Dynamics of carbon monoxide binding to CooA

Geminate carbon monoxide rebinding to a c-type haem

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, ).

CO-sensing mechanisms

Carbon monoxide (CO) has long been known to have dramatic physiological effects on organisms ranging from bacteria to humans, but recently there have a number of suggestions that organisms might have specific sensors for CO. This article reviews the current evidence for a variety of proteins with demonstrated or potential CO-sensing ability. Particular emphasis is placed on the molecular description of CooA, a heme-containing CO sensor from Rhodospirillum rubrum, since its biological role as a CO sensor is clear and we have substantial insight into the basis of its sensing ability.

The C-helix in CooA Rolls upon CO Binding to Ferrous Heme

CooA is a homodimeric transcriptional activator from Rhodospirillum rubrum containing one heme in each subunit. CO binding to the heme in its sensor domain activates CooA, facilitating the binding to DNA by its DNA-binding domain. The C-helix links the two domains and shapes an interface between the subunits. To probe the nature of CO activation, residues at positions 112-121 on the C-helix were replaced by Asn or Gln and their effects were evaluated by resonance Raman spectroscopy and by the measurements of CO binding affinity. The nu(Fe-CO) stretching Raman line in CO-bound wild-type CooA was up-shifted by 6 cm(-1) in the L116Q, G117N, and L120Q mutants, indicating unequivocally that these residues are close to the bound CO. Residues Leu116 and Leu120 from each subunit form contacts with the corresponding residues in the opposite subunit, enabling hydrophobic interactions in the inactive ferrous form. Thus, in the CO-bound activated form, both C-helices appear to roll to direct these residues toward the heme, forming a hydrophobic pocket for the bound CO. The CO affinity is approximately one order of magnitude higher in the L112Q, I115Q, L116Q, G117N, L120Q, and T121N mutants but reduced in A114N mutant. The variation indicates that these residues are close to the heme in the ferrous and/or CO-bound forms and are responsible for CooA activation. A roll-and-slide mechanism is proposed for CO activation of CooA.