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The molybdenum cofactor (Moco) is part of the active site of all molybdenum (Mo)-dependent enzymes, except nitrogenase. Moco consists of molybdopterin (MPT), a phosphorylated pyranopterin with an enedithiolate coordinating Mo and it is synthesized by an evolutionary old multistep pathway. The plant protein Cnx1 from Arabidopsis thaliana catalyzes with its two domains (E and G) the terminal step of Moco biosynthesis, the insertion of Mo into MPT. Recently, the high-resolution MPT-bound structure of the Cnx1 G domain (Cnx1G) has been determined (Kuper, J., Llamas, A., Hecht, H. J., Mendel, R. R., and Schwarz, G. (2004) Nature 430, 803–806). Besides defining the MPT-binding site a novel and unexpected modification of MPT has been identified, adenylated MPT. Here we demonstrate that it is Cnx1G that catalyzes the adenylation of MPT. In vitro synthesized MPT was quantitatively transferred from …