pK_a Determination of Enzyme Bound Coenzyme M With Epoxyalkane: CoM Transferase

Author

• Matthew Stephen McIff, Utah State University

Date of Award

5-2001

Degree Type

Thesis

Degree Name

Departmental Honors

Department

Chemistry and Biochemistry

Abstract

Epoxide metabolism in the aerobic bacterium Xanthobacter strain Py2 proceeds by a coenzyme M (2-mercaptoethanesulfonic acid), NADPH, and NAD⁺ dependent carboxylation reaction that form beta keto acids as products. Epoxide carboxylase, the enzyme catalyzing this reaction, was resolved into four protein components that are required for functional reconstitiution of epoxide carboxylase activity. Component I, the first step in epoxide carboxylation, catalyzes the conversion of R and S-epoxypropane into Rand S-hydroxypropyl-coenzyme M respectively. This reaction consists of an epoxide ring opening, nucleophilic addition of coenzyme M, and oxygen reduction. Coenzyme M, a compound previously found only in the methanogenic Archaea, has been identified as the thiol and central cofactor of aliphatic epoxide carboxylation. Coenzyme M is bound to a Zn metal center on the CoM transferase and serves as a metabolite carrier molecule for further oxidation in the micro metabolic pathway. Free CoM has a pK_a within the range of 9-10 which would render it less effective as a nucleophile under physiological conditions. However, the involvement of CoM in epoxide metabolism suggests that the Zn bound thiolate is stabilized at physiological pH. In this study a pH rate profile was performed where the apparent K_m was determined within a pH range. The pK_a value of the enzyme bound CoM was found to be 7.4. This result suggests that the enzyme binds CoM, through interaction with Zn, and lowers its pK_a. Another pK_a determined at 9.2 suggests that there is an ionizing residue on the CoM transferase also involved in the reaction. It is proposed that this residue is a proton responsible for charge stabilization of the intermediate.

Recommended Citation

McIff, Matthew Stephen, "pK_a Determination of Enzyme Bound Coenzyme M With Epoxyalkane: CoM Transferase" (2001). Undergraduate Honors Capstone Projects. 1074.
https://digitalcommons.usu.edu/honors/1074