Rebinding and relaxation in the myoglobin pocket

Temperature dependence of dynamics of hydrated myoglobin. Comparison of force field calculations with neutron scattering data

Molecular dynamics is used to probe the atomic motions of the carboxy-myoglobin protein as a function of temperature. Simulations of 150 picoseconds in length are carried out on the protein at 20, 60, 100, 180, 220, 240, 260, 280, 300, 320 and 340 K. The simulations attempt to mimic neutron scattering experiments very closely by including a partial hydration shell around the protein. Theoretical elastic, quasielastic and inelastic neutron scattering data are derived from the trajectories and directly compared with experiment. Compared to experiment, the simulation-derived elastic scattering curves show a decrease in intensity as a function of the scattering wavevector, q2. The inelastic and quasielastic spectra show that the inelastic peak is shifted to lower frequency than the experimental value, while quasielastic behavior is in good agreement with experiment. This suggests that the theoretical model is too flexible in the harmonic limit (low temperature), but accurately reproduces high-temperature behavior. Time correlation functions of the intermediate scattering function are determined. At low temperature there is one fast decay process, and at high temperatures there is an additional slow relaxation process that is due to quasielastic scattering. The average atomic fluctuations show that the protein behaves harmonically at low temperatures. At approximately 210 K, a glass-like transition in atomic fluctuations is seen. Above the transition temperature, the atomic fluctuations exhibit both harmonic and anharmonic behavior. Comparison of protein mobility behavior with experiment indicate the fluctuations derived from simulations are larger in the harmonic region. However, the anharmonic region agrees very well with experiment. The anharmonicity is large at all temperatures, with a gradual monotonic increase from 0.5 at 20 K to greater than 0.7 at 340 K without a noticeable change at the glass transition temperature. Heavy-atom dihedral transitions are monitored as a function of temperature. Trends in the type of dihedral transitions that occur with temperature are clearly visible. Dihedral transitions involving backbone atoms occur only above the glass transition temperature. The overall protein behavior results suggest that at low temperatures there is purely vibrational motion with one fast decay process, and above the glass transition temperature there is more anharmonic motion with a fast and a slower relaxation process occurring simultaneously.(ABSTRACT TRUNCATED AT 400 WORDS)

A possible new control mechanism suggested by resonance Raman spectra from a deep ocean fish hemoglobin

The rattail fish, Coryphaenoides armatus, lives at ocean depths of 3000 m. As an adaptation for pumping oxygen into the swim bladder against the extreme pressures at the ocean bottom, the hemoglobin from this fish at low pH exhibits an extraordinarily low affinity for ligands. In this study, continuous wave and time-resolved Raman techniques are used to probe the binding site in this hemoglobin. The findings show an association between the low-affinity material and a highly strained heme-proximal histidine linkage. The transient Raman studies reveal differences in the protein structural dynamics at pH 6 and 8. The emerging picture derived from both this and earlier studies is that in vertebrate hemoglobins the heme-proximal histidine linkage represents a key channel through which species- and solution-dependent variations in the globin are communicated both statically and dynamically to the heme to produce an extensive range of ligand binding properties. Also presented is a new model that relates both intensity and frequency of the resonance Raman band involving the iron-proximal histidine stretching mode to specific protein controlled structural degrees of freedom. There emerges from this model a mechanism whereby modifications in the proximal heme pocket can further reduce the affinity of an already highly strained T state structure of hemoglobin.

Conformational substates and motions in myoglobin. External influences on structure and dynamics

Myoglobin, a simppe dioxygen-storage protein, is a good laboratory for the investigation of the connection between protein structure, dynamics, and function. Fourier-transform infrared spectroscopy on carbon-monoxymyoglobin (MbCO) shows three major CO bands. These bands are excellent probes for the investigation of the structure-function relationship. They have different CO binding kinetics and their CO dipoles form different angles with respect to the heme normal, implying that MbCO exists in three major conformational substates, A0, A1, and A3. The entropies and enthalpies of these substates depend on temperature above approximately 180 K and are influenced by pH, solvent, and pressure. These results suggest that even a protein as simple as Mb can assume a small number of clearly different structures that perform the same function, but with different rates. Moreover, protein structure and dynamics depend strongly on the interaction of the protein with its environment.

