title: Cysteine pK(a) values for the bacterial peroxiredoxin AhpC.
abstract: Salmonella typhimurium AhpC is a founding member of the peroxiredoxin family, a ubiquitous group of cysteine-based peroxidases with high reactivity toward hydrogen peroxide, organic hydroperoxides, and peroxynitrite. For all of the peroxiredoxins, the catalytic cysteine, referred to as the peroxidatic cysteine (C(P)), acts as a nucleophile in attacking the peroxide substrate, forming a cysteine sulfenic acid at the active site. Because thiolates are far stronger nucleophiles than thiol groups, it is generally accepted that cysteine-based peroxidases should exhibit pK(a) values lower than an unperturbed value of 8.3-8.5. In this investigation, several independent approaches were used to assess the pK(a) of the two cysteinyl residues of AhpC. Methods using two different iodoacetamide derivatives yielded unperturbed pK(a) values (7.9-8.7) for both cysteines, apparently due to reactivity with the wrong conformation of C(P) (i.e., locally unfolded and flipped out of the active site), as supported by X-ray crystallographic analyses. A functional pK(a) of 5.94 +/- 0.10 presumably reflecting the titration of C(P) within the fully folded active site was obtained by measuring AhpC competition with horseradish peroxidase for hydrogen peroxide; this value is quite similar to that obtained by analyzing the pH dependence of the epsilon(240) of wild-type AhpC (5.84 +/- 0.02) and similar to those obtained for two typical 2-cysteine peroxiredoxins from Saccharomyces cerevisiae (5.4 and 6.0). Thus, the pK(a) value of AhpC balances the need for a deprotonated thiol (at pH 7, approximately 90% of the C(P) would be deprotonated) with the fact that thiolates with higher pK(a) values are stronger nucleophiles.
authors.affiliation: Department of Biochemistry, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157, USA.
authors.forename: Kimberly J
authors.initials: KJ
authors.lastname: Nelson
authors.forename: Derek
authors.initials: D
authors.lastname: Parsonage
authors.forename: Andrea
authors.initials: A
authors.lastname: Hall
authors.forename: P Andrew
authors.initials: PA
authors.lastname: Karplus
authors.forename: Leslie B
authors.initials: LB
authors.lastname: Poole
mesh_terms: D000042:Absorption; D000478:Alkylation; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D001667:Binding, Competitive; D002021:Buffers; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D003545:Cysteine / Q000737:chemistry* / Q000378:metabolism; D006735:Horseradish Peroxidase / Q000378:metabolism; D006861:Hydrogen Peroxide / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007185:Incubators; D007460:Iodoacetamide / Q000737:chemistry / Q000378:metabolism; D008969:Molecular Sequence Data; D054464:Peroxiredoxins / Q000737:chemistry* / Q000378:metabolism*; D011489:Protein Denaturation; D055550:Protein Stability; D012486:Salmonella typhimurium; D013997:Time Factors
chemical_list: D001426:Bacterial Proteins; D002021:Buffers; D006861:Hydrogen Peroxide; D006735:Horseradish Peroxidase; D054464:Peroxiredoxins; D003545:Cysteine; D007460:Iodoacetamide