PH AND DRIVING-FORCE DEPENDENCE OF INTRAMOLECULAR OXYFERRYL HEME REDUCTION IN MYOGLOBIN

The kinetics of oxyferryl (Fe-IV=O) heme reduction in horst heart myog lobin (Mb) by a(4)LRu(II) (a = NH3; L = NH3, pyridine, isonicotinamide ) bound at the surface His48 were investigated with pulse radiolysis. The observed first-order rate constants (k(obs1)) decreased with incre asing pH and reduction potential for the a(4)LRu centers (E degrees/ R u-III/II = 77, 330, and 400 mV for L = NH3, Pyr, and Isn). Rate-pD dat a obtained in D2O for the a(4)LRu derivative revealed the presence of an equilibrium isotope effect, and a pK(a) of 5.7 (6.2 in D2O) was obt ained for the acid-base group, which is assigned to the distal His64. A mechanism where protonation precedes ET provides a good fit of the k inetic data for the three a(4)LRu derivatives. Marcus theory analysis of the k(ET) (0.74, 1.8, and 3.6 s(-1) for L = Isn. Pyr, and NH3) extr acted from the k(obs1) values yielded a reorganization energy (lambda) of 1.8 for Ru-II --> Fe-IV=O ET in the a(4)LRu derivatives but a lamb da of 2.1 eV for the a(5)Ru derivative, From the latter, it is conclud ed that ET is strongly gated in the a(5)Ru derivative, and this is ass umed to be the major reason for the low reactivity of Fe-IV=O in Mb at high -Delta G degrees.