Dynamics of nitric oxide in the active site of reduced cytochrome c oxidase aa3

Nitric oxide (NO) is involved in the regulation of respiration by acting as a competitive ligand for molecular oxygen at the binuclear active site of cytochrome c oxidase. The dynamics of NO in and near this site are not well understood. We performed flash photolysis studies of NO from heme a3 in cytochrome c oxidase from Paracoccus denitrificans, using femtosecond transient absorption spectroscopy. The formation of the product state--the unliganded heme a3 ground state--occurs in a similar stepwise manner (period approximately 700 fs) as previously observed for carbon monoxide photolysis from this enzyme and interpreted in terms of ballistic ligand motions in the active site on the subpicosecond time scale [Liebl, U., Lipowski, G., Négrerie, M., Lambry, J.-C., Martin, J.-L., and Vos, M. H. (1999) Nature 401, 181-184]. A fraction (approximately 35% at very low NO concentrations) of the dissociated NO recombines with heme a3 in 200-300 ps. The presence of this recombination phase indicates that a transient bond to the second ligand-binding site, a copper atom (CuB), has a short lifetime or may not be formed. Increasing the NO concentration increases the recombination yield on the hundreds of picoseconds time scale. This effect, unprecedented for heme proteins, implies that, apart from the one NO molecule bound to heme a3, a second NO molecule can be accommodated in the active site, even at relatively low (submicromolar) concentrations. Models for NO accommodation in the active site, based on molecular dynamics energy minimizations are presented. Pathways for NO motion and their relevance for the regulation of respiration are discussed.

Coherent reaction dynamics in a bacterial cytochrome c oxidase

Biological reactions in protein complexes involve structural dynamics spanning many orders of magnitude in time. In standard descriptions of catalysis by enzymes, the transition state between reactant and product is reached by thermal, stochastic motion. In the ultrashort time domain, however, the protein moiety and cofactor motions leading to altered conformations can be coherent rather than stochastic in nature. Such coherent motions may play a key role in controlling the accessibility of the transition state and explain the high efficiency of the reaction. Here we present evidence for coherent population transfer to the product state during an ultrafast reaction catalysed by a key enzyme in aerobic organisms. Using the enzyme cytochrome c oxidase aa3 from the bacterium Paracoccus denitrificans, we have studied haem dynamics during the photo-initiated ultrafast transfer of carbon monoxide from haem a3 to CuB by femtosecond spectroscopy. The ground state of the unliganded a3 species is populated in a stepwise manner in time, indicating that the reaction is mainly governed by coherent vibrations (47cm(-1)). The reaction coordinate involves conformational relaxation of the haem group and we suggest that ligand transfer also contributes.

Geminate recombination of nitric oxide to endothelial nitric-oxide synthase and mechanistic implications

The nitric-oxide synthase (NOS) catalyzes the oxidation of L-arginine to L-citrulline and NO through consumption of oxygen bound to the heme. Because NO is produced close to the heme and may bind to it, its subsequent role in a regulatory mechanism should be scrutinized. We therefore examined the kinetics of NO rebinding after photodissociation in the heme pocket of human endothelial NOS by means of time-resolved absorption spectroscopy. We show that geminate recombination of NO indeed occurs and that this process is strongly modulated by L-Arg. This NO rebinding occurs in a multiphasic fashion and spans over 3 orders of magnitude. In both ferric and ferrous states of the heme, a fast nonexponential picosecond geminate rebinding first takes place followed by a slower nanosecond phase. The rates of both phases decreased, whereas their relative amplitudes are changed by the presence of L-Arg; the overall effect is a slow down of NO rebinding. For the isolated oxygenase domain, the picosecond rate is unchanged, but the relative amplitude of the nanosecond binding decreased. We assigned the nanosecond kinetic component to the rebinding of NO that is still located in the protein core but not in the heme pocket. The implications for a mechanism of regulation involving NO binding are discussed.

