Evolution of (Beta/alpha)8 Barrels: Refinement of Enzymatic Efficiency for a New Ortho-Succinylbenzoate Synthase (Osbs) From a Promiscuous Progenitor
Vick, Jacob E.
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https://hdl.handle.net/2142/72334
Description
Title
Evolution of (Beta/alpha)8 Barrels: Refinement of Enzymatic Efficiency for a New Ortho-Succinylbenzoate Synthase (Osbs) From a Promiscuous Progenitor
Author(s)
Vick, Jacob E.
Issue Date
2008
Doctoral Committee Chair(s)
Gerlt, John A.
Department of Study
Biochemistry
Discipline
Biochemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Biochemistry
Abstract
To explore how an enzyme can gain a new function, the L-Ala-D/L-Glu epimerase (AEE) from Escherichia coli was chosen as the starting point for the development of a "new" o-succinylbenzoate synthase (OSBS). AEE and OSBS are members of the enolase superfamily that have a conserved enzyme structure including a (beta/alpha)8-barrel "catalytic domain" and a N-terminal "capping domain" for substrate specificity. Members of the enolase superfamily all catalyze reactions that share the abstraction from a carbon alpha to a carboxylic acid to produce a divalent metal stabilized enolate anion intermediate.
Using structural alignments, the D297G mutation was found to allow a low level of OSBS activity while reducing the efficiency of the AEE reaction [Schmidt, D. M. Z. et al. (2003) Biochemistry 42, 8387--8393]. The D297G mutant allowed an 8 x 107 improvement in catalytic rate by allowing access of the 2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylic acid (SHCHC) substrate by removing unfavorable steric and charge interactions. Anaerobic selection of an error-prone library identified the I19F mutant that allowed for an additional 12-fold increase in catalytic efficiency. The I19F mutant is located in the 20s loop of the capping domain that sequesters the active site from solvent [Vick, J. E., Schmidt, D. M. Z., and Gerlt, J. A. (2005) Biochemistry 44, 11722--11729]. Additional anaerobic selection identified the R24C and L277W mutants that each increased the OSBS efficiency of the I19F/D297G mutant [Vick, J. E. and Gerlt, J. A. (2007) Biochemistry 46, 14589--14598]. These mutants appear to function by altering the interaction between I19F and the substrate/product. Using site-specific randomization, R24W, a single base pair change from R24C, was identified as the greatest improvement of catalytic efficiency to date.
This work allowed a step-wise pathway of changing base pairs to be elucidated that illustrates that a minimal amount of mutations (four from wild-type to I19F/R24W/D297G) are necessary to develop a new function. Additionally, the "new" function of OSBS activity was achieved with changes to substrate specificity, the substitutions allowed the catalytic barrel to remain "hard-wired" to perform acid/base chemistry as both the AEE and OSBS reactions utilized the same catalytic lysines.
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