detected on acyl enzyme formation 

 or reaction of substrates under extreme conditions of pH, there is at present 

 no evidence indicating any significant change in conformational parameters at 

 the normal pH of activity which is 7. 8. The data leads us to believe that there 

 is a distinct conformation change away from the active confornnation as pH is 

 displaced away from 7. 8 either toward basic or acidic values. Reaction with 

 strong specific acylating agents or phosphorylating agents forces the protein 

 back to its active conformation since it reacts only with the active form and 

 stabilizes it. Although confornnational parameters do shift at these pH values, 

 the changes are not an essential aspect of the catalytic function itseK. Further 

 work may prove that small changes in conformation occur also at pH 7.8. We 

 are in fact inclined to suspect the normal occurrence during catalysis of small 

 rearrangements of conformation of the same magnitude as occur in hemoglobin 

 and of equal iniportance. Unfortunately it would be very ennbarrassing to try 

 to defend such a thesis on the basis of present facts. 



Our experiments on hemoglobin and chymotrypsin and the x-ray studies of 

 Muirhead and Perutz^^"*' are thus to some extent disappointing insofar as they 

 do not provide positive evidence for a dynamic involvement of conformation in 

 protein reactions. 



POSSIBLE IMPLICATIONS OF EXISTING PROTEIN 



STRUCTURE -FUNCTION OBSERVATIONS FOR PHOTOSYNTHESIS 



We have purposely selected certain of the results from studies of heme- 

 proteins and chymotrypsin because of their possible bearing on the photosyn- 

 thetic process. None of the work provides much confidence that major con- 

 formation changes or even significant chemical-mechanical interaction will be 

 found essential in the photosynthetic process. On the other hand, the results 

 do not exclude such processes and it would be unwise to ignore even the more 

 extreme implications of such changes at present, particularly in view of the 

 findings of Packer(°) and the Japanese group. ' ^ Let us therefore see what 

 behavior might be predicted even though at this point in time several of our 

 suggestions appear to be extreme. 



There is first the possibility that the spectrum of pigment molecules and 

 the electronic states of the functional atoms or bonds of these groups will be 

 significantly deternnined by the forces of protein folding. The spectrum of the 

 photon-collecting chlorophyll molecules of the bed is close to that of chloro- 

 phyll in solution and these molecules are probably little affected by their loose 

 union with protein and lipoid. On the other hand the fluorescence quantum yield 

 of these chlorophylls is at least tenfold lower than that of chlorophyll in homo- 

 geneous solution so that internal conversion does appear to be influenced by the 

 bonding. ' '' In green plants there are several different kinds of chlorophyll 

 as indicated by spectral changes'-^^) and there is also the P700 pigment which 

 may or may not be chlorophyll. '-^ ' In view of what has been said above about 

 heme spectra, it may be suggested that those forms which differ significantly 

 from normal chlorophyll spectra attain their spectra as a result of distortions 

 supplied by their respective proteins. We are particularly referring here to 



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