Low frequency vibrational modes in proteins: changes induced by point-mutations in the protein-cofactor matrix of bacterial reaction centers

As a step toward understanding their functional role, the low frequency vibrational motions (<300 cm-1) that are coupled to optical excitation of the primary donor bacteriochlorophyll cofactors in the reaction center from Rhodobacter sphaeroides were investigated. The pattern of hydrogen-bonding interaction between these bacteriochlorophylls and the surrounding protein was altered in several ways by mutation of single amino acids. The spectrum of low frequency vibrational modes identified by femtosecond coherence spectroscopy varied strongly between the different reaction center complexes, including between different mutants where the pattern of hydrogen bonds was the same. It is argued that these variations are primarily due to changes in the nature of the individual modes, rather than to changes in the charge distribution in the electronic states involved in the optical excitation. Pronounced effects of point mutations on the low frequency vibrational modes active in a protein-cofactor system have not been reported previously. The changes in frequency observed indicate a strong involvement of the protein in these nuclear motions and demonstrate that the protein matrix can increase or decrease the fluctuations of the cofactor along specific directions.

Spectral equilibration and primary photochemistry in Heliobacillus mobilis at cryogenic temperature

We performed multicolor femtosecond transient absorption measurements on membranes of the photosynthetic bacterium Heliobacillus mobilis at 20 K, by selective excitation at either the red or the blue extreme of the bacteriochlorophyll g Q(Y) band, which is split in three spectral forms (Bchl g 778, 793, and 808) at low temperature. In contrast to room temperature, there is no observable uphill energy transfer upon excitation at the red extreme. This provides a direct experimental confirmation of the expected strong temperature dependence of uphill energy transfer in multichromophore systems. Upon excitation at the blue edge, downhill energy transfer is observed on time ranges varying over 2 orders of magnitude and is discussed in terms of four distinct energy transfer processes: Bchl g 778* --> Bchl g 793* (approximately 50 fs); Bchl g 778* --> Bchl g 808* (approximately 400 fs); Bchl g 793* --> Bchl g 808* (approximately 1.4 ps); and within Bchl g 808* (approximately 7 ps). Surprisingly, the amount of oxidized primary donor P798+ formed on the time scale of picoseconds and tens of picoseconds was found to depend on the excitation conditions: trapping occurs mainly in approximately 80 ps and slower from directly excited Bchl g 808* and can additionally occur in a few picoseconds from Bchl g 778* and Bchl g 793* upon blue excitation. This finding implies that spectral equilibration is not complete prior to charge separation and furthermore is inconsistent with a funnel model, in which P798 is surrounded by long-wavelength pigments. More generally, we discuss to what extent our data bring constraints on the spatial distribution of the different spectral forms of the pigments.

Energy and electron transfer upon selective femtosecond excitation of pigments in membranes of Heliobacillus mobilis

Excitation energy transfer steps in membranes of Heliobacillus mobilis were directly monitored by transient absorption spectroscopy with a time resolution of 30 fs under selective excitation within the inhomogeneously broadened bacteriochlorophyll g QY band. The initial anisotropy was found to be > 0.4, indicating that the pigments are excitonically coupled. After initial decay of this anisotropy in < 50 fs, major sub-picosecond components associated with spectral equilibration were identified, corresponding to uphill energy transfer with a 300 fs time constant (812 nm excitation) and downhill energy transfer with 100 and 500 fs components (770 nm excitation). These equilibrations are ascribed predominantly to single excitation transfer steps, as anisotropy measurements showed that equilibration within spectrally similar pigments occurs on the same time scale as spectral equilibration, a situation which contrasts with that in photosystem I. Downhill energy transfer occurs to a significant extent directly to an energetically heterogeneous population of excited states as well as in a sequential way via gradually lower-lying pools of bacteriochlorophyll g. This finding supports a description in which all pigments, including the bluemost absorbing, are spatially organized in a random way rather than in clusters of spectrally similar species. Spectral equilibration is not entirely completed prior to formation of the primary radical pair P798 + A0-, which was found to proceed in a multiexponential way (time constants of 5 and 30 ps). No indication for the formation of radical species other than P798 + A0- on the time scale up to 100 ps was found.

Picosecond geminate recombination of CO to the complexes calmodulin*heme-CO and calmodulin*heme-CO*melittin

Picosecond CO recombination kinetics have been measured after photodissociation of the artificial complexes calmodulin*heme-CO and calmodulin*heme-CO*melittin. These systems show an enhancement of the geminate fraction of kinetics relative to unbound heme-CO, due in part to fast geminate kinetics (tau=50ps for the initial phase), as well as a decrease in the rate of migration of CO away from the binding site. This indicates that calmodulin provides a complete pocket around the heme group. Rather than competing with the hemes for binding to calmodulin, the melittin seems to act as a cap to further enclose the hemes; melittin increases the affinity of calmodulin for heme-CO, but only weakly affects the CO recombination kinetics.

