Lactate racemase

The lactate racemase enzyme is involved in pyruvate metabolism. It is classified under the isomerase, racemase, epimerase, and enzyme acting on hydroxyl acids and derivatives classes of enzymes. It is found in certain halophilic archaeons, 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, Lactobacillus plantarum, as well as in non-lactic acid bacteria such as Clostridium beijerinckii.

Structure and Properties
The molecular weight of lactate racemase (Lar) differs in the various organisms in which it has been found, ranging from 25,000 to 82,400 g/mol. One molecule of the enzyme contains two iron atoms. The optimal pH for its activity is 5.8-6.2 in L. sake.

Few biochemical studies have been performed on the enzyme because it is highly sensitive to oxidation. The gene sequence encoding the enzyme is not known from any species, and the N-terminal sequence has not been determined.

Enzyme Activity
Lactate racemase catalyzes the inter-conversion of (S)- and (R)-lactate.

In many of the species it is found in, the enzyme acts on the substrate D-lactate and produces L-lactate. There are a few exceptions, such as Lactobacillus plantarum, in which L-lactate is the substrate and D-lactate is produced.

In C. beijerinckii, evidence exists that lactate racemase appears to work through a mechanism that includes an α-carbonyl intermediate that is bound covalently to a sulfhydryl group on the enzyme. Lactate racemase, when purified from L. sake, did not require any cofactors and did not display any lactate dehydrogenase activity. The enzyme was found to be inhibited by AMP, oxamate, atebrin, adenosine monosulfate, cyanide, and some iron-chelating agents4.

The reaction catalyzed by the enzyme was found to reach equilibrium at the point when equimolar concentrations of the D- and L-isomers existed.

In L. plantarum, L-lactate is initially produced, which induces the activity of lactate racemase. Lactate racemase is only expressed in the presence of L-lactate. When the activity of the enzyme is induced by such a presence, D-lactate can be produced. Ultimately, nearly equal amounts of D- and L-lactic acid are produced, allowing for equilibrium to be reached. In this species, lactate racemase activity is repressed by D-lactate. The activity of the lar operon appears to be positively regulated by L-lactate. Therefore, Lar activity appears to be regulated by the ratio of L-lactate/D-lactate.

The six-gene operon (the lar operon) identified in L. plantarum appears to have an association with lactate racemization. The six genes that make up the operon were named larA, larB, larC1, larC2, glpF1, and larE.

Importance
Two pathways appear to exist in L. plantarum for pyruvate to be reduced to lactate. One of them involves the NAD-dependent lactate dehydrogenase that produces D-lactate (LdhD), and the other is through the activity of 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. This 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.