Redox titrations of the nickel hydrogenase of Desulfovibrio gigas /
The hydrogenase from Desulfovibrio giaas catalyzes the
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| Format: | Thesis Book |
| Language: | English |
| Published: |
[Place of publication not identified] :
[publisher not identified] ;
1994.
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| Subjects: | |
| Online Access: | http://proxy.library.tamu.edu/login?url=http://proquest.umi.com/pqdweb?did=741965801&sid=1&Fmt=2&clientId=2945&RQT=309&VName=PQD |
| Summary: | The hydrogenase from Desulfovibrio giaas catalyzes the reversible oxidation Of H2 to protons. The active site is a nickel center with unprecedented redox and magnetic properties. It is found in four magnetic states termed Ni-B, NI-SI, Ni-C, and Ni-R. Ni-B and NI-SI almost certainly correspond to Ni3+ and-Ni2+ respectively. The electronic designations of Ni-C and Ni-R are not known with certainty. The Ni-C state, in the absence of H2,was found to be stable for over 40 hr. This result was surprising since numerous experiments found the Ni-C state to be unstable in the absence of H2. This instability was used as evidence that Ni-C was the catalytic intermediate that reacted with protons to form H2- Our results demonstrate that the Ni-C state is not equilibrium with H2, nor is it the active reductant. The active reducing agent of protons is probably the Ni-R state. Stoichiometric redox titrations of the enzyme were performed with the goal of determining the electronic states of Ni-C and Ni-R. Titrations were monitored at A410 as well as with EPR, simulated using various models that differed in the number of equivalents separating the Ni-B state from Ni-C. Simulations that fit the experimental data best used a model that assumed Ni-C was two electrons mor e reduced than Ni-B. From this analysis, the possible isoelectronic designations of the Ni-C are: (i) a Ni3+Hspecies; (ii) a Nil+H+ species; (iii) a Ni3+fl2-H2 species; and (iv) Ni2+L* species. The enzyme also contains one [Fe3S4)1+/O and two [Fe4S4 ]2+/l+ clusters. An unusual EPR signal, termed the g=2.21 signal, was also followed during the titrations. Analysis revealed that this signal arises from the magnetic interaction of the Ni-C species with the [Fe4S4]1+ cluster with the more negative redox potential. Analysis also suggests that the redox status of this particular cluster controls the reversibility of catalysis. Finally, the most popular mechanism of catalysis was reformulated into a tricyclic mechanism, using the concept of redox microstates. Further insights into the mechanism of catalysis are discussed. |
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| Item Description: | Vita. "Major subject: Chemistry". |
| Physical Description: | xii, 146 leaves : illustrations ; 28 cm. Issued also on microfiche from University Microfilms Inc. |
| Bibliography: | Includes bibliographical references. |