Vibrational dephasing of long- and short-lived primary donor excited states in mutant reaction centers of Rhodobacter sphaeroides

Femtosecond spectroscopy was used to study vibrational dynamics in the first singlet excited state (P*) of the primary donor of bacterial reaction centers (RC)in which primary electron transfer dynamics have been altered by single amino acid modifications. We studied intracytoplasmic RC-only membranes containing Rhodobacter sphaeroides wild-type RCs and RCs bearing mutations in the vicinity of P, where Tyr M210 was modified to His, Leu, and Trp and where Phe L181 was modified to Tyr. These mutations do not change the frequencies of the main low-frequency activated modes, which is consistent with a description in which these modes involve extended regions of the protein. Electron transfer in FL181Y, YM210H, and wild-type RCs at 10 K occurs in approximately 1 ps or less, and damping of the coherences occurs simultaneously with the decay of the P* excited state. These results, and a comparison with YM210L RCs, show that in wild-type RCs the damping is primarily determined by the depletion of P* and not by vibrational dephasing induced by interactions with the bath or nonharmonic coupling. In the YM210L and W mutants, electron transfer occurs on a time scale of hundreds of picoseconds at 10 K. Analysis of the longer-lasting vibrational dynamics in these mutants sets a new lower limit for the intrinsic vibrational dephasing time of 1.2 ps for some modes, but of approximately 2 ps for most activated modes.

Coherent dynamics during the primary electron-transfer reaction in membrane-bound reaction centers of Rhodobacter sphaeroides

The temporal evolution of the near-infrared stimulated emission band of the special pair excited state (P*) in the reaction center of Rhodobacter sphaeroides has been studied in intracytoplasmic membranes of the antenna-deficient RCO1 mutant at 10 K with a resolution of 30 fs. On the 100-fs time scale the emission band gradually shifts to longer wavelengths. After 150 fs the band shifts back to shorter wavelengths and continues to develop on the picosecond time scale in a damped oscillatory manner (most prominent fundamental frequencies around 15 cm-1 and at 92, 122, and 153 cm-1). These phenomena are shown to be due to low-frequency vibrational motions in the P* excited state that conserve their phase on the time scale of electron transfer. These results imply that the vibrational manifold of P* is not thermalized during the electron-transfer reaction in functional reaction centers. The initial Stokes shift dynamics are largely determined by the modes in the 90-160-cm-1 frequency range, which probably involve motions of several chromophores, including the bacteriopheophytin electron acceptor HL.

Ultrafast measurements of geminate recombination of NO with site-specific mutants of human myoglobin

Flash photolysis studies of NO recombination to heme proteins offer a direct probe of protein structural changes on the tens of picoseconds timescale where they can be compared with molecular dynamics simulations. The geminate recombination of NO to site-specific mutants of human myoglobin (Mb) was studied following photodissociation of the MbNO form. Single amino acid changes were introduced at positions Val68, His64, Lys45 and Asp60 because motions of residues at these positions are generally regarded as important for the mechanism of ligand binding. In sharp contrast to the properties of simple porphyrin-NO complexes, the rebinding kinetics are found to be non-exponential for all mutants, even in aqueous solution at 298 K. The Val68 and His64 mutants substantially affect the NO rebinding rates but, surprisingly, so do changes on the protein surface that are further away from the iron. These changes in kinetics occur on a tens of picoseconds timescale, and therefore there is either a fast communication between protein residues over quite long distances or there are subtle differences in protein structure that exert great control over the reaction dynamics. Various models for the rebinding kinetics are evaluated. A model-free approach to data analysis using the maximum entropy method is found to be most useful. This analysis shows that the rate distributions are very different for the mutants, but are generally bimodal.

Direct evidence for the role of haem doming as the primary event in the cooperative transition of haemoglobin

The study of cooperative ligand binding among the four subunits of haemoglobin has played a central role in the understanding of allosteric transitions in a large number of enzymes. Haem iron out-of-plane motion has been suggested to be the trigger for the cooperative transition of haemoglobin. To function as a trigger in a dynamic sense, haem-iron doming must be the first conformational change to occur following ligand dissociation. Here we present the first direct demonstration that haem-iron doming occurs on the same time scale as the breaking of the iron-ligand bond, thus establishing haem-iron doming as the primary event which lead to the R-->T transition in haemoglobin.

