The 2-Å resolution structure of a thermostable ribonuclease A chemically cross-linked between lysine residues 7 and 41

The crystal structure of Lys-7-(dinitrophenylene)-Lys-41-cross-linked ribonuclease A has been determined by molecular replacement and refined by restrained least-squares methods to an R factor of 0.18 at 2.0-Å resolution. Diffraction intensity data were collected by using a conventional diffractometer and an x-ray area detector. Comparison of the thermostable cross-linked protein and the native enzyme shows them to be structurally similar, with a rms difference in backbone and side-chain atoms of 0.52 and 1.34 Å, respectively. Native and modified proteins additionally show 35 common bound solvent sites and similar overall temperature factor behavior, despite localized differences resulting from cross-link introduction, altered crystal pH, or lattice interactions with neighboring molecules. These results are discussed in the context of proposals on the origins of thermostability in the cross-linked enzyme.