Abstract

Abstract The transition element molybdenum (Mo) is of essential importance for (nearly) all biological systems as it is required by enzymes catalyzing diverse key reactions in the global carbon, sulfur and nitrogen metabolism. In all eukaryotic Mo-containing enzymes the metal is complexed by a pterin, thus forming the molybdenum cofactor (Moco) which is the active compound at the catalytic site of these enzymes. Moco consists of Mo covalently bound to a unique tricyclic pterin moiety which is synthesized from GTP by an ancient and conserved biosynthetic pathway, likely being associated with mitochondria. There are four Moco-containing enzymes in plants: (1) nitrate reductase catalyzes the first and rate-limiting step in nitrate assimilation and is structurally similar to the recently identified (2) peroxisomal sulfite oxidase that detoxifies excessive sulfite by simultaneous generation of hydrogen peroxide. (3) Aldehyde oxidase is present in several isoforms, one of which specifically catalyzes the last step of abscisic acid biosynthesis, and (4) xanthine dehydrogenase is a key enzyme of purine degradation and initiation of flowering and leaf senescence. While nitrate reductase and sulfite oxidase immediately become active after incorporation of the Moco, aldehyde oxidase and xanthine dehydrogenase require a post-translational sulfuration for activity. This sulfuration step is catalyzed by the Moco-sulfurase ABA3 , a cytosolic cysteine desulfurase that is regulated by the same stresses like its target enzymes aldehyde oxidase and xanthine dehydrogenase. Besides Moco, aldehyde oxidase and xanthine dehydrogenase also depend on iron-sulfur clusters as prosthetic groups which are synthesised in mitochondria. Analysis of the sta1/atm3 mutant of A. thaliana, which is deficient in a mitochondrial ABC half-size transporter responsible for exporting iron-sulfur clusters into the cytosol, has shown that mitochondria, and in particular the ATM3 transporter, represent a direct crosspoint beween Mo- and iron-homeostasis.

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