Nonexponential relaxation after ligand dissociation from myoglobin: a molecular dynamics simulation

Molecular dynamics simulations of myoglobin after ligand photodissociation show that the out-of-plane motion of the heme iron has a rapid subpicosecond phase followed by a slower nonexponential process involving more global protein relaxation. Individual trajectories show rather different behavior, suggesting there is an inhomogeneous component to the relaxation. The calculated time dependence of the iron motion over 100 ps is in excellent agreement with the frequency shift of band III of the heme group [see Lim, M., Jackson, T. A. & Anfinrud, P. A. (1993) Proc. Natl. Acad. Sci. USA 90, 5801-5804]. If that the barrier to rebinding depends on the out-of-plane iron position, the time dependence obtained from the simulation can explain the nonexponential room-temperature geminate recombination of NO.

Femtosecond spectral evolution of the excited state of bacterial reaction centers at 10 K

The femtosecond spectral evolution of reaction centers of Rhodobacter sphaeroides R-26 was studied at 10 K. Transient spectra in the near infrared region, obtained with 45-fs pulses (pump pulses centered at 870 nm and continuum probe pulses), were analyzed with associated kinetics at specific wavelengths. The t = 0-fs transient spectrum is very rich in structure; it contains separate induced bands at 807 and 796 nm and a bleaching near 760 nm, reflecting strong changes in interaction between all pigments upon formation of the excited state. A complex spectral evolution in the 800-nm region, most notably the bleaching of the 796-nm band, takes place within a few hundred femtosecond--i.e., on a time scale much faster than electron transfer from the primary donor P to the bacteriopheophytin acceptor HL. The remarkable initial spectral features and their evolution are presumably related to the presence of HL, as they were not observed in the DLL mutant of Rhodobacter capsulatus, which lacks this pigment. A simple linear reaction scheme with an intermediate state cannot account for our data; the initial spectral evolution must reflect relaxation processes within the excited state. The importance for primary photochemistry of long distance interactions in the reaction center is discussed.

Direct observation of vibrational coherence in bacterial reaction centers using femtosecond absorption spectroscopy

It is shown that vibrational coherence modulates the femtosecond kinetics of stimulated emission and absorption of reaction centers of purple bacteria. In the DLL mutant of Rhodobacter capsulatus, which lacks the bacteriopheophytin electron acceptor, oscillations with periods of approximately 500 fs and possibly also of approximately 2 ps were observed, which are associated with formation of the excited state. The kinetics, which reflect primary processes in Rhodobacter sphaeroides R-26, were modulated by oscillations with a period of approximately 700 fs at 796 nm and approximately 2 ps at 930 nm. In the latter case, at 930 nm, where the stimulated emission of the excited state, P*, is probed, oscillations could only be resolved when a sufficiently narrow (10 nm) and concomitantly long pump pulse was used. This may indicate that the potential energy surface of the excited state is anharmonic or that low-frequency oscillations are masked when higher frequency modes are also coherently excited, or both. The possibility is discussed that the primary charge separation may be a coherent and adiabatic process coupled to low-frequency vibrational modes.

Ligand binding and protein relaxation in heme proteins: a room temperature analysis of NO geminate recombination

Ultrafast absorption spectroscopy is used to study heme-NO recombination at room temperature in aqueous buffer on time scales where the ligand cannot leave its cage environment. While a single barrier is observed for the cage recombination of NO with heme in the absence of globin, recombination in hemoglobin and myoglobin is nonexponential. Examination of hemoglobin with and without inositol hexaphosphate points to proximal constraints as important determinants of the geminate rebinding kinetics. Molecular dynamics simulations of myoglobin and heme-imidazole subsequent to ligand dissociation were used to investigate the transient behavior of the Fe-proximal histidine coordinate and its possible involvement in geminate recombination. The calculations, in the context of the absorption measurements, are used to formulate a distinction between nonexponential rebinding that results from multiple protein conformations (substates) present at equilibrium or from nonequilibrium relaxation of the protein triggered by a perturbation such as ligand dissociation. The importance of these two processes is expected to depend on the time scale of rebinding relative to equilibrium fluctuations and nonequilibrium relaxation. Since NO rebinding occurs on the picosecond time scale of the calculated myoglobin relaxation, a time-dependent barrier is likely to be an important factor in the observed nonexponential kinetics. The general implications of the present results for ligand binding in heme proteins and its time and temperature dependence are discussed. It appears likely that, at low temperatures, inhomogeneous protein populations play an important role and that as the temperature is raised, relaxation effects become significant as well.