G. Charles Dismukes
Structure-Function Studies of Metal Cluster Enzymes
Photosynthesis and the Environment
a. Water splitting enzyme. The mechanism of water oxidation which is responsible for all of the dioxygen in our atmosphere is being investigated at the atomic level using spectroscopic methods: electron spin resonance spectroscopy to examine the paramagnetic radicals produced during the photochemical charge separation events, NMR spectroscopy to identify the protein coordination environment surrounding the catalytic sites.
b. Water oxidation catalysts. Synthesis of functionally active catalysts for non-photosynthetic water oxidation would greatly benefit the environment by enabling use of dioxygen instead of air in many industrial processes and combustions. There are no commercially available catalysts for this process. Our approach is based on mimicking the structure of the active site of the photosynthetic enzyme which splits water into dioxygen and protons.
Human Immunodeficiency Virus type 1(HIV-1)
The human immunodeficiency virus type 1(HIV-1) is the etiologic agent of AIDS. Replication of the HIV-1 virus, and hence infection, requires DNA synthesis by a retrovirus-encoded RNA-dependent DNA polymerase, or reverse transcriptase (RT). HIV-1-RT is a multi-functional enzyme required, not only, for the synthesis of the double-stranded proviral DNA from the single-stranded retroviral RNA genome, but also, cleavage of the retroviral RNA polymer in the form of a hybrid DNA-RNA intermediate (ribonuclease H or RNAase H activity) that allows transcription of the RNA fragments to proceed. We are working with the pharmaceutical industry to characterize the pair of metal ions that comprise the catalytic site of the RNAase subdomain and several mutant proteins. A second project involves the HIV-1 integrase, an enzyme responsible for integration of the virus' DNA into the host's genome. One of the important goals is to kill the AIDS virus by developing specific inhibitors of the metal sites of these enzymes.
Nature has provided us with two different classes of enzymes, called catalases, which protect us against the destructive effects of hydrogen peroxide which forms during normal cellular biochemistry to an appreciable extent. We study the rare catalase produced by thermophillic bacteria. It has a novel binuclear Mn center at its active site. We are working on the mechanism of catalysis using magnetic resonance methods. This research is aimed at developing both a fundamental understanding of catalysis and a practical application to develop high temperature enzymes for applications in bleaching. The insights discovered from the enzyme studies are being used to synthesize small molecule catalysts for selective oxidations.
The dimanganese enzyme, arginase, is responsible for metabolism of the amino acid L-arginine, a product of protein metabolism. It may also be linked to formation of the intracellular messenger nitric oxide. We are exploring the mechanism of catalysis of this hydrolysis reaction using biochemical and magnetic resonance techniques.
We have a research program in the synthesis of metal complexes as functional models for the enzymes described above. The oxidative class of these enzyme mimics are being developed as potential catalysts for applications in environmentally clean oxidations such as for chlorine-free domestic laundry, paper/pulp bleaching and hydrocarbon oxidations. As an example, we have recently synthesized the first example of the [Mn4O4]6+ cubane to test as a potential model of the photosynthetic water oxidation enzyme. The cubane core is stabilized by coordination of six bidentate diphenylphosphinate ligands between pairs of Mn ions across each of the six faces of the cube.