Ligand binding subsequent to CO photolysis of methionine-modified cytochrome c

Photoacoustic calorimetry studies of CO photo-dissociation from chloramine-T modified horse heart cytochrome-c

Treatment of horse heart Cytochrome-c (Cc) with N-chloro-4-toluosulfonamide (Chloramine-t, CT) results in the oxidation of methionine (Met) residues to the corresponding sulfoxide including the distal heme ligand, Met80. The resulting Fe-sulfoxide coordination is sufficiently labile in the ferrous form to be displaced by gaseous ligands, including CO. Photolysis of the CO-CT-Cc complex provides an opportunity to examine ligand binding dynamics that are associated with a relatively rigid distal heme pocket. In this work, photoacoustic calorimetry (PAC) was utilized to obtain the kinetics as well as enthalpy and molar volume changes subsequent to CO photo-dissociation from CO-CT-Cc. Previous photolysis studies of CO-CT-Cc have led to a proposed model for ligand recombination in which the Met80-sulfoxide and CO recombine with the heme on relatively slow timescales (50 μs and ∼500 μs, respectively). The PAC data presented here reveals two additional kinetic phases with lifetimes of <20 ns and 534 ± 75 ns. The fast phase (<20 ns) is associated with an ΔH of 44 ± 5 kcal mol^-1 and ΔV of -0.5 ± 0.5 mL mol^-1, whereas the slower phase (534 ns) is associated with a small ΔH of 2 ± 3 kcal mol^-1 and ΔV of 1 ± 0.5 mL mol^-1.

Dynamics of the heme-binding bacterial gas-sensing dissimilative nitrate respiration regulator (DNR) and activation barriers for ligand binding and escape

DNR (dissimilative nitrate respiration regulator) is a heme-binding transcription factor that is involved in the regulation of denitrification in Pseudomonas aeruginosa. In the ferrous deoxy state, the heme is 6-coordinate; external NO and CO can replace an internal ligand. Using fluorescence anisotropy, we show that high-affinity sequence-specific DNA binding occurs only when the heme is nitrosylated, consistent with the proposed function of DNR as NO sensor and transcriptional activator. This role is moreover supported by the NO "trapping" properties revealed by ultrafast spectroscopy that are similar to those of other heme-based NO sensor proteins. Dissociated CO-heme pairs rebind in an essentially barrierless way. This process competes with migration out of the heme pocket. The latter process is thermally activated (Ea ∼ 7 kJ/mol). This result is compared with other heme proteins, including the homologous CO sensor/transcription factor CooA, variants of the 5-coordinate mycobacterial sensor DosT and the electron transfer protein cytochrome c. This comparison indicates that thermal activation of ligand escape from the heme pocket is specific for systems where an external ligand replaces an internal one. The origin of this finding and possible implications are discussed.

Ligand Dynamics in an Electron Transfer Protein

Substitution of the heme coordination residue Met-80 of the electron transport protein yeast iso-1-cytochrome c allows external ligands like CO to bind and thus increase the effective redox potential. This mutation, in principle, turns the protein into a quasi-native photoactivable electron donor. We have studied the kinetic and spectral characteristics of geminate recombination of heme and CO in a series of single M80X (X = Ala, Ser, Asp, Arg) mutants, using femtosecond transient absorption spectroscopy. In these proteins, all geminate recombination occurs on the picosecond and early nanosecond time scale, in a multiphasic manner, in which heme relaxation takes place on the same time scale. The extent of geminate recombination varies from >99% (Ala, Ser) to approximately 70% (Arg), the latter value being in principle low enough for electron injection studies. The rates and extent of the CO geminate recombination phases are much higher than in functional ligand-binding proteins like myoglobin, presumably reflecting the rigid and hydrophobic properties of the heme environment, which are optimized for electron transfer. Thus, the dynamics of CO recombination in cytochrome c are a tool for studying the heme pocket, in a similar way as NO in myoglobin. We discuss the differences in the CO kinetics between the mutants in terms of the properties of the heme environment and strategies to enhance the CO escape yield. Experiments on double mutants in which Phe-82 is replaced by Asp or Gly as well as the M80D substitution indicate that such steric changes substantially increase the motional freedom-dissociated CO.