Role of the arginine-45 salt bridge in ligand dissociation from sperm whale carboxymyoglobin as probed by photoacoustic calorimetry

Role of Ionic Strength and pH in Modulating Thermodynamic Profiles Associated with CO Escape from Rice Nonsymbiotic Hemoglobin 1

Type 1 nonsymbiotic hemoglobins are found in a wide variety of land plants and exhibit very high affinities for exogenous gaseous ligands. These proteins are presumed to have a role in protecting plant cells from oxidative stress under etiolated/hypoxic conditions through NO dioxygenase activity. In this study we have employed photoacoustic calorimetry, time-resolved absorption spectroscopy, and classical molecular dynamics simulations in order to elucidate thermodynamics, kinetics, and ligand migration pathways upon CO photodissociation from WT and a H73L mutant of type 1 nonsymbiotic hemoglobin from Oryza sativa (rice). We observe a temperature dependence of the resolved thermodynamic parameters for CO photodissociation from CO-rHb1 which we attribute to temperature dependent formation of a network of electrostatic interactions in the vicinity of the heme propionate groups. We also observe slower ligand escape from the protein matrix under mildly acidic conditions in both the WT and H73L mutant (τ = 134 ± 19 and 90 ± 15 ns). Visualization of transient hydrophobic channels within our classical molecular dynamics trajectories allows us to attribute this phenomenon to a change in the ligand migration pathway which occurs upon protonation of the distal His73, His117, and His152. Protonation of these residues may be relevant to the functioning of the protein in vivo given that etiolation/hypoxia can cause a decrease in intracellular pH in plant cells.

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.

Pyridin-2-yl guanidine derivatives: conformational control induced by intramolecular hydrogen-bonding interactions

The synthesis and conformational analysis of a series of pyridin-2-yl guanidine derivatives using NMR, X-ray crystallography, and B3LYP/6-31+G** theoretical studies are reported. A remarkable difference was observed in the (1)H NMR spectra of the guanidinium salts as compared with their N,N'-di-Boc protected and neutral analogues. This difference corresponds to a 180° change in the dihedral angle between the guanidine/ium moiety and the pyridine ring in the salts as compared to the Boc-protected derivatives, a conclusion that was supported by theoretical studies, X-ray data, and NMR analysis. Moreover, our data sustain the existence of two intramolecular hydrogen-bonding systems: (i) between the pyridine N1 atom and the guanidinium protons in the salts and (ii) within the tert-butyl carbamate groups of the Boc-protected derivatives. To verify that the observed conformational control arises from these intramolecular interactions, a new series of N-Boc-N'-propyl-substituted pyridin-2-yl guanidines were also prepared and studied.

Time resolved thermodynamics associated with ligand photorelease in heme peroxidases and globins: Open access channels versus gated ligand release

Heme proteins represent a diverse class of biomolecules responsible for an extremely diverse array of physiological functions including electron transport, monooxygenation, ligand transport and storage, cellular signaling, respiration, etc. An intriguing aspect of these proteins is that such functional diversity is accomplished using a single type of heme macrocycle based upon iron protoporphyrin IX. The functional diversity originates from a delicate balance of inter-molecular interactions within the protein matrix together with well choreographed dynamics that modulate the heme electronic structure as well as ligand entry/exit pathways from the bulk solvent to the active site. Of particular interest are the dynamics and energetics associated with the entry/exit of ligands as this process plays a significant role in regulating the rates of heme protein activity. Time-resolved photoacoustic calorimetry (PAC) has emerged as a powerful tool through which to probe the underlying energetics associated with small molecule dissociation and release to the bulk solvent in heme proteins on time scales from tens of nanoseconds to several microseconds. In this review, the results of PAC studies on various classes of heme proteins are summarized highlighting how different protein structures affect the thermodynamics of ligand migration. This article is part of a Special Issue entitled: Protein Dynamics: Experimental and Computational Approaches.

