Analysis of lactate racemase (Lar) in lactic acid bacteria (LAB) has been a scientific challenge for many years, as indicated by the numerous contradictory reports on this activity. Recently, genetic and biochemical studies of the Lar system of Lactobacillus plantarum have unveiled the complexity of this particular enzymatic system. Lar activity is associated with LarA and its nickel-containing cofactor, synthesized from nicotinic acid adenine dinucleotide by the three biosynthetic enzymes: LarB, LarC, and LarE. In addition to these core Lar enzymes, a nickel transporter (Lar(MN)QO), a lactic acid channel (LarD) and a transcriptional regulator (LarR) which promotes expression of the lar genes in the presence of excess L-lactate are also part of the Lar system of Lb. plantarum and of many other LAB. These proteins promote racemization of external L-lactate, in addition to carrying out intracellular racemization. This additional outcome suggests that racemization of L-lactate is not only required for cell wall biosynthesis, as reported before, but may have additional roles in lactate production and utilization in LAB. Finally, bioinformatics analyses indicate that some Lar homologs probably catalyze reactions other than lactate racemization.

中文翻译：

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乳酸菌的乳酸外消旋化系统中出乎意料的复杂性。

多年来，乳酸菌（LAB）中乳酸消旋酶（Lar）的分析一直是一项科学挑战，有关该活性的许多相互矛盾的报道都表明了这一点。最近，对植物乳杆菌Lar系统的遗传和生化研究揭示了这种特定酶系统的复杂性。Lar活性与LarA及其含镍辅因子有关，LarA及其含镍辅因子是由烟酸腺嘌呤二核苷酸通过三种生物合成酶LarB，LarC和LarE合成的。除了这些核心Lar酶外，还存在镍转运蛋白（Lar（MN）QO），乳酸通道（LarD）和转录调节剂（LarR），它们在过量L-乳酸存在下促进lar基因的表达。 Lb Lar系统的一部分。plantarum和许多其他LAB。除了进行细胞内消旋作用外，这些蛋白质还促进外部L-乳酸的消旋作用。这一额外的结果表明，L-乳酸的消旋作用不仅是细胞壁生物合成所必需的（如先前报道的那样），而且可能在乳酸菌的生产和利用中具有额外的作用。最后，生物信息学分析表明，某些Lar同源物可能催化除乳酸外消旋作用以外的其他反应。