Natural and chemically induced oligomeric ribonucleases: structural study by immobilized metal ion affinity...

Oligomerization can endow proteins with novel structural and catalytic properties. The native dimer of bovine seminal ribonucleases (BS-RNase) binds, melts and catalyses the hydrolysis of double-stranded ribonucleic acids 30-fold better than its pancreatic homologue, the monomeric RNase A. Chemically induced oligomers of pancreatic RNase A are also found to show an increase in enzyme activity on double-stranded poly(A).poly(U) (Libonati, M. Bertoldi, M. and Sorrentino, S. (1996) Biochem. J. 318, 287-290) and, therefore, can be considered as potential immunosuppressive and cytotoxic agents. We report here a study on the relationship between surface histidine topography in oligomeric forms of these ribonucleases and their catalytic properties. Subtle changes in structure conformation of both BS-RNase and oligomeric RNase A are shown to result in a modification of the affinity of these proteins toward the immobilized transition-metal chelate, IDA-Cu(II). Because, such conformational change has been shown to correlate with an improvement of the newly acquired biological activities upon oligomerization, we can conclude that surface histidines topography constitutes an exquisite probe for the study of protein structure/function relationship.