Bovine Seminal Ribonuclease: Non-hyperbolyc Kinetics in the Second Reaction Step

Bovine seminal ribonuclease is a dimeric enzyme with interchain disulfides linking the 2 subunits. It can be isolated from bull seminal plasma or from bull seminal vesicles, the organ where it is produced. The primary structure of the subunit chain is strictly homologous to that of pancreatic RNase A from the same species. Especially interesting is the observation that the amino acid residues which were found to interact with the substrate at the active site of RNase A are all conserved at identical sequence positions in BS-RNase. Hence, it is not surprising that BS-RNase has the same 2-step mode of action (transphosphorolysis followed by hydrolysis of the cyclic phosphate) and the same bond specificity as RNase A. Investigating both the reaction steps catalysed by BS-RNase with appropriate model substrates, we found that the enzyme displays non-hyperbolic saturation curves, but only for the substrate of the second, rate-limiting step of reaction. This phenomenon appeared overlooked in for the limited range of substrate concentrations used. Non-hyperbolic saturation curves have been occasionally reported for a small number of oligomeric enzymes, but to our knowledge never for a dimeric enzyme.