Bovine seminal ribonuclease (BS-RNase) is the only known dimeric enzyme characterized by an equilibrium between two different 3D structures: MxM, with exchange (or swapping) of the N-terminal 1–20 residues, and M¼M, without exchange. As a consequence, the hinge region 16–22 has a different tertiary structure in the two forms. In the native protein, the equilibrium ratio between MxM and M¼M is about 7 : 3. Kinetic analysis of the swapping pro-cess for a recombinant sample shows that it folds mainly in the M¼M form, then undergoes interconversion into the MxM form, reaching the same 7 : 3equilibrium ratio. Bovine seminal ribonuclease (BS-RNase) is the only known dimeric enzyme characterized by.
