Abstract
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.
This review summarizes our present knowledge of the lactate racemase system of Lactobacillus plantarum, composed of a lactate racemase, three biosynthetic enzymes, a transcriptional regulator, a nickel transporter and a lactic acid channel, and deduces its possible roles in lactic acid bacteria.
Mechanisms for the activation of water, ammonia, and other small molecules by the PC
carbene
P nickel pincer complex were studied computationally with the aid of density functional theory. The ...calculation results indicate that the strongly donating, nucleophilic carbene center can engage in a variety of heterolytic splitting of E−H (E=H, C, N, O) bonds, some of which are reversible. The cleavage of E−H bonds across the Ni=C bond represents a new mode of bond activation by ligand cooperativity in nickel pincer complex. On the basis of the calculations, we also demonstrate that reversible H
2
activation across the Ir=C bond via the PC
carbene
P iridium pincer complex was observed in the experiments, while other E−H (E=C, N, O) bonds were not activated. Our calculations are in good agreement with experimental observations and could provide new insights into ligand cooperativity in nickel pincer complexes.
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