Molybdoenzymes and Molybdenum Cofactor in Plants

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Abstract
The transition element molybdenum is essential for (nearly) all organisms and occurs in more than 30 enzymes catalyzing diverse redox reactions; however, only three Mo-enzymes have been found in plants so far. (1) Nitrate reductase catalyzes the key step in inorganic nitrogen assimilation, (2) aldehyde oxidase(s) recently have been shown to catalyze the last step in the biosynthesis of the phytohormones indole acetic acid and abscisic acid, respectively, and (3) xanthine dehydrogenase is involved in purine catabolism. These enzymes are homodimeric proteins harboring an electron transport chain that involves different prosthetic groups (FAD, heme, or Fe-S, Mo). Among different Mo-enzymes, the alignment of amino acid sequences helps to define regions that are well conserved (domains) and other regions that are highly variable in sequence (interdomain hinge regions). The existence of additional plant Mo-enzymes (like sulfite oxidase) also has to be considered. In this review we focus on structure-function relationships and stress the functional importance of the enzymes for the plant. With the exception of bacterial nitrogenase, Mo-enzymes share a similar pterin compound at their catalytic sites, the molybdenum cofactor. Molybdenum itself seems to be biologically inactive unless it is complexed by the cofactor. This molybdenum cofactor combines with diverse apoproteins where it is responsible for the correct anchoring and positioning of the Mo-center within the holo-enzyme so that the Mo-center can interact with other components of the enzyme's electron transport chain. The organic moiety of the molybdenum cofactor is a unique pterin named molybdopterin. The core structure of molybdopterin is conserved in all organisms. Accordingly, its biosynthetic pathway seems to be conserved because a similar set of cofactor genes has been found in bacteria and higher plants. We describe a model for the biosynthesis of the plant molybdenum cofactor involving the complex interaction of seven proteins.

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