The efficiency of malachite green, free and protein bound, as a photon-to-heat converter

Dye assisted laser inactivation of proteins has been found to be a methodology that can achieve high selectivity. Despite the fact that the methodology is successful, knowledge of the detailed inactivation mechanism would allow full optimization of this technique. Here, pulsed-laser photoacoustic calorimetry is used to study the photophysical properties, principally the heat release behavior, of protein bound malachite green. We found that when bound to bovine serum albumin the dye is a good photon-to-heat converter, but approximately 2.6% of the absorbed photon energy (lambda(exc) = 624 nm) is not released as heat in less than 10 mus. This observation suggests that a mechanism other than simple heat-induced inactivation may be the principle process; a long lived excited triplet state of malachite green (or species derived from it) is postulated to play a major role.

Photolyses of mammalian oxy-hemoglobin studied by nanosecond photoacoustic calorimetry

Enthalpy and conformational volume changes in photolyses of oxy-hemoglobin (HbO(2)) of human, bovine, pig, horse and rabbit are investigated by photoacoustic calorimetry. In the experiment, a pulsed Nd:YAG laser is used as an exciting source, and a PVDF film transducer and a PZT transducer are used to detect the photoacoustic signals. Based on the time scales of the excitation and detection systems as well as the photolysis processes of HbO(2), it can be indicated that the measured enthalpy and conformational volume changes are related to slow geminate recombination and tertiary relaxation in photolyses of HbO(2), which are with the time scale of 30-40 ns and 100-150 ns, respectively. The results show that the enthalpy and conformational volume changes are different for both photolysis processes of HbO(2) and also for various mammals. The different results among the five mammals are analyzed and discussed briefly.

The contribution of heme propionate groups to the conformational dynamics associated with CO photodissociation from horse heart myoglobin

Photoacoustic calorimetry and transient absorption spectroscopy were used to study conformational dynamics associated with CO photodissociation from horse heart myoglobin (Mb) reconstituted with either Fe protoporphyrin IX dimethylester (FePPDME), Fe octaethylporphyrin (FeOEP), or with native Fe protoporphyrin IX (FePPIX). The volume and enthalpy changes associated with the Fe-CO bond dissociation and formation of a transient deoxyMb intermediate for the reconstituted Mbs were found to be similar to those determined for native Mb (DeltaV1 = -2.5+/-0.6 ml mol(-1) and DeltaH1 = 8.1+/-3.0 kcal mol(-1)). The replacement of FePPIX by FeOEP significantly alters the conformational dynamics associated with CO release from protein. Ligand escape from FeOEP reconstituted Mb was determined to be roughly a factor of two faster (tau=330 ns) relative to native protein (tau=700 ns) and accompanying reaction volume and enthalpy changes were also found to be smaller (DeltaV2 = 5.4+/-2.5 ml mol(-1) and DeltaH2 = 0.7+/-2.2 kcal mol(-1)) than those for native Mb (DeltaV2 = 14.3+/-0.8 ml mol(-1) and DeltaH2 = 7.8+/-3.5 kcal mol(-1)). On the other hand, volume and enthalpy changes for CO release from FePPIX or FePPDME reconstituted Mb were nearly identical to those of the native protein. These results suggest that the hydrogen bonding network between heme propionate groups and nearby amino acid residues likely play an important role in regulating ligand diffusion through protein matrix. Disruption of this network leads to a partially open conformation of protein with less restricted ligand access to the heme binding pocket.

