Molybdenum and enzymes
Molybdenum is an essential constituent of enzymes involved in animal and plant metabolism.
The enzyme xanthine oxidase oxidizes hypoxanthine to xanthine and xanthine to uric acid in purine catabolism. Cofactor deficiency leads to abnormal sulfur and xanthine metabolism, i.e., high and low cystine and uric acid concentrations in plasma and urine. Sulfite oxidase catalyzes the oxidation of sulfite to sulfate, and aldehyde oxidase catalyzes the oxidation of aldehydes and various nitrogen-containing aromatic heterocyclic compounds.
Van Gennip, A. H., Abeling, N. G., Stroomer, A. E. M., Overmars, H. and Bakker, H. D., J. Inherit. Metab. Dis., 1994, 17, 142.
Three Mo-enzymes have been found in plants. Nitrate reductase catalyses the reduction of nitrate, the key step in inorganic nitrogen assimilation. Aldehyde oxidase catalyzes the last step in the biosynthesis of the phytohormones indole acetic acid and abscisic acid. Xanthine dehydrogenase is involved in purine catabolism.
With the exception of bacterial nitrogenase, Mo-enzymes share a similar pterin at their catalytic sites, the molybdenum cofactor. Molybdenum is biologically inactive unless it is complexed by the cofactor. The core structure of molybdopterin is conserved in all organisms.
Mendel, R.R., Schwarz, G., Molybdoenzymes and molybdenum cofactor in plants, Critical Reviews In Plant Sciences , 1999, 18, 1, 33-69.
For an excellent up to date review see
Cell biology of molybdenum in plants and humans
The transition element molybdenum (Mo) needs to be complexed by a special cofactor in order to gain catalytic activity. With the exception of bacterial Mo-nitrogenase, where Mo is a constituent of the FeMo-cofactor, Mo is bound to a pterin, thus forming the molybdenum cofactor Moco, which in different variants is the active compound at the catalytic site of all other Mo-containing enzymes. In eukaryotes, the most prominent Mo-enzymes are nitrate reductase, sulfite oxidase, xanthine dehydrogenase, aldehyde oxidase, and the mitochondrial amidoxime reductase. The biosynthesis of Moco involves the complex interaction of six proteins and is a process of four steps, which also requires iron, ATP and copper. After its synthesis, Moco is distributed to the apoproteins of Mo-enzymes by Moco-carrier/binding proteins. A deficiency in the biosynthesis of Moco has lethal consequences for the respective organisms. In humans, Moco deficiency is a severe inherited inborn error in metabolism resulting in severe neurodegeneration in newborns and causing early childhood death. This article is part of a Special Issue entitled: Cell Biology of Metals
Mendel, Ralf R. and Kruse, Tobias, Cell biology of molybdenum in plants and humans, Biochimica et biophysica acta, 2012, 1823, 1568-1579.
The following section reviews of molyboenzymes summarises studies of individual enzymes.