Lactate Racemization and Beyond

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.