We use modern methods of quantum chemistry (density functional methods) to investigate the electronic structures, energetics, and spectroscopy of transition metal complexes at the active sites of metalloproteins. These systems include iron-sulfur and iron-oxo complexes, as well as active site models for manganese and iron superoxide dismutases. Many of these complexes lie at the active sites of important metalloproteins, where they are involved in electron transfer and catalysis. Two important aspects of our work are the extensive use of quantum mechanical geometry optimization for the active site quantum cluster, and the electrostatic representation of complete protein and solvent environments.
The protein electrostatics calculations and analysis are conducted in collaboration with D. Bashford and D. Asthagiri of this department. We are also working with their group in studying the energetics of catalytic dephosphorylation by protein tyrosine phosphatases, important in cell signaling and regulation. In collaboration with F. Gryszpan, calculations of acidities of substituted phenols were used in the design of haptens to generate catalytic antibodies with phosphatase activity, our goal being the neutralization of nerve gases and insecticide toxins.
We also collaborate with E. Getzoff and M. Thompson of this department in the study of photoactive yellow protein (PYP). This contains a chromophore, which undergoes a light cycle, and so acts as a protein photosensor. In a related context, we are studying the mechanism of the blue fluorescence of stilbene when this molecule is bound to an antibody, following on experimental work in the laboratories of R. Lerner, D. Millar, and R. Stevens, and in collaboration with F. Salsbury and C. Brooks. Also, with K. Hahn (Cell Biology), we have begun to investigate improvements in organic fluorescent dyes, which can act as reporters of protein conformational changes or reactions in cells.
We have calculated electronic structures and optimized geometries for active site structures of the iron-oxo enzymes methane monooxygenase (MMO) and ribonucleotide reductase (RNR) in both the oxidized and the two-electron reduced forms. Our extensive studies of structures and energetics for oxidized and reduced MMO have now been submitted for publication. For the two-electron reduced forms, a comparative study of geometries and energetics of native RNR, a mutant RNR, and native MMO shows how the various alternate conformations of the bridging carboxylate ligand (called "the carboxylate shift") arise from different protein environments. These alternative structures constitute a "branch point" for the subsequent reactions with molecular oxygen, and so influence the different catalytic chemistries of these enzymes.
Because iron-sulfur proteins are often electron transfer agents, we seek to understand which features of cluster electronic structure and what interactions with the protein environment determine the wide range of redox potentials in these systems. In some important cases, the redox state and protonation states of the active site are coupled. The Rieske iron-sulfur protein is an important part of the cytochrome bc1 complex, which links electron transfer from ubiquinone to cytochrome c to proton pumping across the inner mitochondrial membrane. The pH dependence of the redox potential of the Rieske iron-sulfur protein is under study using density functional and electrostatics methods.
We are continuing electronic structure calculations for the iron-molybdenum cofactor of the nitrogenase enzyme. This is the 7Fe1Mo active site of the iron-molybdenum protein which binds molecular nitrogen and protons, and catalyzes the multi-electron reduction of molecular nitrogen to two ammonia molecules plus molecular hydrogen. We have calculated the active site geometry, electronic structure, energy and metal spin alignment for a number of likely cluster oxidation and electronic states. Using also observations from Mossbauer spectroscopy, we have identified the probable states which correspond to the "resting form" of the enzyme and the likely candidates for the one-electron reduced and oxidized states. This is a required starting point for studying the catalytic cycle of the enzyme.
Manganese, iron, and c
er-zinc superoxide dismutases
(SOD's) are important
detoxifying agents for superoxide radical anions, converting these
to molecular oxygen and hydrogen peroxide.
Recently, we have
calculated quantum mechanically optimized geometries and
redox potentials, including the coupling between electron
transfer and proton transfer,
for the active sites of
manganese and iron superoxide dismutases, and compared these with
redox potentials derived from reaction kinetics analysis.
Last update: 3/27/99 by TL Comments to lou@scr
s.edu