No abstract

The molybdenum cofactor sulfurase ABA3 from Arabidopsis thaliana is needed for post-translational activation of aldehyde oxidase and xanthine dehydrogenase by transferring a sulfur atom to the desulfo-molybdenum cofactor of these enzymes. ABA3 is a two-domain protein consisting of an NH 2 -terminal NifS-like cysteine desulfurase domain and a C-terminal domain of yet undescribed function. The NH 2 -terminal domain of ABA3 decomposes L-cysteine to yield elemental sulfur, which subsequently is bound as persulfide to a conserved protein cysteinyl residue within this domain. In vivo, activation of aldehyde oxidase and xanthine dehydrogenase also depends on the function of the C-terminal domain, as can be concluded from the A. thaliana aba3/sir3-3 mutant. sir3-3 plants are strongly reduced in aldehyde oxidase and xanthine dehydrogenase activities due to a substitution of arginine 723 by a lysine within the C-terminal domain of the ABA3 protein. Here we present first evidence for the function of the C-terminal domain and show that molybdenum cofactor is bound to this domain with high affinity. Furthermore, cyanide-treated ABA3 C terminus was shown to release thiocyanate, indicating that the molybdenum cofactor bound to the C-terminal domain is present in the sulfurated form. Co-incubation of partially active aldehyde oxidase and xanthine dehydrogenase with ABA3 C terminus carrying sulfurated molybdenum cofactor resulted in stimulation of aldehyde oxidase and xanthine dehydrogenase activity. The data of this work suggest that the C-terminal domain of ABA3 might act as a scaffold protein where prebound desulfo-molybdenum cofactor is converted into sulfurated cofactor prior to activation of aldehyde oxidase and xanthine dehydrogenase.Molybdenum enzymes catalyze diverse redox reactions in the global carbon, nitrogen, and sulfur cycles (1). In all eukaryotic molybdenum enzymes, the molybdenum atom is coordinated by the dithiolene group of molybdopterin, thus forming the molybdenum cofactor (Moco) 2 (2). According to the coordination chemistry of the molybdenum ligand, eukaryotic molybdenum enzymes can be divided into two groups; Moco with two additional oxo-ligands and a protein-derived cysteinyl sulfur is bound by enzymes of the sulfite oxidase family, whereas enzymes of the xanthine oxidase family have one oxygen, one inorganic sulfur, and one hydroxyl group ligated to the pterin-chelated molybdenum of the active enzyme. Among the four different molybdenum enzymes known in higher plants, sulfite oxidase and nitrate reductase belong to the sulfite oxidase family, whereas aldehyde oxidase (AO) and xanthine dehydrogenase (XDH) are members of the xanthine oxidase family (3). Although it is believed that all of these molybdenum enzymes basically incorporate the same type of Moco, only AO and XDH, but not enzymes of the sulfite oxidase family, require a final enzyme-dependent post-translational modification of the molybdenum center for activity (4). During this modification step, an oxo-ligand of the Moco in inactive AO and XDH...

No abstract

No abstract

No abstract