Publication Date

5-2017

Date of Final Oral Examination (Defense)

11-11-2016

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Chemistry

Department

Chemistry

Major Advisor

Rajesh Nagarajan, Ph. D.

Advisor

Henry A. Charlier, Ph.D.

Advisor

Eric Brown, Ph.D.

Abstract

Resistance to antibiotics has become a major challenge in today’s society for treating bacterial infections. Inhibition of quorum sensing has a potential to be a non-antibiotic based therapeutic that could be used to fight these bacterial infections. Quorum sensing is a cell density dependent, intercellular communication mechanism that bacteria use to synchronize behavior such as virulence and resistance to antibiotics. If this switch from planktonic to communal behavior can be inhibited, the bacteria will be less virulent. One possible way to accomplish this is by inhibiting the enzymes that are responsible for making the quorum sensing signaling molecules in Gram-negative bacteria – acyl-homoserine lactone (AHL) synthases. Since AHL synthases are mostly uncharacterized, understanding how these enzymes recognize its acyl-substrate would be beneficial to designing effective quorum sensing inhibitors. The focus of this thesis is to investigate the substrate recognition mechanism in BjaI, an acyl-homoserine lactone synthase found in soybean symbiont Bradyrhizobium japonicum. BjaI was chosen because it can be used to highlight the differences between acyl-ACP vs acyl-CoA utilizing AHL synthases. We found that all of our single point mutations within either of the substrate binding pockets, S-adenosyl-L-methionine and isovaleryl-CoA, were detrimental to enzyme activity. Kinetic constants were measured for the native and other similar non-native acyl-CoAs as well as their respective alkyl-CoA inhibitors. For too long (> six carbons) and too short (< four carbons) acyl-CoAs, we found that BjaI rejected nonspecific substrates at the binding step. However, for substrates that are structurally similar to isovaleryl-CoA, BjaI uses a combination of both the binding and catalytic steps to reject the nonspecific substrate. The tools used in this study should open new doors to designing effective quorum sensing inhibitors.

Available for download on Wednesday, May 22, 2019

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