Cyclic electron transfer within the [Zn-myoglobin, cytochrome b(5)] complex

To characterize the electrostatic complex formed between myoglobin (Mb) and cytochrome bs (Febs), we have performed flash photolysis triplet-quenching and electron-transfer (ET) measurements of the interaction between Zn deuteroporphyrin (ZnD)-substituted Mb (sperm whale) (ZnDMb) and Feb(5)(trypsin-solubilized, bovine) at pH values between 6 and 7.5. For pH values between pH 6 and pH 7.5, the quenching rate constant (Delta k) varies linearly with [Fe(3+)b(5)]. The slope (M) obtained from plots of Delta k versus [Fe(3+)b(5)] is strongly dependent on pH (M = 140 x 10(6) M(-1) s(-1) at pH 6 and M = 2.4 x 10(6) M(-1) s(-1) at pH 7.5). The triplet decay profiles remain exponential throughout these titrations. Together, these results indicate that the association constant obeys the inequality, K-a less than or equal to 3000 M(-1) and that the lower limit for the rate constant for dissociation of the (3)DA complex of (k(off))(min) = 10(6) s(-1) at pH 6 and (k(off))(min) = 10(4) s(-1) at pH 7.5. Transient absorption measurements have shown that this quenching of (3)ZnDMb by Fe(3+)b(5) can be attributed to intracomplex (ZnD)-Zn-3 --> Fe3+P ET and that the transient absorbance changes observed at the D-3/D isosbestic points represent the time evolution of the (D(+)A(-)), [ZnD(+)Mb, Fe(2+)b(5)] intermediate, I. The long-time behavior of the progress curves (t greater than or equal to 20 ms) collected during a titration of Fe(3+)b(5) by ZnDMb (reverse titration protocol) is neither purely second-order nor purely first-order but rather resembles a mixed-order process involving both the (D(+)A(-)) complex and its dissociated components. Modeling this data indicates that the D(+)A(-) complex product must dissociate with a rate constant slower than that of the precursor, DA, complex. Theoretical studies of the protein pair by Brownian dynamics simulations show that Mb has a broad reactive surface which encompasses the ''hemisphere'' that includes the exposed heme edge. The most stable complexes occur when bs is bound at one of two subdomains within this hemisphere. The kinetics measurements and calculations taken together allow us to discuss the relative importance of global and local electrostatics in regulating protein-protein recognition and reactivity.