O2 and CO reactions with heme proteins: quantum yields and geminate recombination on picosecond time scales

Picosecond time-resolved absorption spectroscopy and low-temperature studies have been undertaken in order to understand the nature of the intrinsic quantum yields and geminate recombination of carbon monoxide and oxygen to hemoglobin and myoglobin. We find that the photoproduct yields at 40 ps and long times (minutes) after photolysis at 8 K are similar; however, the yield of oxygen photoproducts is 0.4 +/- 0.1 while the yield of carbon monoxide photoproducts is 1.0 +/- 0.1 for both myoglobin and hemoglobin. Measurements in the Soret, near-infrared, and far-IR are used to quantitate the photoproduct yields. These results call into question previous cryogenic kinetic studies of O2 recombination. Significant subnanosecond geminate recombination is observed in oxyhemoglobin down to 150 K, while below 100 K this geminate recombination disappears. The lower photoproduct yields for oxyheme protein complexes can be attributed to both subnanosecond and subpicosecond recombination events which are ligand and protein dynamics dependent.

Hydration-dependent conformational states of hemoglobin. Equilibrium and kinetic behavior

The equilibrium and kinetics of methemoglobin conversion to hemichrome induced by dehydration were investigated by visible absorption spectroscopy. Below about 0.20 g water per g hemoglobin only hemichrome was present in the sample; above this value, an increasing proportion of methemoglobin appeared with the increase in hydration. The transition between the two derivatives showed a time-dependent biphasic behavior and was observed to be reversible. The rates obtained for the transition of methemoglobin to hemichrome were 0.31 and 1.93 min-1 and for hemichrome to methemoglobin 0.05 and 0.47 min-1. We suggest that hemichrome is a reversible conformational state of hemoglobin and that the two rates observed for the transition between the two derivatives reflect the alpha- and beta-chains of hemoglobin.

E.p.r. studies of photolysis of nitrosyl haemoglobin at low temperatures

Photolysis of HbNO has been studied from 6.2 K to 15.5 K by electron spin resonance during and after continuous illumination. Non-exponential kinetics of both dissociation and reassociation of NO was observed. The prolonged illumination separates the fast and slow ligands. This picture is consistent with NO tunnelling from two sites at different distances from the bound position. This result is obtained using a model of a sum of two exponentials or of conformational substates.

Inhomogeneous broadening in spectral bands of carbonmonoxymyoglobin. The connection between spectral and functional heterogeneity

The rebinding kinetics of CO to myoglobin after flash photolysis is nonexponential in time below approximately 180 K; the kinetics is governed by a distribution of enthalpic barriers. This distribution results from inhomogeneities in the protein conformation, referred to as conformational substates. Hole-burning experiments on the Soret and IR CO-stretch bands test the assumption that an inhomogeneous distribution of conformational substates results in inhomogeneously broadened spectra. CO was slowly photolyzed at different wavelengths in the Soret band at 10 K. Both the Soret band and the CO-stretch band A1, centered at 1,945 cm-1, shift during photolysis, demonstrating that different wavelengths excite different parts of the distributed population. We have also done kinetic hole-burning experiments by measuring peak shifts in the Soret and A1 bands as the CO molecules rebind. The shifts indicate that the spectral and enthalpic distributions are correlated. In the A1 band, the spectral and enthalpic distributions are highly correlated while in the Soret the correlation is weak. From the peak shifts in the spectral and kinetic hole-burning experiments the inhomogeneous broadening is estimated to be approximately 15% of the total width in the Soret band and approximately 60% in A1. We have previously measured the tilt angle alpha between the bound CO and the heme normal (Ormos, P., D. Braunstein, H. Frauenfelder, M. K. Hong, S.-L. Lin, T. B. Sauke, and R. D. Young. 1988. Proc. Natl. Acad. Sci. USA. 85:8492-8496) and observed a wave number dependence of the tilt angles within the CO-stretch A bands. Thus the spectral and enthalpic distributions of the A bands are coupled to a heterogeneity of the structure.

Temperature-derivative spectroscopy: a tool for protein dynamics

A relaxation method that measures the derivative of a population with respect to temperature is introduced and used to study the recombination of CO to sperm whale myoglobin after a photolyzing flash. Measurement of the geminate process in the infrared CO-stretch bands shows distributed activation enthalpies with different distributions for each band, transitions between two bands that correspond to photolyzed ligands, and kinetic hole burning. The data are well described by gaussian enthalpy distributions; the results match and complement those of isothermal methods. The temperature-derivative technique is further used to explore the recombination of CO from outside the heme pocket.

