Geminate carbon monoxide rebinding to a c-type haem

Abstract

A chemically modified form of cytochrome c(cyt. c), termed carboxymethyl cytochrome c(cm cyt. c), possesses a vacant sixth coordination site to the haem iron that is available to bind external ligands. We present data on the rapid flash photolysis of CO from the ferrous haem iron of cm cyt. c and describe the kinetics and spectral transitions that accompany the recombination. This was achieved using 30-femtosecond laser pulses and a white light continuum to monitor spectral transitions. Whereas the photo-dissociation quantum yield is close to 1, the yield of CO escape from the protein (the apparent quantum yield, varphi) relative to myoglobin (varphi=1) is small due to rapid geminate recombination of CO. On ligand photo-dissociation the haem undergoes a spin-state transition from low-spin ferrous CO bound to penta-coordinate high-spin. Subsequently the system reverts to the CO bound form. The data were fitted with a minimum number of exponentials using global analysis. Recombination of CO with the haem iron of cm cyt. c is multiphasic (tau=16 ps, 120 ps and 1 ns), involving three spectrally distinct components. The fraction of haem (0.11) not recombining with CO within 4 ns is similar to the value of varphi(0.12) measured on the same preparation by the "pulse method" (M. Brunori, G. Giacometti, E. Antonini and J. Wyman, Proc. Natl. Acad. Sci. USA, 1973, 70, 3141-3144, ). This implies that no further geminate recombination occurs at t>4 ns. This unusually efficient CO-haem geminate recombination indicates the sterically hindered ("caged") nature of the distal haem pocket in cm cyt. c from which it is difficult for CO to escape. The large geminate phase may be contrasted with the behaviour of myoglobin in which geminate recombination is small. This is in general agreement with the well-documented extensive structural dynamics in myoglobin that allow ligand passage, and a higher structural rigidity in cyt. c imposed by the restraints of minimising reorganisation energy for electron transfer (M. Brunori, D. Bourgeois and D. Vallone, J. Struct. Biol., 2004, 147, 223-234, ). The high pH ferrous form of cm cyt. c is a low-spin species having a lysine bound to the central iron atom of the haem (M. Brunori, M. Wilson and E. Antonini, J. Biol. Chem., 1972, 247, 6076-6081; G. Silkstone, G. Stanway, P. Brzezinski and M. Wilson, Biophys. Chem., 2002, 98, 65-77, ). This high pH (pH approximately 8) form of deoxy cm cyt. c undergoes photo-dissociation of lysine (although the proximal histidine is possible) after photo-excitation. Recombination occurs with a time constant (tau) of approximately 7 ps. This is similar to that observed for the geminate rebinding of the Met80 residue in native ferrous cyt. c(tau approximately 6 ps) following its photo-dissociation (S. Cianetti, M. Negrerie, M. Vos, J.-L. Martin and S. Kruglik, J. Am. Chem. Soc., 2004, 126, 13 932-13 933; W. Wang, X. Ye, A. Demidov, F. Rosca, T. Sjodin, W. Cao, M. Sheeran and P. Champion, J. Phys. Chem., 2000, 104, 10 789-10 801, ).