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].