Relationships between nonhyperbolic kinetics and dimeric structure in ribonucleases.

Dimers of bovine pancreatic RNase A give nonhyperbolic saturation curves for the substrate of the second, rate-limiting step of the reaction. Under the same conditions, the monomeric native enzyme shows Michaelis-Menten kinetics. Naturally dimeric bovine seminal RNase, which has been found to give nonhyperbolic saturation curves, loses this property upon monomerization. It is proposed that when RNase monomers are arranged in a quaternary structure, they assume a conformation which enables them to be modulated in their catalytic activities. A correlation is suggested between this effect and the quaternary structure proposed for both of these dimeric ribonucleases, in which composite active sites are generated by the mutual exchange of the NH2-terminal ends of the two monomers.