Extremely fast hydrogen exchange of ribonuclease‐(1–118) as compared with native RNase A and its implication for the conformational energy state

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Abstract
RNase‐(1–118) containing native disulfide bonds is similar in fold to native RNase A but not of lowest Gibbs energy as compared with the isomers containing non‐native disulfide bonds. The present n.m.r. studies have indicated a dramatic increase in the exchange rate of all of the ‘protected’ amide protons of RNase‐(1–118) over RNase A. A calculation shows a large increase in the rate of ‘opening’ of the structure. The exchange rate of the protected amide protons of RNase‐(1–120) is slower than RNase‐(1–118) but much faster than RNase A. Binding with a synthetic complementing fragment (114–124) markedly reduces the exchange rate of 20 to 25 amide protons of RNase‐(1–118). It has previously been shown that binding with a complementing fragment of RNase‐(1–118) generates a lowest Gibbs energy state. Thus, using available thermodynamic information for interpretation, we suggest that a) removal of six carboxy terminal residues of RNase A would disrupt coupling between these residues and those distant in the structure (loss of extra stabilizing energy), b) this would, in turn, alter the enthalpy‐entropy compensation in such a way that the magnitude of Gibbs energy change favoring folding is significantly reduced without a large change of fold and c) in this activated state the molecule would be highly motile.