Influence of peptide dipoles and hydrogen bonds on reactive cysteine p K a values in fission yeast DJ ‐1

Cysteine residues with depressed p K a values are critical for the functions of many proteins. Several types of interactions can stabilize cysteine thiolate anions, including hydrogen bonds between thiol(ate)s and nearby residues as well as electrostatic interactions involving charged residues or dipoles. Dipolar stabilization of thiolates by peptide groups has been suggested to play a particularly important role near the N ‐termini of α‐helices. Using a combination of X ‐ray crystallography, site‐directed mutagenesis and spectroscopic methods, we show that the reactive cysteine residue ( C ys111) in S chizosaccharomyces pombe DJ ‐1 experiences a 0.6 unit depression of its thiol p K a as a consequence of a hydrogen bond donated by a threonine side chain ( T hr114) to a nearby peptide carbonyl oxygen at the N ‐terminus of an α‐helix. This extended hydrogen bonded interaction is consistent with a sum of dipoles model whereby the distal hydrogen bond polarizes and strengthens the direct hydrogen bond between the proximal amide hydrogen and the cysteine thiol(ate). Therefore, our results suggest that the local dipolar enhancement of hydrogen bonds can appreciably stabilize cysteine thiolate formation. However, the substitution of a valine residue with a proline at the i + 3 position has only a minor effect (0.3 units) on the p K a of Cys111. As proline has a reduced peptide dipole moment, this small effect suggests that a more extended helix macrodipolar effect does not play a major role in this system. Database Structural models and diffraction data are available in the Protein Data Bank under the accession number(s) 4GDH (wild‐type SpDJ‐1), 4GE0 (T114P SpDJ‐1) and 4GE3 (T114V SpDJ‐1) Structured digital abstractSpDJ‐1  and  SpDJ‐1   bind  by  x‐ray crystallography  ( View interaction )