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The Fe-heme cofactor "biochip" performs an incredible range of biological
functions, from oxygen and electron transport to signal transduction and
multi-electron redox catalysis. Because of the sluggish redox response
of proteins, measurements are typically obtained by slow, equilibrium titrations
which can't differentiate kinetic or reorganizational factors. We are using
new fast electrochemical techniques to initiate and follow multi-electron
protein catalysis. Likewise, we are developing photoactive hemeproteins
as a way to look at very short-lived species generated during redox transformations.
Our intent is to use these techniques to address long-standing issues in
bioinorganic chemistry about redox control and catalysis at heme sites:
for example, how does the heme coordination environment affect and control
redox transformations in the time domain; and what are the sequential steps
in heme-based oxidoreductase catalysis, such as in the multi-electron reduction
of dioxygen.
We are interested in both reductive and oxidative transformations at the heme active sites. Reductive catalysis involves multiple-electron reductive cleavage of substrate-oxygen bonds while still Fe-bound, ultimately yielding reduced substrates and water. Examples of such biocatalysts are nitrite and sulfite reductases, or cytochrome c oxidase. Oxidative catalysis results from high-valent heme intermediates formed by direct reaction with hydrogen peroxide as in the peroxidases or, paradoxically, by reduction of dioxygen as in cytochrome P450.
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