Side-chain interactions governing the pairing of half-cystine residues in ribonuclease.

Fully reduced ribonuclease (RNase), devoid of demonstrable secondary or tertiary structure, may be oxidized by molecular oxygen to yield a product which is indistinguishable by physical measurements or enzymatic activity from the native enzyme (2-4). Since reduced RNase contains eight sulfhydryl groups, 105 possible arrangements with four disulfide bridges may occur (5, 6), yet the native enzyme can be recovered in nearly quantitative yields after such oxidations. It is apparent, then, that certain interactions among elements of the primary structure must serve as guides for the unique pairing of sulfhydryl groups. An attempt has been made to study these interactions by carrying out oxidations of reduced RNase in the presence of various reagents known to influence interor intramolecular bonding. Enzymatically inactive derivatives were produced, quite similar to native RNase in physical properties, which could, under appropriate circumstances, be rearranged to the native enzyme. These materials are believed to be a mixture representing some, if not all, of the possible isomeric configurations of disulfide bonding in the molecule. The observation that this mixture of materials can be readily converted to the native structure is taken as evidence that the unique secondary and tertiary structure of RNase is, thermodynamically, the most stable configuration.