title: The role of histidine-42 in the oxidation-reduction mechanism of Chromatium vinosum high potential iron-sulfur protein.
abstract: The second order rate constants for the oxidation of high potential iron-sulfur protein (Hipip) of Chromatium vinosum by ferricyanide were determined as a function of ionic strength and pH. From the ionic strength results, calculations were done to correct the rate constant at each pH for the electrostatic interactions between Hipip and ferricyanide. The electrostatic corrections are necessary since the charge of the protein changes as a function of pH and can mask the ionization of mechanistically important amino acid residues. An apparent pKa congruent to 7 was obtained from electrostatically corrected rate-pH profile, indicating the possible participation of histidine. Perturbation difference spectroscopic studies of Hipip as a function of pH also gave apparent pKa values of 6.9 and 6.7 for the reduced and oxidized protein, respectively. That it was indeed His 42 (the only His in the polypeptide) that was responsible for the kinetic and spectroscopic pKa values was demonstrated by modification of His 42 of Hipip by the histidine selective reagent diethylpyrocarbonate. No modification of Tyr 19 could be detected. It is concluded that either deprotonation or modification of His 42 results in the destabilization of the reduced cluster and thus a faster rate of oxidation. This work provides the first experimental evidence of the 'squeeze effect' mechanism (Carter, C.W., Jr., Kraut, J., Freer, S.T. and Alden, R.A. (1974) J. Biol. Chem. 249, 6339--6346) in which the polypeptide directly modulates the stability of the iron-sulfur cluster.
mesh_terms: D002844:Chromatium / Q000378:metabolism*; D004227:Dithionite / Q000494:pharmacology; D005292:Ferricyanides / Q000494:pharmacology; D006639:Histidine / Q000494:pharmacology*; D006863:Hydrogen-Ion Concentration; D007506:Iron-Sulfur Proteins / Q000378:metabolism*; D007700:Kinetics; D008667:Metalloproteins / Q000378:metabolism*; D009994:Osmolar Concentration; D010084:Oxidation-Reduction; D012241:Rhodopseudomonas / Q000378:metabolism*; D013053:Spectrophotometry
chemical_list: D005292:Ferricyanides; D007506:Iron-Sulfur Proteins; D008667:Metalloproteins; D004227:Dithionite; D006639:Histidine; C028033:potassium ferricyanide