Hydrogen exchange in RNase A: neutron diffraction study.

Abstract

H exchange was studied in a single crystal of bovine pancreatic RNase A (EC 3.1.27.5) in the course of a neutron structure investigation. Refinement of the occupancies of amide H residues provided information about the kind of isotope present in each site and also provided estimates of the errors associated with the measurement. Of the 120 peptide amide H residues, 28 were at least partially protected from exchange during .apprx. 1 yr required for crystal preparation and data collection. Most of the protected H were involved in H bonds with main-chain carbonyl groups. A contiguous region of the .beta.-sheet containing residues 75, 106-109, 116 and 118 had a large number of protected H residues, indicating its low flexibility and the lack of accessibility to solvent. Residues 11-13 from the .alpha.-helix near the amino terminus were protected, in good agreement with a model of cooperative unwinding of this helix, starting from the free (amino) end.