Photoacoustic characterization of protein dynamics following CO photodetachment from fully reduced bovine cytochrome c oxidase

We report a protein conformational change following carbon monoxide photodetachment from fully reduced bovine cytochrome c oxidase that is hypothesized to be associated with changes in ligand mobility through a dioxygen access channel in the protein. Although not resolved by earlier photoacoustic or optical studies on this adduct, utilization of slightly lower temperatures revealed a process with a kinetic lifetime of about 70 ns at 10 degrees C. We measure an enthalpy change of about 8 kcal/mol in 0.050 M HEPES buffer that becomes less endothermic (DeltaH approximately 2 kcal/mol) at higher ionic strength. The volume contraction of about -0.7 mL/mol associated with the process almost doubles in higher ionic strength buffer systems. Measurements of samples in phosphate buffer systems are similar and appear to display the same subtle ionic strength dependence. Both the isolation of this photoacoustic signal component and the possible dependence on ionic strength of the thermodynamic parameters derived from its analysis appear analogous to and consistent with prior photoacoustic results monitoring CO photodetachment from the camphor complex of cytochrome P-450. Accordingly, we consider a similar model in which a conformational change results in movement of an exposed charged group or groups towards the interior of the protein, out of contact with solvent, as in the closing of a salt bridge.

Characterization of carbon monoxide photodissociation from Fe(II)LPO with photoacoustic calorimetry

Lactoperoxidase belongs to a family of mammalian peroxidases that catalyze the oxidation of halides and small organic molecules in the presence of H2O2. We have used photoacoustic calorimetry to characterize thermodynamic parameters associated with ligand dissociation from bovine milk lactoperoxidase. Upon CO photorelease, a prompt (tau < 50 ns) exothermic volume contraction (DeltaH = -20 +/- 7 kcal mol-1 and DeltaH = -2 +/- 1 mL mol-1) was measured at pH 7.0 and 4.0, whereas an endothermic expansion (DeltaH = 30 +/- 13 kcal mol-1 and DeltaV = 9 +/- 2 mL mol(-1)) was observed at pH 10.0 and 7.0 in the presence of 500 mM NaCl. We attribute the observed volume and enthalpy changes to electrostriction arising from changes in the charge distribution associated with a reorganization of the heme binding pocket upon ligand dissociation. It is likely that cleavage of the Fe-CO bond is accompanied by distortion of a salt bridge between Arg557 and the heme propionate group, resulting in the observed electrostriction due to changes in charge distribution.

Spectroscopic and Photothermal Study of Myoglobin Conformational Changes in the Presence of Sodium Dodecyl Sulfate

Interactions between sodium dodecyl sulfate (SDS) and horse heart myoglobin (Mb) at surfactant concentrations below the critical micelle concentration have been studied using steady-state and transient absorption spectroscopies and photoacoustic calorimetry. SDS binding to Mb induces a heme transition from high-spin five-coordinate to low-spin six-coordinate in met- and deoxyMb, with the distal His residue likely to be the sixth ligand. The transition is complete at an SDS concentration of approximately 350 microM and approximately 700 microM for met- and deoxyMb, respectively. DeltaG(H(2)O) and m values determined from equilibrium SDS-induced unfolding curves indicate similar stability of met- and deoxyMb toward unfolding; however, the larger m value for the deoxyMb equilibrium intermediate indicates that its structure differs from that of metMb. Results from transient absorption spectroscopy show that CO rebinding to Fe(2+)-Mb in the presence of SDS is a biphasic process with the rate constant of the first process approximately 5.5 x 10(3) s(-1), whereas the second process displays a rate similar to that for CO rebinding to native Mb (k(obs) = 7.14 x 10(2) s(-1)) at 1 mM CO. Results of photoacoustic calorimetry show that CO dissociation from deoxyMb occurs more than 10 times faster in the presence of SDS than in native Mb. These data suggest that the heme binding pocket is more solvent-exposed in the SDS-induced equilibrium intermediate relative to native Mb, which is likely due to the electrostatic and hydrophobic interactions between surfactant molecules and the protein matrix.

