Muconate lactonizing enzymes (EC 5.5.1.1, muconate cycloisomerase I, cis,cis-muconate-lactonizing enzyme, cis,cis-muconate cycloisomerase, 4-carboxymethyl-4-hydroxyisocrotonolactone lyase (decyclizing), CatB, MCI, MLE, 2,5-dihydro-5-oxofuran-2-acetate lyase (decyclizing)) are involved in the breakdown of lignin-derived aromatics, catechol and protocatechuate, to citric acid cycle intermediates as a part of the β-ketoadipate pathway in soil microbes.

[2][3][4] MLEs have an identifying structure made up of two proteins and two Magnesium ions as well as various classes depending on whether it is bacterial or eukaryotic.

[9][10] The structure of the Muconate lactonizing enzymes (MLEs) consists of a seven-bladed beta propeller with various modifications based on the type of class that it belongs to.

[11] Muconate lactonizing enzymes (MLEs) have the opposite type of reaction mechanism compared to Mandelate racemase (MR), it being the reverse of beta-elimination.

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.

[8]  A microorganism named Pseudomona putida loses its ability to grow due to the deletion in catB gene for muconate lactonizing enzyme.

The point mutation that resulted into the variants, Ser271Ala and Ile54Val, for Cl-muconating enzyme showed a significant decrease in the dehalogenation activity.

[1] One advantage of the mutation in muconate lactonizing enzyme from Asp to Asn or from Glu to Gln is that it can help to understand the effect of the metal ligand on the catalytic process and the binding site.

Even though Muconate lactonizing enzyme and Mandelate Racemase catalyse different chemical reactions that is required by Pseudomonas putida for the catabolic processes, they are structurally very similar to one another.

Based on the Nature International Journal of Science, this characteristic of homologous structure is said to have been evolved from a common ancestor.

[9] According to the book, Enzymatic Mechanism, by Perry A. Frey and Dexter B. Northrop, Muconate lactonizing enzymes and Mandelate Racemase are both the member of enolase superfamily.

Even though both the enzyme are different in their chemical reactions, they both have the same end product that leads to extracting a proton from an alpha carbon to a carboxylate ion.