Date of Award

1986

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Abstract

The biological activation of hydrogen is catalyzed by hydrogenases. An understanding of the mechanism by which hydrogenases act requires the elucidation of the nature and type of metal centers present in the enzymes. Room temperature UV-visible absorption, low temperature magnetic circular dichroism and electron paramagnetic resonance spectroscopies have been used to characterize the electronic and magnetic properties of the iron-sulfur and nickel centers in hydrogenases from Clostridium pasteurianum, Methanobacterium thermoautotrophicum, and Desulfovibrio gigas. In addition, another Ni-containing enzyme, carbon monoxide dehydrogenase from Clostridium thermoaceticum, and D. gigas Ni-substituted rubredoxin were investigated. The results provide evidence for the presence of conventional ferredoxin type 4Fe-4S centers in the bidirectional and uptake Fe-only hydrogenases from C. pasteurianum. They also indicate that the hydrogen activating sites in these enzymes are novel Fe-S clusters that differ in both their magnetic and electronic properties from those characterized thus far. Spectroscopic studies of M. thermoautotrophicum and D. gigas hydrogenases have facilitated characterization of the type and properties of multiple Fe-S centers in these enzymes. In addition, low temperature MCD spectroscopy is shown to be uniquely capable of investigating and characterizing the electronic transitions associated with paramagnetic Ni in hydrogenases. Evidence is presented to show that a monomeric Ni center is the active site in these hydrogenases and that four different paramagnetic Ni species can be observed during the activation cycle. The role and redox states of Ni in the catalytic mechanism are discussed in light of the spectroscopic investigations. It is proposed that the hydrogen binding site is a Ni(III) center that undergoes a redox change upon photolytic cleavage of the Ni-H bond at low temperatures. Spectroscopic studies of the metal centers in C. thermoaceticum CO dehydrogenase show the presence of multiple 4Fe-4S clusters in addition to a novel cluster which could be a mixed Ni-Fe-S cluster. Low temperature MCD studies of D. gigas Ni-substituted rubredoxin reveal the electronic and magnetic properties that should be expected for Ni(II) in a biological tetracoordinated S rich environment.

Pages

338

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