Function of MoaB Proteins in the Biosynthesis of the Molybdenum and Tungsten Cofactors

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Abstract
Molybdenum (Mo) and tungsten (W) enzymes catalyze important redox reactions in the global carbon, nitrogen, and sulfur cycles. Except in nitrogenases both metals are exclusively associated with a unique metal-binding pterin (MPT) that is synthesized by a conserved multistep biosynthetic pathway, which ends with the insertion and thereby biological activation of the respective element. Although the biosynthesis of Mo cofactors has been intensively studied in various systems, the biogenesis of W-containing enzymes, mostly found in archaea, is poorly understood. Here, we describe the function of the Pyrococcus furiosus MoaB protein that is homologous to bacterial (such as MogA) and eukaryotic proteins (such as Cnx1) involved in the final steps of Mo cofactor synthesis. MoaB reconstituted the function of the homologous Escherichia coli MogA protein and catalyzes the adenylylation of MPT in a Mg2+ and ATP-dependent way. At room temperature reaction velocity was similar to that of the previously described plant Cnx1G domain, but it was increased up to 20-fold at 80 °C. Metal and nucleotide specificity for MPT adenylylation is well conserved between W and Mo cofactor synthesis. Thermostability of MoaB is believed to rely on its hexameric structure, whereas homologous mesophilic MogA-related proteins form trimers. Comparison of P. furiosus MoaB to E. coli MoaB and MogA revealed that only MogA is able to catalyze MPT adenylylation, whereas E. coli MoaB is inactive. In summary, MogA, Cnx1G, and MoaB proteins exhibit the same adenylyl transfer activity essential for metal insertion in W or Mo cofactor maturation.

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