The History of the Molybdenum Cofactor-A Personal View

Abstract

The transition element molybdenum (Mo) is an essential micronutrient for plants, animals, and microorganisms, where it forms part of the active center of Mo enzymes. To gain biological activity in the cell, Mo has to be complexed by a pterin scaffold to form the molybdenum cofactor (Moco). Mo enzymes and Moco are found in all kingdoms of life, where they perform vital transformations in the metabolism of nitrogen, sulfur, and carbon compounds. In this review, I recall the history of Moco in a personal view, starting with the genetics of Moco in the 1960s and 1970s, followed by Moco biochemistry and the description of its chemical structure in the 1980s. When I review the elucidation of Moco biosynthesis in the 1990s and the early 2000s, I do it mainly for eukaryotes, as I worked with plants, human cells, and filamentous fungi. Finally, I briefly touch upon human Moco deficiency and whether there is life without Moco.

Keywords: gephyrin; molybdenum; molybdenum cofactor biosynthesis; molybdopterin; nitrate reductase.

Figure 1

Structures of PrecursorZ, molybdopterin (MPT), Moco, and their derivatives. PrecursorZ is cyclic pyranopterin monophosphate (cPMP), and CompoundZ is its air-oxidized product. MPT and Moco are both 5,6,7,8-tetrahydropyranopterins. FormB is the air-oxidized degradation product of MPT and Moco, whereas FormA is the iodine-oxidized derivative. FormA and FormB are shown in their phosphorylated forms. Urothione is the degradation product of Moco in humans, which is excreted in the urine.

Figure 2

Models for the Moco biosynthesis pathway. (a) Model for the Moco biosynthesis pathway in E. coli, published by the Rajagopalan group in 1992. The proteins are named according to their functions. Modified after [81]. (b) Model for the Moco biosynthesis pathway in plants, published by Mendel in 1992 [84]. The proteins are named in the Cnx nomenclature. CnxA is the molybdate insertase. DHNPT stands for dihydro neopterin triphosphate. Nia is the designation for the nitrate reductase monomer. Modified after [84].

Figure 3

Pathway of Moco biosynthesis in eukaryotes. The pathway can be divided into four steps, each being characterized by its main features as given in italics on the right side. The names for the proteins from the plant Arabidopsis thaliana (green), humans (red), and E. coli (black) catalyzing the respective steps are given. In GTP, the C8 atom of the purine is labeled with a star. This carbon is inserted between the 2′ and 3′ ribose carbon atoms, thus forming the new C1′ position in the four-carbon side chain of the pterin (labeled with a star in cPMP). The in vivo sulfur (X-S) source for Cnx5 and MOCS3 is probably cysteine. Steps three and four in eukaryotes are catalyzed by the individual domains of Cnx1 (G and E) or Gephyrin (G and E). Functional properties such as [Fe–S] clusters in Cnx2 and Mocs1A, the use of S-adenosyl methionine (SAM), adenylation, and sulfuration of the small subunit of MPT synthase (Cnx7 and Mocs2B, respectively) are indicated. In Cnx5, MoeBD denotes the MoeB-like domain and RLD the rhodanese-like domain. Modified after Mendel and Kruse [93].

Figure 4

Model for the Moco biosynthesis pathway in plants, published by Schwarz et al. in the year 2000 [129]. Cnx1 is shown to be bound to an actin filament. An unidentified molybdate transport system was proposed that interacts with Cnx1 to facilitate molybdate channeling to the E domain, given that a mutation in this protein results in a molybdate-repairable phenotype. The conversion of precursorZ cPMP) to MPT by the MPT synthase (Cnx6 and Cnx7) is shown. MPT is highly sensitive to oxidation; therefore, it was suggested that the rapid conversion of precursorZ to Moco should occur in a multienzyme complex anchored by Cnx1 on the cytoskeleton. Modified after Schwarz et al. [129].

Figure 5

Present model for the biosynthesis, distribution, and maturation of Moco in plant cells. Biosynthesis starts with the conversion of GTP to cPMP in the mitochondria. The dependence of Cnx2 on [Fe–S] is indicated. The transporter Atm3 is assumed to be involved in the export of cPMP to the cytosol [97], where MPT-synthase, consisting of Cnx6 and Cnx7, is sulfurated by Cnx5, with the primary sulfur donor of Cnx5 (X-S) being unknown. The individual reactions of the Mo insertase Cnx1 and its products (Moco, pyrophosphate, and AMP) are indicated. Molybdate for this reaction is supplied by the vacuolar exporter Mot2 (a Mot2-mutant leads to a shortage of molybdate in the cytosol and the accumulation of molybdate in the vacuole) [146]. Later, Mot2 was renamed as Mot1.2 [111]. The Mot1-transporter is the cellular Mo-importer and resides in the plasma membrane [111]. Mature Moco can be either bound to a Moco-binding protein (MoBP) or directly to the Mo enzymes. The Moco sulfurase ABA3 generates a protein-bound persulfide, which is the source of the terminal sulfur ligand of Moco enzymes in the XDH/AO family. Similarly to Cnx2, xanthine dehydrogenase and aldehyde oxidase also depend on [Fe–S] from mitochondria. Modified after Mendel and Kruse [93].