CRYSTAL-STRUCTURE OF 2 COVALENT NUCLEOSIDE DERIVATIVES OF RIBONUCLEASE-A

Crystal structures of two forms of ribonuclease A with deoxynucleosides covalently bound to respectively Hisl2 and Hisl 19 have been solved. One form, T-H12-RNase, has a deoxythymidine bound to Nє2of Hisl2, while the other one, U-H119-RNase, has a deoxyuridine bound to Nδtof Hisl 19. The two crystal forms are nearly isomorphous, with two molecules in the asymmetric unit. However, the modified ribonucleases differ both in their enzymatic activities and in the conformation of the catalytic site and of the deoxynucleoside-histidine moiety. T-H12-RNase is characterized by complete loss of enzymatic activity; in this form the deoxynucleoside completely blocks the catalytic site and forms intramolecular contacts with residues associated with both the B1 and B2 sites. U-H119-RNase retains 1% of the activity of the unmodified enzyme, and in this form His119 adopts a different orientation, corresponding to the alternate conformation reported for this residue; the deoxynucleoside-histidine moiety points out of the active site and does not form any contacts with the rest of the protein, thus allowing partial access to the catalytic site. On the basis of these structures, we propose possible mechanisms for the reactions of bromoacetamido nucleosides with ribonuclease A. © 1990, American Chemical Society. All rights reserved.