The majority of molybdenum-containing enzymes utilise a molybdenum cofactor (MoCF or Moco) consisting of a Mo atom coordinated via a cis-dithiolene moiety to molybdopterin (MPT). MoCF is ubiquitous in nature, and the pathway for MoCF biosynthesis is conserved in all three domains of life. MoCF-containing enzymes function as oxidoreductases in carbon, nitrogen, and sulphur metabolism [1, 2].

In Escherichia coli, biosynthesis of MoCF is a three stage process. It begins with the MoaA and MoaC conversion of GTP to the meta-stable pterin intermediate precursor Z. The second stage involves MPT synthase (MoaD and MoaE), which converts precursor Z to MPT; MoeB is involved in the recycling of MPT synthase. The final step in MoCF synthesis is the attachment of mononuclear Mo to MPT, a process that requires MoeA and which is enhanced by MogA in an Mg2 ATP-dependent manner [3]. MoCF is the active co-factor in eukaryotic and some prokaryotic molybdo-enzymes, but the majority of bacterial enzymes requiring MoCF, need a modification of MTP for it to be active; MobA is involved in the attachment of a nucleotide monophosphate to MPT resulting in the MGD co-factor, the active co-factor for most prokaryotic molybdo-enzymes. Bacterial two-hybrid studies have revealed the close interactions between MoeA, MogA, and MobA in the synthesis of MoCF [4]. Moreover the close functional association of MoeA and MogA in the synthesis of MoCF is supported by fact that the known eukaryotic homologues to MoeA and MogA exist as fusion proteins: CNX1 (Q39054) of Arabidopsis thaliana (Mouse-ear cress), mammalian Gephryin (e.g. Q9NQX3) and Drosophila melanogaster (Fruit fly) Cinnamon (P39205) [5].

This domain is found in molybdopterin cofactor oxidoreductases, such as in the C-terminal of Mo-containing sulphite oxidase, which catalyses the conversion of sulphite to sulphate, the terminal step in the oxidative degradation of cysteine and methionine [6]. This domain is involved in dimer formation, and has an Ig-fold structure [7].

1. Cell biology of molybdenum. Biochim. Biophys. Acta 1763, 621-35
2. Molybdenum and tungsten in biology. Trends Biochem. Sci. 27, 360-7
3. Mutational analysis of Escherichia coli MoeA: two functional activities map to the active site cleft. Biochemistry 46, 78-86
4. In vivo interactions between gene products involved in the final stages of molybdenum cofactor biosynthesis in Escherichia coli. J. Biol. Chem. 277, 48199-204
5. Molybdenum co-factor biosynthesis: the Arabidopsis thaliana cDNA cnx1 encodes a multifunctional two-domain protein homologous to a mammalian neuroprotein, the insect protein Cinnamon and three Escherichia coli proteins. Plant J. 8, 751-62
6. Molecular basis of sulfite oxidase deficiency from the structure of sulfite oxidase. Cell 91, 973-83

The majority of molybdenum-containing enzymes utilise a molybdenum cofactor (MoCF or Moco) consis
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