Date of Award

4-15-2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Dr. Giovanni Gadda - Committee Chair

Second Advisor

Dr. Al Baumstark - Committee Member

Third Advisor

Dr. Dabney White Dixon - Committee Member

Abstract

Choline oxidase (E.C. 1.1.3.17) from Arthrobacter globiformis catalyzes the four-electron oxidation of choline to glycine betaine (N,N,N-trimethylglycine) via two sequential, FAD-dependent reactions in which betaine aldehyde is formed as an enzyme-bound intermediate. In each oxidative half-reaction, molecular oxygen acts as electron acceptor and is converted into hydrogen peroxide. Biochemical, structural, and mechanistic studies on the wild-type and a number of mutant variants of choline oxidase have recently been carried out, allowing for the depiction of the mechanism of alcohol oxidation catalyzed by the enzyme. Catalysis by choline oxidase is initiated by the removal of the hydroxyl proton of alcohol substrate by a catalytic base in the enzyme-substrate complex, yielding the formation of the alkoxide species. In this dissertation, the roles of His351 and conserved His466 were investigated. The results presented demonstrate that His351 is involved in the stabilization of the transition state for the hydride transfer reaction and contributes to substrate binding. His466 is likely to be a catalytic base in choline oxidase due to its dramatic effect on enzymatic activity. Comparison of choline oxidase and other enzymes within its superfamily reveals the presence of a conserved His-Asn pair within the active site of enzymes. Therefore, the role of the conserved Asn510 in choline oxidase was examined in this study. The results presented here establish the importance of Asn510 in both the reductive and oxidative half-reactions. The lost of ability to form a hydrogen bond interaction between the side chain at position 510 with neighboring residues such as His466 resulted in a change from stepwise to concerted mechanism for the cleavages of OH and CH bonds of choline, as seen in the Asn510Ala mutant. Finally, the steady-state kinetic mechanism of pyranose 2-oxidase in the pH range from 5.5 to 8.5 was investigated. It was found that pH exerts significant effects on enzyme mechanism. This study has established the involvement of the residues in the initiation of enzyme catalysis and the stabilization of the alkoxide intermediate in choline oxidase. In addition, this work demonstrates the first instance in which the kinetic mechanism of a flavin-dependent oxidase is governed by pH.

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Chemistry Commons

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