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Here we report the structure of acireductone dioxygenase (ARD), the first determined for a new family of metalloenzymes. ARD represents a branch point in the methionine salvage pathway leading from methylthioadenosine to methionine and has been shown to catalyze different reactions depending on the type of metal ion bound in the active site. The solution structure of nickel-containing ARD (Ni-ARD) was determined using NMR methods. X-ray absorption spectroscopy, assignment of hyperfine shifted NMR resonances and conserved domain homology were used to model the metal-binding site because of the paramagnetism of the bound Ni2+. Although there is no structure in the Protein Data Bank within 3 A r.m.s deviation of that of Ni-ARD, the enzyme active site is located in a conserved double-stranded b-helix domain. Furthermore, the proposed Ni-ARD active site shows significant post-facto structural homology to the active sites of several metalloenzymes in the cupin superfamily.

The downstream prion-like protein (doppel, or Dpl) is a paralog of the cellular prion protein, PrP C . The two proteins have Ϸ25% sequence identity, but seem to have distinct physiologic roles. Unlike PrP C , Dpl does not support prion replication; instead, overexpression of Dpl in the brain seems to cause a completely different neurodegenerative disease. We report the solution structure of a fragment of recombinant mouse Dpl (residues 26 -157) containing a globular domain with three helices and a small amount of ␤-structure. Overall, the topology of Dpl is very similar to that of PrP C . Significant differences include a marked kink in one of the helices in Dpl, and a different orientation of the two short ␤-strands. Although the two proteins most likely arose through duplication of a single ancestral gene, the relationship is now so distant that only the structures retain similarity; the functions have diversified along with the sequence.doppel protein structure ͉ NMR ͉ protein structures ͉ prion protein ͉ prion diseases