Date of Award

12-2021

Degree Name

Master of Science

Department

Chemistry

First Advisor

Dr. Frederick Stull

Second Advisor

Dr. David Huffman

Third Advisor

Dr. Todd Barkman

Keywords

Enzymology, oxidative stress resistance, RcIA, HOSCN, bacteria

Access Setting

Masters Thesis-Open Access

Abstract

Reactive chlorine species (RCS) - hypochlorous acid (HOCl), hypothiocyanous acid (HOSCN), and chloramines - are powerful antimicrobial oxidants generated by the innate immune system. Bacteria developed defense mechanisms against RCS, which are highly toxic for invading pathogens. One such defense system involves RclA, an enzyme from the flavin disulfide reductase family, which is massively upregulated upon exposure of Escherichia coli to RCS. Recent research suggests that RclA functions as a Cu(II) reductase to provide resistance against RCS. However, the exact mechanism used by RclA was subject to debate. In this study, we demonstrate that RclA is unlikely to function as a Cu(II) reductase. Instead, we show that RclA provides resistance against RCS by directly detoxifying HOSCN, reducing it to non-toxic thiocyanate (SCN-) using NAD(P)H via a ping-pong kinetic mechanism. RclA catalyzes the reduction of HOSCN with kcat of 182 s-1, and the specificity constant (kcat/Km) of 9.12 x 107 M-1 s-1, indicating that RclA is a potent and highly specific HOSCN reductase. HOSCN is a primarily sulfhydryl-reactive oxidant, and RclA has two conserved cysteine residues at its active site. Thus, we hypothesize that RclA detoxifies HOSCN before it can target other thiols in the cell, and the enzyme’s active site cysteines are critical for its HOSCN reductase activity. Our study provides a rationale for how bacteria can cope with RCS produced by the immune system. Targeting RclA in bacteria found in oral cavity or lung airway fluids may make pathogenic bacteria more susceptible to the immune response, thus allowing the creation of new strategies for controlling pathogenic infections.

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