Date of Completion

2015

Document Type

Honors College Thesis

Department

Biochemistry

Type of Thesis

Honors College, College of Arts and Science Honors

First Advisor

Robert Hondal

Keywords

selenium, selenocysteine, seleno effect, thio effect, thioredoxin reductase

Abstract

The purpose of my research is to determine if the substitution of selenium for sulfur will cause rate acceleration of the thiol-disulfide exchange reaction in the C-terminal redox center of Plasmodium falciparum thioredoxin reductase (PfTR) in a position specific manner. P. falciparum is the protist that causes most serious form of malaria and is responsible for the majority of malaria related deaths. While mammalian thioredoxin reductase (mTR) contains selenium in its C-terminal redox center, which accelerates the rate of reaction, PfTR functions in the absence of selenium. The results of my studies show that the introduction of selenium via selenocysteine to the C-terminal redox center of PfTR impacts the rate of this reaction positively when substituted at Cys540 and negatively at Cys535. This data can be explained mechanistically by the use of a disulfide electrophilic activation mechanism by PfTR. Specifically, the disulfide bond between Cys540 and Cys535 in the C-terminal redox center of PfTR is primed for attack by the thiolate of Cys93 of the N-terminal redox center because of a hydrogen bonding interaction between Cys535 and an adjacent histidine residue. This interaction polarizes the Cys540–Cys535 disulfide, making the sulfur atom of Cys540 highly electrophilic. The differential effects on rate caused by selenium substitution of each of the two sulfur atoms of the C-terminal disulfide represents a “seleno effect”, similar to the observed “thio effect” in phosphodiesterases when sulfur substitutes for oxygen in non-bridging oxygen atoms of phosphodiesters. Thioredoxin reductase is essential in all organisms and its loss of function is deadly for the host. Due to the differences in structure between PfTR and mTR, the mechanism proposed represents a potential drug target in the fight against malaria.

Creative Commons License

Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License.

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