Muconate lactonizing enzymes (, 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. Some bacterial species are also capable of dehalogenating chloroaromatic compounds by the action of chloromuconate lactonizing enzymes. MLEs consist of several strands which have variable reaction favorable parts therefore the configuration of the strands affect its ability to accept protons. The bacterial MLEs belong to the enolase superfamily, several structures from which are known. 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. The reaction mechanism that MLEs undergo are the reverse of beta-elimination in which the enolate alpha-carbon is protonated. MLEs can undergo mutations caused by a deletion of catB structural genes which can cause some bacteria to lose its functions such as the ability to grow. Additional mutations to MLEs can cause its structure and function to alter and could cause the conformation to change therefore making it an inactive enzyme that is unable to bind its substrate. There is another enzyme called Mandelate Racemase that is very similar to MLEs in the structural way as well as them both being a part of the enolase superfamily. They both have the same end product even though they undergo different chemical reactions in order to reach the end product.
Structure
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. There are three classes of MLEs, them being the bacterial MLEs, bacterial CMLEs, and eukaryotic MLEs/CMLEs. The bacteria MLEs are made up of a TIM barrel and are arranged in the Syn stereochemistry, while the bacterial CMLEs are arranged in the Anti stereochemistry. When it comes to Eukaryotic MLEs/CMLEs, they are arranged in the Syn stereochemistry. Eukaryotic MLEs/CMLEs have not had a sequence similarity to any other families of enzymes, but the bacteria MLEs have been found to be similar to the Enolase superfamily, and the bacterial CMLEs are similar to the Class II fumarases.
The structure itself consists of two protein molecules consisting of a chain of amino acids and two chemicals being the two Magnesium ions.
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.