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Structure of the molybdopterin-bound Cnx1G domain links molybdenum and copper metabolism


The molybdenum cofactor is part of the active site of all molybdenum-dependent enzymes1, except nitrogenase. The molybdenum cofactor consists of molybdopterin, a phosphorylated pyranopterin2, with an ene-dithiolate coordinating molybdenum. The same pyranopterin-based cofactor is involved in metal coordination of the homologous tungsten-containing enzymes found in archea3. The molybdenum cofactor is synthesized by a highly conserved biosynthetic pathway4. In plants, the multidomain protein Cnx1 catalyses the insertion of molybdenum into molybdopterin. The Cnx1 G domain (Cnx1G), whose crystal structure has been determined in its apo form, binds molybdopterin with high affinity and participates in the catalysis of molybdenum insertion. Here we present two high-resolution crystal structures of Cnx1G in complex with molybdopterin and with adenylated molybdopterin (molybdopterin–AMP), a mechanistically important intermediate. Molybdopterin–AMP is the reaction product of Cnx1G and is subsequently processed in a magnesium-dependent reaction by the amino-terminal E domain of Cnx1 to yield active molybdenum cofactor. The unexpected identification of copper bound to the molybdopterin dithiolate sulphurs in both structures, coupled with the observed copper inhibition of Cnx1G activity, provides a molecular link between molybdenum and copper metabolism.

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Figure 1: Structure of the Cnx1G–MPT and Ser583Ala–MPT–AMP complexes.
Figure 2: Mg2+- and Cnx1E-dependent Moco synthesis by Cnx1G–MPT complexes.
Figure 3: Cu–MPT dithiolate complex.
Figure 4: Hypothetical mechanism of the molybdenum insertion reaction.

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We thank R. N. Pau for inspiring discussion; T. Otte and F. Koenig for technical assistance; the staff at beamlines BW6 at DESY and PSF-BL2 at BESSY; and V. Wray for critically reading the manuscript. This work was supported by grants from the Deutsche Forschungsgemeinschaft (to H.J.H., R.R.M. and G.S.), and the Fonds der Chemischen Industrie and the Fritz Thyssen Stiftung (to R.R.M.).

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Correspondence to Günter Schwarz.

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Supplementary information

Supplementary Figure 1

Coordination of Cu-MPT and Cu-MPT-AMP. (DOC 196 kb)

Supplementary Figure 2

Structural comparison between Cnx1G and S583A. (DOC 867 kb)

Supplementary Figure 3

Structural comparison of MPT-AMP. (DOC 210 kb)

Supplementary Figure 4

Structural comparison between S583A and the Cnx1E-homologous MoeA. (DOC 1155 kb)

Supplementary Table 1

Data collection and refinement statistics. (DOC 32 kb)

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Kuper, J., Llamas, A., Hecht, HJ. et al. Structure of the molybdopterin-bound Cnx1G domain links molybdenum and copper metabolism. Nature 430, 803–806 (2004).

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