Metastable intermediates in myoglobin at low pH

Resonance Raman and optical absorption spectra of ligand-free (deoxy) myoglobin and CO-bound myoglobin (MbCO) at pH 2.6 have been measured by using continuous-flow/rapid-mixing techniques. The spectra of deoxy myoglobin at low pH within 6 ms of the pH drop demonstrate that the iron-histidine bond has been ruptured but that the heme is still five-coordinate. Comparison with data from model complexes indicates that a weak-field ligand, such as a water molecule, is coordinated at the fifth position. The Raman spectrum of MbCO at low pH has an Fe-CO stretching mode that is characteristic of a six-coordinate heme with an unhindered Fe-CO moiety. Immediately following the pH drop in this case, there is no indication that the iron-proximal histidine bond is broken. Three different structural changes are detected at low pH: (i) the iron-proximal histidine (F8) bond in ligand-free myoglobin is broken and replaced by a weak-field ligand, (ii) the distal pocket in MbCO is opened, and (iii) protein constraints on the heme group in MbCO are relaxed. Previous conclusions that the kinetics of CO-binding in hemoproteins at low pH is modified by rupturing the iron-proximal histidine bond are supported by these new results which, however, demand a more complete reevaluation of the phenomenon.

Conformational substates of myoglobin detected by extrinsic dynamic fluorescence studies

The extent of conformational substates of two apomyoglobins, i.e., sperm whale and tuna apomyoglobin, was investigated by examining the fluorescence decay in the frequency domain of the extrinsic fluorophore TNS [6-(p-toluidino)-2-naphthalenesulfonic acid] bound to the heme binding site. Data analysis was performed in terms of a continuous, unimodal lifetime distribution having a Lorentzian shape. The results were compared with those for the free fluorophore in an isotropic nonviscous solvent. The incorporation of TNS into the protein matrix resulted in a broadening of the lifetime distribution due to the microenvironmental heterogeneity generated by structural fluctuations. The larger width of lifetime distribution observed for TNS bound to tuna apomyoglobin was related to a more extended conformational space accessible to the fluorophore in this protein compared to sperm whale myoglobin. A temperature increase from 15 to 40 degrees C produced a further broadening of the lifetime distributions of TNS bound to both proteins. This result can be explained by assuming the existence of conformational substates at high energy content or separated by high energy barriers, which are not populated at low temperature. The overall picture emerging from the reported data is that the lifetime distributions of TNS bound to apomyoglobins are determined largely by the number of conformational substates accessible to the protein matrix and, to a lesser extent, by the interconversion rates among these states.

Integer-spin electron paramagnetic resonance of iron proteins

A quantitative interpretation is presented for EPR spectra from integer-spin metal centers having large zero-field splittings. Integer-spin, or non-Kramers, centers are common in metalloproteins and many give EPR signals, but a quantitative understanding has been lacking until now. Heterogeneity of the metal's local environment will result in a significant spread in zero-field splittings and in broadened EPR signals. Using the spin Hamiltonian Hs = S.D.S + beta S.g.B and some simple assumptions about the nature of the zero-field parameter distributions, a lineshape model was devised which allows accurate simulation of single crystal and frozen solution spectra. The model was tested on single crystals of magnetically dilute ferrous fluosilicate. Data and analyses from proteins and active-site models are presented with the microwave field B1 either parallel or perpendicular to B. Quantitative agreement of observed and predicted signal intensities is found for the two B1 orientations. Methods of spin quantitation are given and are shown to predict an unknown concentration relative to a standard with known concentration. The fact that the standard may be either a non-Kramers or a Kramers center is further proof of the model's validity. The magnitude of the splitting in zero magnetic field is of critical importance; it affects not only the chance of signal observation, but also the quantitation accuracy. Experiments taken at microwave frequencies of 9 and 35 GHz demonstrate the need for high-frequency data as only a fraction of the molecules give signals at 9 GHz.