Laser-induced time-resolved photoacoustic calorimetry study on photo-dissociation of human and bovine oxyhemoglobin

The dynamics of the enthalpy and volume changes related to the photo-dissociation of oxygen from human and bovine oxyhemoglobin are investigated by nanosecond time-resolved photoacoustic calorimetry (PAC). The values of enthalpy and volume change associated with the above process are deltaH = 37.8 +/- 3 kcal/mol, deltaV = 5.0 +/- 1 ml/mol for human HbO(2); and deltaH = 35.7 +/- 3.5 kcal/mol, deltaV = 4.8 +/- 1 ml/mol for bovine HbO(2), respectively. A possible explanation for the similar values between both human and bovine oxyhemoglobin is proposed. In addition, the PAC results for human HbO(2) and HbCO are compared and discussed.

Co-C bond dissociation energy and reaction volume change of 2',5'-dideoxyadenosylcobalamin studied by laser-induced time-resolved photoacoustic calorimetry

Time resolved photoacoustic calorimetry (PAC) was applied to a study of the photolysis of a coenzyme B(12) analog 2',5'-dideoxyadenosylcobalamin, which lacks an -OH group at the 2' position of ribofuranose ring. In aqueous solution, we report for the first time the quantum yield Phi(d) (0.25+/-0.02), Co-C bond dissociation energy (BDE; 31.8+/-2.5 kcal mol(-1)) and reaction volume change deltaV(R) (6.5+/-0.5 ml mol(-1)) due to conformation changes of the corrin ring and its side chains accompanying the cleavage of the Co-C bond. These values for the analog are very similar to those for the natural cofactor. Based our results and previous studies, a possible explanation for the similarity in their structure and properties versus the large difference in their enzymatic activity is discussed.

Time-resolved photothermal methods: accessing time-resolved thermodynamics of photoinduced processes in chemistry and biology

Photothermal methods are currently being employed in a variety of research areas, ranging from materials science to environmental monitoring. Despite the common term which they are collected under, the implementations of these techniques are as diverse as the fields of application. In this review, we concentrate on the recent applications of time-resolved methods in photochemistry and photobiology.

Protein relaxation in the photodissociation of myoglobin-CO complexes

Laser-induced optoacoustic spectroscopy has been applied to the study of the photodissociation of myoglobin-CO complexes. Time-resolved optoacoustic signals have been measured from aqueous solutions of horse myoglobin-CO complex (hMbCO) at pH 3.5 and 8, and of sperm whale myoglobin-CO complex (swMbCO) at pH 8, in the temperature range 273-300 K. The signal of hMbCO at pH 8 exhibits three components. The first, which is faster than 20 ns and is associated with a reaction enthalpy of 61 kJ mol(-1), corresponds to Fe-CO bond breakage. The second component has a decay time of 80 ns at 293 K and is associated with an exothermic protein relaxation (-13 kJ mol(-1)) and a volume change of -3 ml mol(-1). The relaxation, which involves a state where the photo-dissociated CO is still in a protein docking site, is thermally activated, with an activation enthalpy of 51 kJ mol(-1). The third component has a decay time of 800 ns at 293 K and an activation enthalpy of 39 kJ mol(-1), and is associated with an endothermic process (26 kJ mol(-1)) and an expansion of 19 ml mol(-1). This process is ascribed to the migration of the photodissociated CO to the bulk solvent. At acidic pH, the latter process becomes faster (230 ns) and the volume change decreases. These features are correlated with the presence of an open form of the protein. swMbCO exhibits two components only, due to the overlap of the two fastest processes. The first involves a reaction enthalpy of 49 kJ mol(-1) and a volume contraction of -4.9 ml mol(-1). The second component (900 ns at 293 K, activation enthalpy 45 kJ mol(-1)) is associated with a reaction enthalpy of 38 kJ mol(-1) and a volume expansion of 15.3 ml mol(-1). These experimental findings have been interpreted by means of a new model, which also takes into account both laser flash photolysis results and structural information. The model is based on a two-dimensional scheme which describes both protein relaxation and the CO pathway following photodissociation.

