Structure and spectroscopy of metallo-lactamase active sites

This study clarifies the relationship between the active-site geometry of the bimetallic enzyme, zinc-beta-lactamase, and the electronic spectra of the cobalt-substituted enzyme. Ab initio quantum methods were used to study both the structure and the spectroscopy of the active sites of metallo-beta-lactamases. Theoretically optimized structures for the cobalt-substituted enzyme indicate that the coordination number of the two cobalt atoms remains the same as those of the zinc atoms in the crystal structure. Transition energies and intensities for the bimetallic active site were calculated by multiconfiguration self-consistent-field methods and identified with a metal center. The visible spectrum line positions and intensities calculated using correlation energy corrections derived from second-order perturbation calculations on model systems were in good agreement with the experimental data, demonstrating that the first shell of ligands determines the spectra. Visible transition energies are predicted at both metal centers, but the calculated intensities suggest that contributions from the four-coordinate site dominate the visible spectrum. The charge-transfer excitation from the cysteine to the open-shell cobalt cation results in at least 20 ligand-to-metal charge-transfer (LMCT) lines. The lowest two energy transitions are assigned to the observed 330-nm absorption, which is usually attributed experimentally to the entire LMCT transition. However, the intensities of the higher energy LMCT transitions are predicted to be much more intense.