Hookworm activation-associated secreted proteins can be structurally classified into at least three different groups. The hallmark feature of Group 1 activation-associated secreted proteins is a prominent equatorial groove, which is inferred to form a ligand binding site. Furthermore, a conserved tandem histidine motif is located in the centre of the groove and believed to provide or support a yet to be determined catalytic activity. Here, we report three-dimensional crystal structures of Na-ASP-2, an L3-secreted activationassociated secreted protein from the human hookworm Necator americanus, which demonstrate transition metal binding ability of the conserved tandem histidine motif. We further identified moderate phosphohydrolase activity of recombinant Na-ASP-2, which relates to the tandem histidine motif. By panning a random 12-mer peptide phage library, we identified a peptide with high similarity to the human calcium-activated potassium channel SK3, and confirm binding of the synthetic peptide to recombinant Na-ASP-2 by differential scanning fluorimetry. Potential binding modes of the peptide to Na-ASP-2 were studied by molecular dynamics simulations which clearly identify a preferred topology of the Na-ASP-2:SK3 peptide complex. KeywordsActivation-associated secreted proteins, Host-parasite interactions, Pathogenesis-related proteins, Protein structure, SCP/TAPS proteins Database Coordinates and structure factors have been deposited with the PDB, accession numbers 4nui, 4nuk, 4nun and 4nuo.

Mycobacterial carbonic anhydrases, such as the essential protein Rv3588c, are attractive drug targets since they constitute a different class of carbonic anhydrases than those found in humans.

The mycobacterial enzyme Rv1284 is a member of the b-carbonic anhydrase family that is considered essential for survival of the pathogen. The active site cavity of this dimeric protein is characterized by an exceptionally small volume and harbours a catalytic zinc ion coordinated by two cysteine and one histidine residue side chains. Using the natural products polycarpine and emodin as chemical probes in crystallographic experiments and stopped-flow enzyme assays, we report that the catalytic activity can be reversibly inhibited by oxidation. Oxidative conditions lead to the removal of one of the active site cysteine residues from the coordination sphere of the catalytic metal ion by engagement in a disulfide bond with another cysteine residue close by. The subsequent loss of the metal ion, which is supported by crystallographic analysis, may thus render the protein catalytically inactive. The oxidative inhibition of Rv1284 can be reversed by exposing the protein to reducing conditions. Because the physical size of the chemical probes used in the present study substantially exceeds the active site volume, we hypothesized that these compounds exert their effects from a surface-bound location and identified Tyr120 as a critical residue for oxidative inactivation. These findings link conditions of oxidative stress to pH homeostasis of the pathogen. Because oxidative stress and acidification are defence mechanisms employed by the innate immune system of the host, we suggest that Rv1284 may be a component of the mycobacterial survival strategy.Database Atomic coordinates and structure factors have been deposited in the Protein Data Bank under accession numbers 4yf4, 4yf5 and 4yf6.

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