Date of Award:

12-2010

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Chemistry and Biochemistry

Advisor/Chair:

Scott A. Ensign

Abstract

Enantiopure alcohols are valuable intermediates in fine organic synthesis, in particular for preparation of biologically active compounds. The necessity of preparing single enantiomer drugs in an optically pure form has triggered much research, especially in the pharmaceutical industry. The biocatalytical production of chiral alcohols by alcohol dehydrogenase enzymes is characterized by the asymmetric reduction of the corresponding ketones, usually with high degree of stereoselectivity. The commercial value of the enzymes as stereoselective biocatalysts has been a significant driving force in understanding features that control their mechanism of catalysis and stereoselectivity. This work focuses on two enantiocomplementary dehydrogenase enzymes ((R)- and 2-(S)-hydroxypropyl-CoM (HPC) dehydrogenases (DH)) of the epoxide carboxylation pathway in Xanthobacter autotrophicus strain Py2. The main goal of this dissertation is to kinetically, mechanistically and structurally characterize S-HPCDH and through the comparison studies with R-HPCDH reveal the basis for high degree of stereoselectivity exhibited by both enzymes. Analysis of the molecular structure of R-HPCDH and the homology model of S-HPCDH suggests a mechanism of substrate specificity in which the binding of the substrate sulfonate moiety at distinct sites on each stereoselective enzyme directs the orientation of the appropriate substrate enantiomer for the hydride abstraction. The positively charged residues responsible for binding the CoM moiety of the substrate were identified in R-HPCDH (Arg152 and Arg196), and in S-HPCDH (Arg211 and Lys214). Site-directed mutagenesis confirmed their importance in binding and orienting physiological substrates, but not the substrates lacking the CoM moiety. Extensive kinetic and mechanistic characterization of S-HPCDH reveals its key catalytic features similar to those of R-HPCDH, but also points out a few important differences. Furthermore, the role of the methionine residues flanking the substrate in the active site of both dehydrogenases was investigated. Substitution of these residues to alanine resulted in enzymes with significantly altered catalytic parameters and suggested their importance in binding and catalysis. Additionally, the X-ray crystal structures of the Met187Ala and Met192Ala mutants of R-HPCDH have revealed their role as "gate keepers," protecting the active site from the surrounding solvent. Kinetic analysis of Met187Leu and Met192Leu mutants implied a structural, rather than catalytic function of the methionines. It is proposed that steric clashes of the terminal methyl group of the HPC substrates with the nicotinamide ring of NAD+ are a major determinant of the enantioselectivity in S-HPCDH. This research provides the first side-by-side characterization of a pair of short-chain dehydrogenase/reductase (SDR) enzymes expressed simultaneously to act on two enantiomers of the same alcohol produced in a metabolic pathway. The R-HPCDH and S-HPCDH enzymes are distinguished from all other known members of the SDR family in using the novel sulfonate functional group of coenzyme M as a handle for chiral discrimination. These results provide a standard for examining the molecular basis of stereoselectivity in other such enzyme pairs.

Comments

This work made publicly available electronically on October 1, 2010.

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