Resonance raman investigations of site-directed mutants of myoglobin: effects of distal histidine replacement

The resonance Raman spectra of met-, deoxy-, and (carbonmonoxy)myoglobin (MbCO) are studied as a function of amino acid replacement at the distal histidine-E7 position. The synthetic wild type is found to be spectroscopically identical with the native material. The methionine and glycine replacements do not affect the met or deoxy spectra but do lead to distinct changes in the nu Fe-CO region of the MbCO spectrum. The native MbCO displays a pH-dependent population redistribution of the nu Fe-CO modes, while the analogous population in the mutant systems is found to be pH independent. This indicates that histidine-E7 is the titratable group in native MbCO. Moreover, the pH dependence of the population dynamics is found to be inconsistent with a simple two-state Henderson-Hasselbalch analysis. Instead, we suggest a four-state model involving the coupling of histidine protonation and conformational change. Within this model, the pK of the distal histidine is found to be 6.0 in the "open" configuration and 3.8 in the "closed" conformation. This corresponds to a 3 kcal/mol destabilization of the positively charged distal histidine within the hydrophobic pocket and suggests how protonation can lead to a larger population of the "open" conformation. At pH 7, the pocket is found to be "open" approximately 3% of the time. Further work, involving both IR and Raman measurements, allows the electron-nuclear coupling strengths of the various nu Fe-CO and nu C-O Raman modes to be determined. The slowly rebinding conformational state, corresponding to nu Fe-CO = 518 cm-1 (nu C-O = 1932 cm-1), displays unusually weak coupling of the Fe-CO mode to the Soret transition. Studies of the nu Fe-CO region as a function of temperature reveal that the equilibria between the conformational states are quenched in both the native and glycine mutant below the freezing point of the solvent. Unusual line narrowing of the nu Fe-CO modes at the phase transition is also observed in all samples studied. This line narrowing stands in marked contrast to the other heme Raman modes and suggests that Fe-CO librational motion and/or distal pocket vibrational (or conformational) excitations are involved in the line broadening at room temperature.

Closed-time distribution of ionic channels. Analytical solution to a one-dimensional defect-diffusion model

A one-dimensional version of the model recently proposed by Läuger (1988) to explain the closed-time distribution of ionic channels in cell membranes is solved analytically. While the probability density f(t) for closed-time lengths may show a well-defined exponential behavior at short times, a power-law decay is predicted at long times. The influence of an additional random distribution of defects in the current-conducting protein is investigated and found to be dominating at long times. Explicit expressions that may be used for fitting experimental data are given for the closed-time distribution. Some of the available data are discussed and shown to be in good agreement with the predictions of the model.

Picosecond tryptophan fluorescence of thioredoxin: evidence for discrete species in slow exchange

The steady-state tryptophan fluorescence and time-resolved tryptophan fluorescence of Escherichia coli thioredoxin, calf thymus thioredoxin, and yeast thioredoxin have been studied. In all proteins, the tryptophan residues undergo strong static and dynamic quenching, probably due to charge-transfer interactions with the nearby sulfur atoms of the active cysteines. The use of a high-resolution photon counting instrument, with a time response of 60 ps full width at half-maximum, allowed the detection of fluorescence lifetimes ranging from a few tens of picoseconds to 10 ns. The data were analyzed both by classical nonlinear least squares and by a new method of entropy maximization (MEM) for the recovery of lifetime distributions. Simulations representative of the experimental data were used to test the MEM analysis. Strong support was obtained in this way for a small number of averaged discrete species in the fluorescence decays. Wavelength studies show that each of these components spreads over closely spaced excited states, while the temperature studies indicate that they do not exchange significantly on the nanosecond time scale. The oxidized form of thioredoxin is characterized by a high content of a very short lifetime below 70 ps, the amplitude of which is sharply decreased upon reduction. On the other hand, the fluorescence anisotropy decays indicate that reduction causes an increase of the very fast tryptophan rotations in an otherwise relatively rigid structure. While the calf thymus and E. coli proteins have mostly similar dynamical fluorescence properties, the yeast thioredoxin differs in many respects.

Multiple conformational states in myoglobin revealed by frequency domain fluorometry

