Human DJ-1, a disease-associated protein that protects cells from oxidative stress, contains an oxidation-sensitive cysteine (C106) that is essential for its cytoprotective activity. The origin of C106 reactivity is obscure, due in part to the absence of an experimentally determined pK a value for this residue. We have used atomic-resolution X-ray crystallography and UV spectroscopy to show that C106 has a depressed pK a of 5.4 ( 0.1 and that the C106 thiolate accepts a hydrogen bond from a protonated glutamic acid side chain (E18). X-ray crystal structures and cysteine pK a analysis of several site-directed substitutions at residue 18 demonstrate that the protonated carboxylic acid side chain of E18 is required for the maximal stabilization of the C106 thiolate. A nearby arginine residue (R48) participates in a guanidinium stacking interaction with R28 from the other monomer in the DJ-1 dimer and elevates the pK a of C106 by binding an anion that electrostatically suppresses thiol ionization. Our results show that the ionizable residues (E18, R48, and R28) surrounding C106 affect its pK a in a way that is contrary to expectations based on the typical ionization behavior of glutamic acid and arginine. Lastly, a search of the Protein Data Bank (PDB) produces several candidate hydrogen-bonded aspartic/glutamic acid-cysteine interactions, which we propose are particularly common in the DJ-1 superfamily.Cysteine thiolates are potent nucleophiles that are used by many proteins for catalysis, metal binding, or to facilitate post-translational modification. However, the solution pK a value of cysteine (8.3) is not within the optimal physiological pH range of most proteins. To render cysteine residues reactive, the thiol pK a must be depressed by stabilization of the conjugate thiolate anion by the protein environment. Multiple types of stabilizing interactions can decrease cysteine pK a values, including electrostatic complementarity with nearby cations (1, 2), the R-helix macropole (3), and hydrogen bonding to the thiol (4).The Parkinsonism-associated protein DJ-1 contains an oxidation-sensitive cysteine residue (C106 in human DJ-1) of unknown pK a that is required for the cytoprotective activity of the protein (5, 6). DJ-1 is a homodimeric protein of 189 amino acid monomers that is found both in the cytoplasm (7) and in the mitochondria (5, 8) and protects cells from various types of oxidative stress (5,(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). Although C106 has been shown to be essential for the protective function of DJ-1 in cell culture (5, 16) and Drosophila melanogaster (6), the specific biochemical activity of the protein that requires this cysteine residue is uncertain. Identifying the structural determinants of C106 ionization (and hence reactivity) is an essential first step in understanding how this conserved cysteine residue contributes to the neuroprotective activity of DJ-1.The reactive cysteine in human DJ-1 is one of the most highly conserved residues in the DJ-1 superfamily (19,20). Crystal s...

Page 7432. Due to a typographical error made in the final typesetting of this article, the third and fourths rows from the bottom of Table 1 should begin "rmsd for angle distances (Å)" and "rmsd for bond lengths (Å)", respectively.