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Background  Alkyl hydroperoxidase activity provides an important antioxidant defense for bacterial cells. The catalytic mechanism requires two peroxidases, AhpC and AhpD, where AhpD plays the role of an essential adaptor protein.    Results  The crystal structure of a putative AhpD from  Pseudomonas aeruginosa  has been determined at 1.9 Å. The protein has an all-helical fold with a chain topology similar to a known AhpD from  Mycobacterium tuberculosis  despite a low overall sequence identity of 9%. A conserved two α-helical motif responsible for function is present in both. However, in the  P. aeruginosa  protein, helices H3, H4 of this motif are located at the N-terminal part of the chain, while in  M. tuberculosis  AhpD, the corresponding helices H8, H9 are situated at the C-terminus. Residues 24-62 of the putative catalytic region of  P. aeruginosa  have a higher sequence identity of 33% where the functional activity is supplied by a proton relay system of five residues, Glu36, Cys48, Tyr50, Cys51, and His55, and one structural water molecule. A comparison of five other related hypothetical proteins from various species, assigned to the alkyl hydroperoxidase D-like protein family, shows they contain the same conserved structural motif and catalytic sequence Cys-X-X-Cys. We have shown that AhpD from  P. aeruginosa  exhibits a weak ability to reduce H 2 O 2  as tested using a ferrous oxidation-xylenol orange (FOX) assay, and this activity is blocked by thiol alkylating reagents.    Conclusion  Thus, this hypothetical protein was assigned to the AhpD-like protein family with peroxidase-related activity. The functional relationship of specific oligomeric structures of AhpD-like structural family is discussed.

Crystal structure of alkyl hydroperoxidase D like protein PA0269 from Pseudomonas aeruginosa: Homology of the AhpD-like structural family