The transition element molybdenum (Mo) is essential for (nearly)
all organisms and occurs in more than 40 enzymes catalysing
diverse redox reactions, however, only four of them have been
found in plants. (1) Nitrate reductase catalyses the key step
in inorganic nitrogen assimilation, (2) aldehyde oxidase(s)
have been shown to catalyse the last step in the biosynthesis
of the phytohormone abscisic acid, (3) xanthine dehydrogenase
is involved in purine catabolism and stress reactions, and (4)
sulphite oxidase is probably involved in detoxifying excess
sulphite. Among Mo-enzymes, the alignment of amino acid sequences
permits domains that are well conserved to be defined. With
the exception of bacterial nitrogenase, Mo-enzymes share a similar
pterin compound at their catalytic sites, the molybdenum cofactor.
Mo 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-centre within the holo-enzyme so that
the Mo-centre can interact with other components of the enzyme’s
electron transport chain. A model for the three-step biosynthesis
of Moco involving the complex interaction of six proteins will
be described. A putative Moco-storage protein distributing Moco
to the apoproteins of Mo-enzymes will be discussed. After insertion,
xanthine dehydrogenase and aldehyde oxidase, but not nitrate
reductase and sulphite oxidase, require the addition of a terminal
sulphur ligand to their Mo-site, which is catalysed by the sulphur
transferase ABA3.
Key words: Key words: Abscisic acid biosynthesis, aldehyde oxidase, molybdenum cofactor, nitrate reductase, sulphite oxidase, xanthine dehydrogenase.
Source:
Journal of Experimental Botany, Vol. 53, No. 375, pp. 1689-1698, August 1, 2002
© 2002 Oxford University Press