The nickel-pincer nucleotide (NPN) is a novel metallocofactor required for lactate racemase and similar activities. The name pincer derives from the metal ion being tri-coordinated in a planar orientation. These complexes are common in synthetic organic chemistry; however, the NPN cofactor is the first nickel pincer complex to be identified in Nature. Since its discovery in 2015, much work has been done to improve our understanding of the function and biosynthesis of this cofactor. Three enzymes, LarB, LarE, and LarC are involved in the biosynthesis of this cofactor. LarB adds a second carboxyl group to the pyridinium ring of the precursor nicotinic acid adenine dinucleotide (NaAD) and hydrolyzes the phosphoanhydride bond to form the product, pyridinum-3,5-biscarboxylic acid mononucleotide. LarE adds two molecules of sulfur which results in pyridinium-3,5-bisthiocarboxylic acid mononucleotide (P2TMN). Finally, LarC completes the synthesis of the mature NPN cofactor by inserting the nickel ion. Previously, LarC was shown to be a CTP dependent enzyme, but the function of this cofactor was not clear. Through mass spectrometry analysis and activity assay, I discovered a reaction intermediate, CMP-P2TMN, which provides insight on the role of CTP in the reaction. I speculate that the function of this adduct is to position the substrate correctly for the metal ion insertion. Working toward a better understanding of this process, I have obtained a preliminary cryo-electron microscopic protein structure of LarC from Moorella thermoacetica. The NPN biosynthetic pathway and larA genes are found in almost a quarter of the analyzed prokaryotic genomes. The current process used to screen the functionality of these predicted homologs uses an in vitro method that is time consuming and error prone. I developed an efficient alternative method to confirm the roles of biosynthesis protein homologs and to generate active NPN-containing proteins by implementing a co-expression system in genetically tractable Escherichia coli.
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