Solution structure and dynamics of ribonuclease Sa

We have used NMR methods to characterize the structure and dynamics of ribo nuclease Sa in solution. The solution structure of RNase Sa was obtained us ing the distance constraints provided by 2,276 NOES and the C6-C96 disulfid e bond. The 40 resulting structures are well determined; their mean pairwis e RMSD is 0.76 Angstrom (backbone) and 1.26 Angstrom (heavy atoms). The sol ution structures are similar to previously determined crystal structures, e specially in the secondary structure, but exhibit new features: the loop co mposed of Pro 45 to Ser 48 adopts distinct conformations and the rings of t yrosines 51, 52, and 55 have reduced flipping rates. Amide protons with gre atly reduced exchange rates are found predominantly in interior beta -stran ds and the alpha -helix, but also in the external 3/10 helix and edge beta -strand linked by the disulfide bond. Analysis of N-15 relaxation experimen ts (R1, R2, and NOE) at 600 MHz revealed five segments, consisting of resid ues 1-5, 28-31, 46-50, 60-65, 74-77, retaining flexibility in solution. The change in conformation entropy for RNase SA folding is smaller than previo usly believed, since the native protein is more flexible in solution than i n a crystal. (C) 2001 Wiley-Liss, Inc.