Lactate racemase
Encyclopedia
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
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, Lactobacillus plantarum, as well as in non-lactic acid bacteria such as Clostridium beijerinckii.
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.
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.
. 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.
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
is involved in pyruvate metabolism. It is classified under the isomerase
Isomerase
In biochemistry, an isomerase is an enzyme that catalyzes the structural rearrangement of isomers. Isomerases thus catalyze reactions of the formwhere B is an isomer of A.-Nomenclature:...
, racemase, epimerase, and enzyme acting on hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
acids and derivatives classes of enzymes. It is found in certain halophilic
Halophile
Halophiles are extremophile organisms that thrive in environments with very high concentrations of salt. The name comes from the Greek for "salt-loving". While the term is perhaps most often applied to some halophiles classified into the Archaea domain, there are also bacterial halophiles and some...
archaea, such as Haloarcula marismortui and in a few species of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, such as several Lactobacillus
Lactobacillus
Lactobacillus is a genus of Gram-positive facultative anaerobic or microaerophilic rod-shaped bacteria. They are a major part of the lactic acid bacteria group, named as such because most of its members convert lactose and other sugars to lactic acid. They are common and usually benign...
species (which produce D- and L-lactate
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
), 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 pHPH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
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
Lactate dehydrogenase
Lactate dehydrogenase is an enzyme present in a wide variety of organisms, including plants and animals.Lactate dehydrogenases exist in four distinct enzyme classes. Two of them are cytochrome c-dependent enzymes, each acting on either D-lactate or L-lactate...
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
Operon
In genetics, an operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo trans-splicing to create...
(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 lactateLactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
. 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.