REACTION OF THE HANSENULA-ANOMALA FLAVOCYTOCHROME B(2) AND CYTOCHROMEB(2) CORE WITH INORGANIC OUTER-SPHERE REDOX COMPOUNDS

The oxidation of reduced cytochrome b(2) core and flavocytochrome b(2) by three inorganic outer sphere compounds, Fe(CN)6(3-), Co(phen)(3)(3 +) and Mn(CyDTA) (H2O)(-), has been studied by stopped-flow. The react ion with Fe(CN)(6)(3-) is very rapid; the second order rate constants at 10 degrees C (pH 7) and I = 0.02 M are k = 1 x 10(8) M(-1) s(-1) an d 1 x 10(7) M(-1) s(-1) for cytochrome b(2) core and flavocytochrome b (2), respectively. The reaction between cytochrome b(2) core and Co(ph en)(3)(3+), too fast at pH 7.0, has been characterized at 10 degrees C and pH 4.0; the second order rate constant is k = 2 x 10(7) M(-1) s(- 1) and becomes 4 x 10(8) M(-1) s(-1) at pH 6.5. The reaction between f lavocytochrome b(2) and Co(phen)(3)(3+) has a second order rate consta nt k = 2 x 10(7) M(-1) s(-1) at pH 7.0, 10 degrees C. The oxidation of both proteins by Mn(CyDTA)(H2O)(-) is characterized by a second order rate constant k = 2.8 x 10(6) M(-1) s(-1) and 2.3 x 10(5) M(-1) s(-1) for cytochrome b(2) core and flavocytochrome b(2), respectively, at p H 7.0 and 10 degrees C. The reactivity of the b(2) heme towards the ou ter sphere oxidants is higher than that reported for heme c in bacteri al and eukaryotic cytochromes c. The larger Delta E and the larger acc essibility of the b(2) heme can account for this result. The flavodehy drogenase domain seems to modulate the electron transfer also to these inorganic compounds, as found previously in the case of macromolecula r electron acceptors.