Effect of molybdate and tungstate on the biosynthesis of CO dehydrogenase and the molybdopterin cytosine‐dinucleotide‐type of molybdenum cofactor in Hydrogenophaga pseudoflava

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Abstract
The molybdenum‐containing iron‐sulfur flavoprotein CO dehydrogenase is expressed in a catalytically fully competent form during heterotrophic growth of the aerobic bacterium Hydrogenophaga pseudoflava with pyruvate plus CO. We have adopted these conditions for studying the effect of molybdate (Mo) and tungstate (W) on the biosynthesis of CO dehydrogenase and its molybdopterin (MPT) cytosine‐dinucleotide‐(MCD)‐type molybdenum cofactor. W was taken up by the Mo transport system and, therefore, interfered with Mo transport in an antagonistic way. Depletion of Mo from the growth medium as well as inclusion of excess W both resulted in the absence of intracellular Mo and led to the biosynthesis of CO dehydrogenase species of proper L2M2S2 subunit structure that carried the two 2Fe : 2S type‐I and type‐II centers and two FAD molecules. EPR, ultraviolet/visible and CD spectroscopies established the full functionality of the cofactors. Due to the absence of the Mo‐MCD cofactor, the enzyme species were catalytically inactive. Unexpectedly, the following cytidine nucleotides were present in inactive CO dehydrogenase : CDP, dCDP, CMP, dCMP, CTP or dCTP. The sum of cytidine nucleotides was two/mol enzyme. The binding specificities of inactive CO dehydrogenase for cytidine nucleotides (oxy > deoxy; diphosphate > monophosphate > triphosphate), and the absence of MPT suggest that, in active CO dehydrogenase, the cytidine diphosphate moiety of Mo‐MCD provides the strongest interactions with the protein and determines the specificity for the type of nucleotide. In H. pseudoflava, the biosynthesis of MPT (identified as form A) was independent of Mo. Mo was, however, strictly required for the conversion of MPT to MCD (identified as form‐A−CMP) as well as the insertion of Mo‐MCD into CO dehydrogenase. These data support a model for the involvement of Mo in the biosynthesis of the Mo‐MCD cofactor and of fully functional CO dehydrogenase in which the synthesis and insertion of Mo‐MCD require Mo, and protein synthesis including integration of the FeS‐centers and FAD are independent of Mo.

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