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The structure of ribonuclease A has been refined jointly with the neutron and
X-ray data extending to 2.0 A. The results of an earlier X-ray refinement
provided the starting model [Wlodawer, A., Bott, R., & Sjölin, L. (1982) J.
Biol. Chem. 257, 1325-1332]. The final R factors were 0.159 (X-ray) and 0.183
(neutron) for a model containing all of the atoms expected in the protein, 128
waters, and a phosphate molecule in the active site. The joint refinement
necessitated modifications in the orientation of a number of side chains,
including the catalytically active lysine-41, which is now thought to form a
salt link to the phosphate. Major modifications of the previous model of the
bound solvent were necessary. The refinement of all atom occupancies with the
neutron data only provided the information about the amide hydrogen exchange. A
fourth of all amide hydrogens were found to be at least partially protected from
exchange after a year of exchange with D2O.
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