Volume and enthalpy profiles of CO rebinding to horse heart myoglobin

Carbon monoxide binding to myoglobin was characterized using the photothermal beam deflection method. The volume and enthalpy changes coupled to CO dissociation were found to be 9.3+/-0.8 mL x mol(-1) and 7.4+/-2.8 kcal x mol(-1), respectively. The corresponding values observed for CO rebinding have the same magnitude but opposite sign: Delta V=-8.6+/-0.9 mL x mol(-1) and Delta H=-5.8+/-2.9 kcal x mol(-1). Ligand rebinding occurs as a single conformational step with a rate constant of 5 x 10(5) M(-1) s(-1) and with activation enthalpy of 7.1+/-0.8 kcal x mol(-1) and activation entropy of -22.4+/-2.8 cal x mol(-1) K(-1). Activation parameters for the ligand binding correspond to the activation parameters previously obtained using the transient absorption methods. Hence, at room temperature the CO binding to Mb can be described as a two-state model and the observed volume contraction occurs during CO-Fe bond formation. Comparing these results with CO dissociation reactions, for which two discrete intermediates were characterized, indicates differences in mechanism by which the protein modulates ligand association and dissociation.

Volume and enthalpy profiles of CO binding to Fe(II) tetrakis-(4-sulfonatophenyl)porphyrin

The focus of this study is to examine volume and enthalpy profiles of ligand binding associated with CO-Fe(II) tetrakis-(4-sulfonato phenyl)-porphyrin (COFe(II)4SP) in aqueous solution. Temperature dependent photothermal beam deflection was employed to probe the overall enthalpy and volume changes associated with CO-photolysis and recombination. The analysis demonstrates that ligand recombination occurs with a pseudo first order rate constant of (2.5+/-0.2)x10(4) s(-1) (at 25 degrees C) with a corresponding volume decrease of 6+/-1 ml/mol. The activation enthalpy (DeltaH(double dagger)) and volume (DeltaV(double dagger)) change for CO recombination (determined from temperature/pressure dependent transient absorption spectroscopy) are found to be 3.9 kcal/mol and 8.2 ml/mol, respectively. These data are consistent with a mechanism in which photolysis yields a five-coordinate high spin (H(2)O)Fe(II)4SP complex that recombines in a single step to form the low spin (CO)(H(2)O)Fe(II)4SP complex. Base elimination, often associated with CO photolysis from hemes, is not observed in this system. The overall volume changes suggest a transition state with significant high spin character. Furthermore, these results demonstrate the utility of coupling photothermal techniques with variable pressure/temperature transient absorption spectroscopy to probe heme reaction dynamics.

Dynamics of structure and energy of horse carboxymyoglobin after photodissociation of carbon monoxide

The energetics and structural volume changes after photodissociation of carboxymyoglobin are quantitatively investigated by laser-induced transient grating (TG) and photoacoustic calorimetric techniques. Various origins of the TG signal are distinguished: the phase grating signals due to temperature change, due to absorption spectrum change, and due to volume change. We found a new kinetics of approximately 700 ns (at room temperature), which was not observed by the flash photolysis technique. This kinetics should be attributed to the intermediate between the geminate pair and the fully dissociated state. The enthalpy of an intermediate species is determined to be 61 +/- 10 kJ/mol, which is smaller than the expected Fe-CO bond energy. The volume of MbCO slightly contracts (5 +/- 3 cm(3)/mol) during this process. CO is fully released from the protein by an exponential kinetics from 25 to -2 degrees C. During this escaping process, the volume expands by 14.7 +/- 2 cm(3)/mol at room temperature and 14 +/- 10 kJ/mol is released, which should represent the protein relaxation and the solvation of the CO (the enthalpy of this final state is 47 +/- 10 kJ/mol). A potential barrier between the intermediate and the fully dissociated state is DeltaH(*) = 41.3 kJ/mol and DeltaS(*) = 13.6 J mol(-1) K(-1). The TG experiment under a high wavenumber reveals that the volume expansion depends on the temperature from 25 to -2 degrees C. The volume changes and the energies of the intermediate species are discussed.

