Protease activity of 1,10-phenanthroline-copper(I). Targeted scission of the catalytic site of carbonic anhydrase

To investigate the mechanism of scission of proteins by the chemical cleaving agent 1,10-phenanthroline-copper, the active sires of human carbonic anydrase I and bovine carbonic anhydrase II have been targeted for cleavage by a tight binding: sulfonamide inhibitor tethered to the metal complex. The inhibitor-phenanthroline-copper conjugate binds to the carbonic anhydrases with sub-micromolar K-d's and, upon addition of a reducing agent, causes scission specifically within the active site of the enzymes to yield a discrete set of cleavage fragments. N- and C-terminal sequencing and mass spectrometric analysis of several fragments indicate that the C-terminal cleavage fragments have free amino groups al their N termini, thereby allowing facile location of the cut sites through standard Edman degradation. The N-terminal cleavage fragments do not have a free carboxyl group at their C termini. It is proposed that scission occurs by abstraction of H at C-alpha, followed by oxidation at C-alpha by the neighboring cupric ion and cleavage of the C-alpha-C(O) bond to give an N-terminal fragment containing a C-terminal acyl amide, and an unstable C-terminal fragment containing an N-terminal isocyanate group which undergoes hydrolysis to a free amino terminus. Modeling of the inhibitor-phenanthroline-copper conjugate within the active site of human carbonic anhydrase I shows that the sites of cleavage that have been identified are fully consistent with the available structural data.