Electron-transfer studies with the CuA domain of Thermus thermophilus cytochrome ba3.

Claire E.Sutter, Ralf Langen, Donita Sanders, Stephen M. Lawrence, Pernilla Wittung, Angel J. Di Billio, Michael J. Hill, James A Fee, John H Richards, Jay Winkler and Bo G. Malmstrom. Inorg Chim Acta243, 141-145(1996).

Summary:Flash photolysis has been used to initiate electron transfer from excited tris(2,2'-bipyridyl)ruthenium(II) to the CuA site of the soluble domain from subunit II of Thermus thermophilus cytochrome ba3. Luminescence quenching of the excited state of the ruthenium (II) complex was observed at low protein concentrations (20-200 uM, CuA domain) with second- order rate constants of 2.9 x 10^3 and 1.3 X 10^1 M-1 s-1 at low and high ionic strength respectively. Transient absorption measurements demonstrate that 10-20% of the quenching arises. from electron transfer (EI). At high protein concentrations (>250 uM CuA) and low ionic strength (5 mM Tris, pH 8.1), the quenching rate saturates due to ground state complex formation; a first-order rate constant of 1.5 x 10^5 sec-1 estimated for ET in the complex. Given the high driving force involved (-Delta G = 1.1 eV), it is possible that these ET reactions occur in the inverted driving-force regime. Spectroscopic measurements indicate that the T. thermophilus CuA domain and horse heart cytochrome c form a complex at low ionic strength with an apparent dissociation constant, Kd of ~ 5uM.