Our research concerns the role and assembly of transition metal centers in metalloenzymes and metalloproteins. Metal centers constitute the active sites of at least one third of all enzymes and determining the assembly mechanism of metallocenters and the electronic and structural properties of metal centers that confer selective and specific catalytic activity present fascinating challenges to inorganic chemists. In our work we use a range of spectroscopic techniques to probe the characteristic properties of transition metal centers, i.e. color, paramagnetism, etc. These include electron paramagnetic resonance (EPR), resonance Raman, FTIR, and UV/visible/near-IR absorption and natural and magnetically induced circular dichroism (CD and MCD). We also collaborate with other groups for Mossbauer, electron-nuclear double resonance (ENDOR), X-ray absorption and X-ray crystallography studies of proteins prepared in our laboratory. The information content of these techniques is often complementary and leads to a detailed information concerning the electronic, magnetic, and structural properties of metal centers imbedded in a large polypeptide macromolecule. The goal is detailed molecular-level understanding of the mechanism of assembly and/or the role of the metal center(s) in catalysis, regulation or electron transfer.
Our research program currently focuses primarily on the assembly and functions of biological iron-sulfur centers, clusters of non-heme iron and inorganic sulfide involving [Fe2S2], [Fe3S4], [Fe4S4], and [Fe8S7] core units that are generally attached to the protein via cysteinate ligation. Fe-S clusters are present in more than 300 different types of enzymes or proteins and play crucial roles in fundamental life processes such as respiration, photosynthesis and nitrogen fixation. As such they constitute one of the most ancient, ubiquitous and structurally and functionally diverse classes of biological prosthetic groups. Consequently the process of Fe-S cluster biosynthesis is essential to almost all forms of life and is remarkably conserved in prokaryotic and eukaryotic organisms. Moreover, a molecular-level understanding of iron-sulfur cluster biogenesis is crucial for understanding a variety of human diseases involving anemias, myopathies and ataxias that arise from defects in Fe-S cluster biogenesis proteins. Research on Fe-S cluster biosynthesis in the Johnson group currently involves investigating the molecular mechanisms of Fe-S cluster assembly on scaffold proteins and the subsequent intact cluster transfer to acceptor or carrier proteins. In addition we are investigating the mechanistic roles of Fe-S clusters in two rapidly emerging classes of Fe-S enzymes involved with disulfide reduction and radical generation. Funding for this work is provided by the National Institutes of Health.
