The lactate racemase enzyme (Lar) (EC 5.1.2.1) interconverts the D- and L-enantiomers of lactic acid. It is classified under the isomerase, racemase, epimerase, and enzyme acting on hydroxyl acids and derivatives classes of enzymes.^[1] It is found in certain halophilic archaea, such as Haloarcula marismortui, and in a few species of bacteria, such as several Lactobacillus species (which produce D- and L-lactate) including Lactobacillus sakei, Lactobacillus curvatus, and Lactobacillus plantarum, as well as in non-lactic acid bacteria such as Clostridium beijerinckii. ^[2] The gene encoding lactate racemase in L. plantarum was identified as larA and shown to be associated with a widespread maturation system involving larB, larC1, larC2, and larE.^[3] The optimal pH for its activity is 5.8-6.2 in L. sakei.^[4]

Structure and properties

The molecular weight of lactate racemase differs in the various organisms in which it has been found, ranging from 25,000 to 82,400 g/mol.^[5] The structure of the enzyme from L. plantarum was solved by Jian Hu and Robert P. Hausinger of Michigan State University and co-workers there and elsewhere.^[6] The protein contains a previously unknown covalently-linked nickel-pincer nucleotide (NPN) cofactor (pyridinium 3-thioamide-5-thiocarboxylic acid mononucleotide), where the nickel atom is bound to C4 of the pyridinium ring and two sulfur atoms. This cofactor participates in a proton-coupled hydride-transfer mechanism.^[7]

There have been a number of recent studies on NPN cofactor synthesis by the LarB, LarE, and LarC proteins. LarB is a carboxylase/hydrolase of nicotinamide adenine dinucleotide (NAD), providing pyridinium-3,5-dicarboxylic acid mononucleotide and adenosine monophosphate (AMP).^[8] LarE is an ATP-dependent sulfur transferase that converts the two substrate carboxyl groups into thioacids by sacrificing the sulfur atoms of a cysteine residue in the protein.^[9] Finally, LarC inserts nickel into the organic ligand by a CTP-dependent process to complete synthesis of the NPN cofactor.^[10]

Enzyme activity

In many of the species containing lactate racemase, the physiological role of the enzyme is to convert substrate D-lactate into L-lactate. In other species, such as L. plantarum, the cellular role is to transform L-lactate into D-lactate for incorporation into the cell wall.^[2]

The in vitro reaction catalyzed by the enzyme reaches equilibrium at the point where approximately equimolar concentrations of the D- and L-isomers exist.^[4]

L. plantarum initially produces L-lactate, which induces the activity of lactate racemase. By contrast, D-lactate represses lactate racemase activity in this species. Therefore, Lar activity appears to be regulated by the ratio of L-lactate/D-lactate. L. plantarum LarA represents a new type of nickel-dependent enzyme, due to its novel nickel-pincer ligand ligand cofactor.^[6]

Importance

Two pathways appear to exist in L. plantarum for transforming pyruvate into D-lactate. One of them involves the NAD-dependent lactate dehydrogenase that directly produces D-lactate (LdhD), and the other is through the sequential activities of an L-specific lactate dehydrogenase followed by lactate racemase. If the LdhD enzyme is inactivated or inhibited, lactate racemase provides the bacterium with a rescue pathway for the production of D-lactate.^[2] This pathway is significant because the production of D-lactate in L. plantarum is linked to the biosynthesis of the cell wall. Mutants lacking LdhD activity that also had the lar operon deleted only produced L-lactate, and peptidoglycan biosynthesis was not able to occur.

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