期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2016
卷号：113
期号：20
页码：5598-5603
DOI：10.1073/pnas.1600486113
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：The lactate racemase enzyme (LarA) of Lactobacillus plantarum harbors a (SCS)Ni(II) pincer complex derived from nicotinic acid. Synthesis of the enzyme-bound cofactor requires LarB, LarC, and LarE, which are widely distributed in microorganisms. The functions of the accessory proteins are unknown, but the LarB C terminus resembles aminoimidazole ribonucleotide carboxylase/mutase, LarC binds Ni and could act in Ni delivery or storage, and LarE is a putative ATP-using enzyme of the pyrophosphatase-loop superfamily. Here, we show that LarB carboxylates the pyridinium ring of nicotinic acid adenine dinucleotide (NaAD) and cleaves the phosphoanhydride bond to release AMP. The resulting biscarboxylic acid intermediate is transformed into a bisthiocarboxylic acid species by two single-turnover reactions in which sacrificial desulfurization of LarE converts its conserved Cys176 into dehydroalanine. Our results identify a previously unidentified metabolic pathway from NaAD using unprecedented carboxylase and sulfur transferase reactions to form the organic component of the (SCS)Ni(II) pincer cofactor of LarA. In species where larA is absent, this pathway could be used to generate a pincer complex in other enzymes.