Photoacoustic calorimetry of proteins

We have described two examples of time-resolved photoacoustic calorimetry for the study of heme protein transient intermediates. Before photoacoustic calorimetry, determining thermodynamic information on short-lived intermediates was difficult. Along with being sensitive to enthalpic and volume changes, photoacoustic calorimetry can detect conformational changes in a time-resolved manner. In complex protein systems, the interpretation of the structural origins of a conformational change is sometimes difficult. Site-directed mutagenesis has been used successfully to identify the residues that play important roles in the ligand binding to both Mb and cytochrome P450cam. In both systems the hydration state of salt bridges gave rise to volume changes that were identified through mutagenesis of the residues involved. With its increasing popularity and the power of site-directed mutagenesis, time-resolved photoacoustic calorimetry is fast becoming a technique to probe conformational dynamics in proteins.

Triangular kinetic schemes applied to the stability of a heme-globin complex

Horse heart apomyoglobin traps the heme released from Aplysia californica myoglobin. The kinetics fit a triangular mechanism for a bipbasic reaction. Laplacian solutions for differential equations appropriate to triangular kinetic schemes involving up to four rate constants are elaborated and confirmed. Two general schemes and two special cases are considered. In the first scheme, a rearrangement of the starting material is concurrent with product formation. In the second scheme, the starting material forms two products in equilibrium at two different rates. A general equation for the absorbance-time curve is derived for these triangular schemes, from which rate constants can be estimated. Changes in instantaneous rate versus time are employed to analyze the absorption versus time plots and the curvature of a first-order rate analysis. Aplysia metmyoglobin equilibrates between slow donor (pentacoordinate, which lacks the axial water molecule) and fast donor (bexacoordinate). No heme release was observed for deoxy, oxy, carbonyl, or azide derivatives of the Aplysia myoglobin, or when the distal HisE7 of the apohemoprotein is replaced by leucine or valine. This suggest a role for hydrophobicity of the active site, and for a trans effect of the axial ligand in determining the stability of the embedded prosthetic heme.

Volume and enthalpy changes in the early steps of bacteriorhodopsin photocycle studied by time-resolved photoacoustics

We have studied the photoinduced volume changes, energetics, and kinetics in the early steps of the bacteriorhodopsin (BR) photocycle with pulsed, time-resolved photoacoustics. Our data show that there are two volume changes. The fast volume change ( < or = 200 ns) is an expansion (2.5 +/- 0.3 A3/molecule) and is observed exclusively in the purple membrane (PM), vanishing in the 3-[(3-cholamidopropyl)-dimethylammonio] -1-propane-sulfonate-sulfonate-solubilized BR sample; the slow change (approximately 1 micros) is a volume contraction (-3.7 +/- 0.3 A3/molecule). The fast expansion is assigned to the restructuring of the aggregated BR in the PM, and the 1-micros contraction to the change in hydrogen bonding of water at Asp 212 (Kandori et al. 1995. J. Am. Chem. Soc. 117:2118-2119). The formation of the K intermediate releases most of the absorbed energy as heat, with delta Hk = -36 +/- 8 kJ/mol. The activation energy of the K --> L step is 49 +/- 6 kJ/mol, but the enthalpy change is small, -4 +/- 10 kJ/mol. On the time scale we studied, the primary photochemical kinetics, enthalpy, and volume changes are not affected by substituting the solvent D2O for H2O. Comparing data on monomeric and aggregated BR, we conclude that the functional unit for the photocycle is the BR monomer, because both the kinetics (rate constant and activation energy) and the enthalpy changes are independent of its aggregation state.

