Function On a large scale, MLEs catalyze the bacterial β-ketoadipate pathways by catabolizing aromatic lignin found in plants to intermediates found in the Krebs Cycle. Some MLEs can be halogenated with Cl and perform slightly different functions in the microbe. Halogenated MLEs can remove Cl from halogenated aromatics allowing for the decomposition of 2,4-dichlorophenoxyacetate. This unique function allows for these microbes to be used in bioremediation, decreasing the toxicity in herbicide infested soil. More specifically, MLEs have multiple strands. Some strands contain more reaction favorable portions according to Quantum Mechanics/Molecular Mechanics (QM/MM) analysis. The second strand of the MLE contains a basic residue that allows for proton acceptance on the Lys-162 or Lys-168 dependent upon which configuration it is in, anti or syn respectively. It is supported that the basic residues on the second strand are used in the formation of an energy surface versus the sixth strand. MLEs found in Mycobacterium smegmatis are anti-MLEs meaning they produce an anti-product of muconolactone (muconate lactone) while Pseudomonas fluorescens uses syn-MLEs to produce a syn-product. Mechanism and Action Muconate lactonizing enzymes (MLEs) have the opposite type of reaction mechanism compared to Mandelate racemase (MR), it being the reverse of beta-elimination. Therefore, an alpha-carbon of the enolate gets protonated instead of deprotonated. But this protonation is a thermodynamically favorable step in the reaction. Also, just as in MR, in MLEs the formation of the enolate intermediate still is the central catalytic problem therefore being the rate limiting step. Moreover, MLEs can facilitate catalysis by attaching the substrate and therefore increasing the nucleophilicity of the carboxylate in order to produce lactone. Muconate lactonizing enzyme actions to catalyze same 1,2 addition-elimination reaction. This can be done with or without a metal cofactor. In the soil microbes, Cis, cis- muconates (Substrate) is converted into muconolactones (product) by MLEs. This chemical reaction is part of β-ketoadipate pathway, an aerobic catabolic pathway, which breaks down aromatic compounds like lignin to an intermediate in citric acid cycle . β-ketoadipate pathway has two main branches :- 1) catechol branch and 2) protocatechuate branch. Catechol branch consists of cis, cis- muconate lactonizing enzyme, whereas a protocatechuate branch consists of carboxy -cis, cis-mucontate lactonizing enzyme. Both the reactions form Succinate + acetyl CoA, which leads into the citric acid cycle. On the other hand, Mandelate racemaseactions to catalyze the inversion of configuration at the alpha-carbon by creating a carbanionic intermediate. Mutation The