o-Succinylbenzoate synthase (OSBS) from Amycolatopsis, a member of the
enolase superfamily, catalyzes the Mn2+-dependent exergonic
dehydration of 2-succinyl-6R-hydroxy-2,4-cyclohexadiene-1R-carboxylate (
SHCHC) to 4-(2'-carboxylphenyl)-4-oxobutyrate (
o-succinylbenzoate or OSB) in the
menaquinone biosynthetic pathway. This
enzyme first was identified as an N-acylamino
acid racemase (NAAAR), with the optimal substrates being the enantiomers of
N-acetyl methionine. This laboratory subsequently discovered that this
protein is a much better catalyst of the OSBS reaction, with the value of k(cat)/K(M), for
dehydration, 2.5 x 10(5) M(-1) s(-1), greatly exceeding that for 1,1-proton transfer using the enantiomers of
N-acetylmethionine as substrate, 3.1 x 10(2) M(-1) s(-1) [Palmer, D. R., Garrett, J. B., Sharma, V., Meganathan, R., Babbitt, P. C., and Gerlt, J. A. (1999) Biochemistry 38, 4252-8]. The efficiency of the promiscuous NAAAR reaction is enhanced with alternate substrates whose structures mimic that of the
SHCHC substrate for the OSBS reaction, for example, the value of k(cat)/K(M) for the enantiomers of N-succinyl phenylglycine, 2.0 x 10(5) M(-1) s(-1), is comparable to that for the OSBS reaction. The mechanisms of the NAAAR and OSBS reactions have been explored using mutants of Lys 163 and Lys 263 (K163A/R/S and K263A/R/S), the putative
acid/base catalysts identified by sequence alignments with other OSBSs, including the structurally characterized OSBS from Escherichia coli. Although none of the mutants display detectable OSBS or NAAAR activities, K163R and K163S catalyze stereospecific exchange of the alpha-
hydrogen of N-succinyl-(S)-phenylglycine with
solvent hydrogen, and K263R and K263 catalyze the stereospecific exchange the alpha-
hydrogen of N-succinyl-(R)-phenylglycine, consistent with formation of a Mn2+-stabilized enolate
anion intermediate. The rates of the exchange reactions catalyzed by the wild-type
enzyme exceed those for racemization. That this
enzyme can catalyze two different reactions, each involving a stabilized enediolate
anion intermediate, supports the hypothesis that evolution of function in the
enolase superfamily proceeds by pathways involving functional promiscuity.