Thiol/disulfide exchange between small heat shock protein 25 and glutathione

Murine small heat shock protein 25 (Hsp25) carries a single Cys-residue at position 141 of its amino acid sequence. In glutathione redox buffers, Hsp25 equilibrates between reduced protein (PSH), mixed disulfide (PSSG) and protein dimer (PSSP) forms. At highly oxidative conditions, native Hsp25 predominantly forms PSSP while denatured Hsp25 forms PSSG. Conversion of PSSP to PSSG correlates with urea and temperature denaturation of tertiary and/or quaternary structure of Hsp25. At pH 7.5, 25 degreesC, the second-order rate constant for the formation of PSSP in the reaction of native PSH with GSSG is 20.1+/-1.4 M-1 min-1. This is approximately 3-fold lower than the reaction velocity of GSSG with a typical, unhindered thiol of pKa 8.6. At redox equilibrium, the fractions of PSSP, PSSG, and PSH depend on the concentration of GSH and less on the ratio [GSH]/[GSSG] (R). At a constant R, the fractions of PSSG and PSH species depend similarly on GSH concentration, being approximately equal in glutathione redox buffers with low R. It is concluded that in oligomeric complexes, Hsp25 subunits in vitro form stable dimers, in which the reacting -SH groups are in a proximity to form intersubunit disulfide bonds. Within a reaction of one of these -SH groups with GSSG, steric hindrances and electrostatic repulsion complicate penetration of another reduced or oxidized glutathione molecule to the reaction site.