ELECTRON-TRANSFER IN HEMOPROTEINS .10. THE EFFECT OF PH, IONIC-STRENGTH, AND ZINC IONS ON THE RATE OF FERRICYTOCHROME-C REDUCTION BY PIG-HEART OXYMYOGLOBIN

The rate of the redox reaction between porcine oxymyoglobin and ferric ytochrome c was measured as a function of ionic strength in the pH ran ge of 5-8. At low ionic strength (< 0.1 M), a sigmoidal pH profile was obtained, with pK(a,obs) almost-equal-to 5.7, which was attributed to His-119 in the active site of myoglobin. The reaction rate decreased drastically when the ionic strength increased from 0 to 0.1 M. The slo pe of the IgK(a,obs) vs. square-root I /(1 + square-root I) plot varie d with pH and was greater at pH less-than-or-equal-to 6 than at pH 7.5 , which is due to His-119 deprotonation. At high ionic strength (> 0.1 M), ttie rate of electron transfer in the system was low, did not var y with pH, and decreased slightly as the ionic strength increased from 0.1 to 1 M. The principal role of local electrostatic interactions in the formation of the productive electron-transfer complex between myo globin and cytochrome c was demonstrated. Zinc ion binding to His-119 inhibited electron transfer at a ionic strength of 0.01 M in a manner similar to that observed at high ionic strength, although the reacting protein charges were not shielded, and His-119 retained its positive charge. This points to a direct role of His-119 in the electron transf er mechanism. The effects of pH, ionic strength, and zinc ions on the reaction between porcine oxymyoglobin and cytochrome c were compared w ith those reported for sperm whale oxymyoglobin. Differences in electr ostatic and steric characteristics of both myoglobins were invoked to explain the higher electron-donating efficiency of porcine myoglobin.