Lactate Racemization and Beyond

Abstract

Enzymatic racemization of lactate has been reported in several bacterial species, including Lactobacillus species. The role of lactate racemase (Lar) is still a matter of debate and is probably dependent if the species in which it is found is a lactate producer, a lactate consumer, or both. A transcriptomic experiment revealed the involvement of two operons of 9 genes in lactate racemization in L. plantarum: the larR (MN) QO and the larABCDE operons. The lactate racemase, LarA, has been shown to harbour a tethered nickel pincer complex, which we call Nickel Pincer Nucleotide or NPN in this review. This cofactor seems well adapted to catalyse lactate racemisation by a hydride transfer mechanism. The cofactor is synthesized from nicotinic acid adenine dinucleotide by the NPN biosynthetic enzymes: LarB, LarC, and LarE. LarD is an aquaglyceroporin, LarR a transcriptional regulator, and Lar (MN) QO a three-component nickel transporter. Lactate racemase gene was reported to be widespread in bacterial and archaeal genomes. We suggest that many other enzymatic functions are present in the LarA superfamily of enzymes in addition to lactate racemization. The Lactate Racemization Operon Enzymatic racemization of lactate was first reported in 1936 in Clostridium beijerinckii (formerly C. butylicum) [1] and was then detected in Staphylococcus aureus (formerly S. ureae), in Lactobacillus sakei (formerly L. sake) [2], in L. plantarum [3], and in several other Lactobacillus sp. [4], as well as in the rumen bacteria Selenomonas ruminantium and Megasphaera elsdenii [5,6] and in halophilic archaea [7]. The role of lactate racemase (Lar) is still a matter of debate and is probably dependent on the species in which it is found. In bacteria consuming lactate, like M. elsdenii, Lar enables the consumption of both Land D-lactate isomers, despite the presence of only one D-or L-lactate dehydrogenase [6]. In lactic acid bacteria, the presence of a lactate racemase is less straightforward, as lactate is a waste product of fermentation and its racemization is not required for growth. In L. plantarum, Lar was suggested to be used as a rescue pathway for the formation of D-lactate, which is incorporated in the cell wall and confers to the bacteria a resistance to the vancomycin antibiotic [8,9]. This role can be extended to the growth conditions in which the bacteria perform malolactic fermentation, producing only L-lactate [10]. However, the presence of the lactic acid channel LarD suggests that this is probably not the only role Lar fulfils in lactic acid bacteria. Lactate racemization of externally produced L-lactate in D-lactate consuming bacteria could be a way for the lactate racemizing bacteria to generate a proton gradient across its cell wall, if a hypothetical transporter of L-lactate was present in these species [10]. The identity of the Lar enzyme remained unknown until a transcriptomic experiment performed in 2014 revealed the involvement of two operons of 9 genes in lactate racemization in L. plantarum [9,11]. These genes were shown to encode a nickel transport system Lar(MN)QO, a nickel-dependent lactate racemase LarA, a set of three proteins of unknown function LarB, LarC and LarE necessary for the maturation of the racemase, a lactic acid channel LarD [12], and a transcriptional regulator LarR responding to the enantiomeric excess of L-lactate [13]. These proteins constitute the lactate racemase system found in L. plantarum and in many other Lactobacillus species (Figure 1) [10]. Figure 1: The Lar system of L. plantarum.