The Reactivity of Certain Functional Groups in 



Ribonuclease A towards Substitution by i-Fluoro-2,4- 



dinitrobenzene. Inactivation of the Enzyme by 



Substitution at the Lysine Residue in Position 41 



C. H. W. HiRS, MiRjAM Halmann,* and Jadwiga H. Kycia 



Department of Biology, Brookhaven National Laboratory, 

 Upton, L.L, NY., U.S.A. 



Introduction 



In this communication some of the initial results obtained in a study 

 of the dinitrophenylation of ribonuclease A will be presented. Publication 

 of the preliminary findings is warranted because some new information 

 has been brought to light concerning the manner in which ribonuclease A 

 interacts with its substrate. 



While the reactivity of the functional groups in proteins has continued 

 to be the subject of comprehensive study, a more extensive development 

 of knowledge in this area of interest will have to await the elucidation of 

 the structures of the macromolecules involved. When it became possible 

 to put forward a primary structural formula for ribonuclease A [i, 2] 

 studies of the reactivity of certain functional groups in this protein became 

 of interest as a means with which to probe the secondary and tertiary 

 structure of the ribonuclease A molecule in solution. It was hoped that 

 information about the secondary and tertiary structure made available in 

 this way would also prove useful in delineating some of the structural 

 features associated with the catalytic activity of this protein. 



It has become common practice to attribute variations in the reactivity 

 of the amino acid side-chains in proteins to their degree of accessibility 

 to reagents. Residues reacting more slowly than others of the same type 

 are considered to be "masked" or "buried". When the reactivity of such 

 residues returns to the anticipated level through exposure of the protein 

 to agents such as extremes of pH, urea in high concentrations, etc., capable 

 of causing the disruption of the secondary and tertiary structure, it is usual 

 to attribute the effect to "unmasking". While emphasizing the importance 

 of steric effects such views tend to draw attention away from the possibility 



* Present address: Israel Institute for Biological Sciences, Nes Ziona, Israel. 