Electrostatic control of the substrate access channel in cytochrome P-450cam

Camphor binding to ferric cytochrome P-450cam is a two-step process. The first step corresponds to the diffusion of camphor into the heme pocket, and the second one corresponds to an observable spin transition of the heme iron. In this paper, electrostatic interactions that may control the opening of the structure to allow substrate access to the buried and not solvent-exposed active site were examined. The electrostatic interactions occurring at the protein surface were weakened by increasing the ionic strength of the medium with sodium salts and strengthened by decreasing the dielectric constant of the medium with ethylene glycol as a cosolvent. The results obtained with the wild-type protein were compared to those obtained with the site-directed mutant of cytochrome P-450cam in which the Arg 186-Asp 251 and Lys 178-Asp 251 salt bridges, located at the entrance of the proposed access channel, were suppressed by replacing Asp 251 with an asparagine residue. Over a range of sodium chloride concentrations from 0 to 400 mM, camphor binding is favored, as seen in the variation in the first step dissociation equilibrium constant, K1d, which decreases from 49.5 to 24 microM, respectively. Addition of ethylene glycol favors the dissociation of the substrate-bound complex. The addition of sodium to the ethylene glycol-containing samples reverses the effect of the cosolvent. Removal of the Arg 186-Asp 251 and Lys 178-Asp 251 salt bridges results in an alteration in camphor binding in which K1d is equal to 34 microM without sodium.(ABSTRACT TRUNCATED AT 250 WORDS)

A photoacoustic calorimetry study of horse carboxymyoglobin on the 10-nanosecond time scale

The development of a photoacoustic calorimeter with a time resolution of 10 ns is presented, and the dynamics of the enthalpy and volume changes found in the photodissociation of CO from horse carboxymyoglobin are examined. With this enhanced time resolution a new transient species, the lifetime of which is 29 ns at 20 degrees C, is observed in the ligand dissociation process.

CO recombination to human myoglobin mutants in glycerol-water solutions

The kinetics of CO recombination to site-specific mutants of human myoglobin have been studied by flash photolysis in the temperature range 250-320 K on the nanosecond to second time scale in 75% glycerol at pH 7. The mutants were constructed to examine specific proposals concerning the roles of Lys 45, Asp 60, and Val 68 in the ligand binding process. It is found that ligand recombination is nonexponential for all the mutants and that both the geminate amplitude and rate show large variations. The results are interpreted in terms of specific models connecting the dynamics and structure. It is shown that removal of the charged group at position 45 does not substantially affect the barrier height for escape or entry of the ligand; therefore the breakage of the salt bridge linking Lys 45, Asp 60, and a heme propionate is ruled out as the rate-determining barrier for this process. On the other hand, it is found that the escape barrier decreases roughly as size of the residue at position 68 increases, in the order Ala > Val > Asn > Leu. The residue at position 68 is also a major contributor to the final barrier to rebinding, but the barrier height shows no correlation with residue size and is more dependent on the stereochemistry of the residue. A molecular mechanism for ligand binding that is consistent with the results is discussed, and supporting evidence for this mechanism is examined.

Picosecond phase grating spectroscopy of hemoglobin and myoglobin: energetics and dynamics of global protein motion

Phase grating spectroscopy has been used to follow the optically triggered tertiary structural changes of carboxymyoglobin (MbCO) and carboxyhemoglobin (HbCO). Probe wavelength and temperature dependencies have shown that the grating signal arises from nonthermal density changes induced by the protein structural changes. The material displaced through the protein structural changes leads to the excitation of coherent acoustic modes of the surrounding water. The coupling of the structural changes to the fluid hydrodynamics demonstrates that a global change in the protein structure is occurring in less than 30 ps. The global relaxation is on the same time scale as the local changes in structure in the vicinity of the heme pocket. The observed dynamics for global relaxation and correspondence between the local and global structural changes provides evidence for the involvement of collective modes in the propagation of the initial tertiary conformational changes. The energetics can also be derived from the acoustic signal. For MbCO, the photodissociation process is endothermic by 21 +/- 2 kcal/mol, which corresponds closely to the expected Fe-CO bond enthalpy. In contrast, HbCO dissipates approximately 10 kcal/mol more energy relative to myoglobin during its initial tertiary structural relaxation. The difference in energetics indicates that significantly more energy is stored in the hemoglobin structure and is believed to be related to the quaternary structure of hemoglobin not present in the monomeric form of myoglobin. These findings provide new insight into the biomechanics of conformational changes in proteins and lend support to theoretical models invoking stored strain energy as the driving force for large amplitude correlated motions.

