Document Type

Dissertation

Degree

Doctor of Philosophy (PhD)

Major/Program

Biomedical Sciences

First Advisor's Name

Prof. Barry P Rosen

First Advisor's Committee Title

Committee chair

Second Advisor's Name

Dr. Masafumi Yoshinaga

Second Advisor's Committee Title

Co-Committee chair

Third Advisor's Name

Dr. Jeremy Chambers

Third Advisor's Committee Title

Committee member

Fourth Advisor's Name

Prof. Yong Cai

Fourth Advisor's Committee Title

Committee member

Fifth Advisor's Name

Dr. Konstantinos Kavallieratos

Fifth Advisor's Committee Title

Committee member

Keywords

ArsI, C-As lyase, roxarsone, MSMA, organoarsenical herbicides, organoarsenical growth promoters, type I extradiol dioxygenases, arsenite, mass spectrometry, HPLC

Date of Defense

6-26-2017

Abstract

Organoarsenicals such as methylarsenical methylarsenate (MAs(V)) and aromatic arsenicals including roxarsone (4-hydroxy-3-nitrophenylarsenate or Rox(V)) have been extensively used as an herbicide and growth enhancers in animal husbandry, respectively. They undergo environmental degradation to more toxic inorganic arsenite (As(III)) that contaminates crops and drinking water. We previously identified a bacterial gene (arsI) responsible for aerobic MAs(III) demethylation. The gene product, ArsI, is a Fe(II)-dependent extradiol dioxygenase that cleaves the carbon-arsenic (C-As) bond in MAs(III) and trivalent aromatic arsenicals. The objective of this study was to elucidate the ArsI mechanism. Using isothermal titration calorimetry, we determined the dissociation constants (Kd) and ligand-to-protein stoichiometries (N) of ArsI for Fe(II), MAs(III) and aromatic phenyl arsenite. Using a combination of methods including chemical modification, site-directed mutagenesis, and fluorescent spectroscopy, we demonstrated that amino acid residues predicted to participate in Fe(II)-binding (His5-His62-Glu115) and substrate binding (Cys96-Cys97) are all involved in catalysis. Finally, the products of Rox(III) degradation were identified as As(III) and 4-hydroxy-2-nitrophenol, demonstrating that ArsI is a dioxygenase that incorporates one oxygen atom from dioxygen into the carbon and the other to the arsenic to catalyze the cleavage of the C-As bond. These results augment our understanding of the mechanism of this novel C-As lyase.

Identifier

FIDC001926

ORCID

orcid.org/0000-0003-2651-810X

Available for download on Friday, July 13, 2018

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