Role of XDHC in Molybdenum cofactor insertion into xanthine dehydrogenase of Rhodobacter capsulatus.

Rhodobacter capsulatus xanthine dehydrogenase (XDH) is composed of two subunits, XDHA and XDHB. Immediately downstream of xdhB, a third gene was identified, designated xdhC, which is cotranscribed with xdhAB. Interposon mutagenesis revealed that the xdhC gene product is required for XDH activity. However, XDHC is not a subunit of active XDH, which forms an alpha2beta2 heterotetramer in R. capsulatus. It was shown that XDHC neither is a transcriptional regulator for xdh gene expression nor influences XDH stability. To analyze the function of XDHC for XDH in R. capsulatus, inactive XDH was purified from an xdhC mutant strain. Analysis of the molybdenum cofactor content of this enzyme demonstrated that in the absence of XDHC, no molybdopterin cofactor MPT is present in the XDHAB tetramer. In contrast, absorption spectra of inactive XDH isolated from the xdhC mutant revealed the presence of iron-sulfur clusters and flavin adenine dinucleotide, demonstrating that XDHC is not required for the insertion of these cofactors. The absence of MPT from XDH isolated from an xdhC mutant indicates that XDHC either acts as a specific MPT insertase or might be a specific chaperone facilitating the insertion of MPT and/or folding of XDH during or after cofactor insertion.