Analysis of photoacoustic waveforms using the nonlinear least squares method

Pulsed-laser photoacoustics is a technique which measures photoinduced enthalpic and volumetric changes on the nano- and microsecond timescales. Analysis of photoacoustic data generally requires deconvolution for a sum of exponentials, a procedure which has been developed extensively in the field of time-resolved fluorescence decay. Initial efforts to adapt an iterative nonlinear least squares computer program, utilizing the Marquardt algorithm, from the fluorescence field to photoacoustics indicated that significant modifications were needed. The major problem arises from the wide range of transient decay times which must be addressed by the photoacoustic technique. We describe an alternative approach to numerical convolution with exponential decays, developed to overcome the problems. Instead of using an approximation method (Simpson's rule) for evaluating the convolution integral, we construct a continuous instrumental response function by quadratic fitting of the discrete data and evaluate the convolution integral directly, without approximations. The success and limitations of this quadratic-fit convolution program are then demonstrated using simulated data. Finally, the program is applied to the analysis of experimental data to compare the resolution capabilities of two commercially available transducers. The advantages of a broadband, heavily damped transducer are shown for a standard organic photochemical system, the quenching of the triplet state of benzophenone by 2,5-dimethyl-2,4-hexadiene.

Direct observation of global protein motion in hemoglobin and myoglobin on picosecond time scales

Picosecond phase-grating spectroscopy is highly sensitive to density changes and provides a new holographic approach to the study of protein dynamics. Photodissociation of carbon monoxide from heme proteins induces a well-defined transition from a ligated to a deoxy structure that is important to hemoglobin and myoglobin functionality. Grating spectroscopy was used to observe protein-driven density waves on a picosecond time scale after carbon monoxide dissociation. This result demonstrates that global tertiary structure changes of proteins occur on an extremely fast time scale and provides new insight into the biomechanics of deterministic protein motion.

Photoacoustic calorimetric study of the conversion of rhodopsin and isorhodopsin to lumirhodopsin

The enthalpy and volume changes for the conversion of rhodopsin and isorhodopsin to lumirhodopsin have been investigated by time-resolved photoacoustic calorimetry. The conversion of rhodopsin to lumirhodopsin is endothermic by 3.9 +/- 5.9 kcal/mol and is accompanied by an increase in volume of 29.1 +/- 0.8 mL/mol. The lumirhodopsins produced from rhodopsin and isorhodopsin are energetically equivalent.

Binding mode of azide to ferric Aplysia limacina myoglobin. Crystallographic analysis at 1.9 A resolution

The binding mode of azide to the ferric form of Aplysia limacina myoglobin has been studied by X-ray crystallography. The three-dimensional structure of the complex has been refined at 1.9 A resolution to a crystallographic R-factor of 13.9%, including 126 ordered solvent molecules. Azide binds to the heme iron, at the sixth co-ordination position, and is oriented towards the outer part of the distal site crevice. This orientation is stabilized by an ionic interaction with the side-chain of Arg66 (E10) which, from an outer orientation in the 'aquo-met' ligand-free myoglobin, folds back towards the distal site in the presence of the anionic ligand. In the absence of a hydrogen bond donor residue at the distal E7 position in Aplysia limacina myoglobin, a different polar residue, Arg66 at the E10 topological position, has been selected by molecular evolution in order to grant ligand stabilization.