The tryptophanyl fluorescence decays of two myoglobins, i.e., sperm whale and tuna myoglobin, have been examined in the frequency domain with an apparatus which utilizes the harmonic content of a mode-locked laser. Data analysis was performed in terms of continuous distribution of lifetime having a Lorentzian shape. Data relative to sperm whale myoglobin, which possesses two tryptophanyl residues, i.e., Trp-A-5 and -A-12, provided a broad lifetime distribution including decay rates from a few picoseconds to about 10 ns. By contrast, the tryptophanyl lifetime distribution of tuna myoglobin, which contains only Trp-A-12, showed two well-separated and narrow Lorentzian components having centers at about 50 ps and 3.37 ns, respectively. In both cases, the chi 2 obtained from distribution analysis was lower than that provided by a fit using the sum of exponential components. The long-lived components present in the fluorescence decay of the two myoglobins do not correspond to any of those observed for the apoproteins at neutral pH. The tryptophanyl lifetime distribution of sperm whale apomyoglobin consists of two separated Lorentzian components centered at 2.25 and 5.4 ns, whereas that of tuna apomyoglobin consists of a single Lorentzian component, whose center is at 2.19 ns. Acidification of apomyoglobin to pH 3.5 produced a shift of the distribution centers toward longer lifetimes.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural basis of hierarchical multiple substates of a protein. I: Introduction

A computer experiment of protein dynamics is carried out, which consists of two steps: (1) A Monte Carlo simulation of thermal fluctuations in the native state of a globular protein, bovine pancreatic trypsin inhibitor; and (2) a simulation of the quick freezing of fluctuating conformations into energy minima by minimization of the energy of a number of conformations sampled in the Monte Carlo simulation. From the analysis of results of the computer experiment is obtained the following picture of protein dynamics: multiple energy minima exist in the native state, and they are distributed in clusters in the conformational space. The dynamics has a hierarchical structure which has at least two levels. In the first level, dynamics is restricted within one of the clusters of minima. In the second, transitions occur among the clusters. Local parts of a protein molecule, side chains and local main chain segments, can take multiple locally stable conformations in the native state. Many minima result from combinations of these multiple local conformations. The hierarchical structure in the dynamics comes from interactions among the local parts. Protein molecules have two types of flexibility, each associated with elastic and plastic deformations, respectively.

Orientation of carbon monoxide and structure-function relationship in carbonmonoxymyoglobin

Fourier transform infrared spectroscopy of the CO stretch bands in carbonmonoxymyoglobin (MbCO) reveals three major bands implying that MbCO exists in three major substates, A0, A1, and A3. After photolysis at low temperatures the CO is in the heme pocket, and the resulting CO stretch bands represent the B substates. Photoselection experiments determine the orientation of CO in the A (bound) and B (photolyzed) substates: Small fractions of MbCO are photolyzed at 10 K with linearly polarized light at 540 nm. The resulting linear dichroism in the A and B IR bands yields the tilt angle between the heme normal and CO. The average angles are as follows: alpha (A0) = 15 degrees +/- 3 degrees; alpha (A1) = 28 degrees +/- 2 degrees, and alpha (A3) = 33 degrees +/- 4 degrees. The A bands are inhomogeneously broadened; the angle alpha shows a wavenumber dependence within the A bands. The wavenumber dependence is interpreted as a distribution of the tilt angle within the individually inhomogeneous A substates, thus providing a structural parameter to characterize the distribution of the conformational substates. The B substates exhibit no induced linear dichroism; in the photolyzed substates the ligand is randomly oriented with respect to the heme plane. The present results together with earlier data on static and kinetic properties of CO binding to Mb establish relations among spectroscopic, structural, energetic, and functional parameters.

Ligand binding to synthetic mutant myoglobin (His-E7----Gly): role of the distal histidine

Low-temperature flash photolysis with IR and visible spectroscopy was used to probe the influence of the distal histidine His-64(E7) of sperm-whale myoglobin (Mb) on the orientation of bound carbon monoxide (CO) and on the kinetics of CO rebinding. The synthesis and high-level expression of a sperm-whale myoglobin gene in Escherichia coli permits the efficient substitution of the distal histidine through site-directed mutagenesis. Substitution of His-E7 with glycine [GlyE7]Mb bound with CO (CO[GlyE7]Mb) results in one broad bound-CO IR stretch band, v(C-O), centered at 1973 cm-1 at 10 K, in contrast to three distinct bands for native and synthetic wild-type MbCO at 1966, 1945, and 1929 cm-1. After flash photolysis at 10 K, the unbound state of CO[GlyE7]Mb exhibits two CO stretch bands, whereas MbCO has three. Fourier transform IR spectroscopy measurements of the linear dichroism after photoselective flash photolysis of CO bound to [GlyE7]Mb at 10 K reveals the bound CO to be oriented at an angle of alpha = 20 degrees +/- 2 degrees with respect to the heme normal. Flash photolysis data from 10 to 300 K provide evidence for a larger distal pocket and a smaller enthalpy barrier (by approximately 4 kJ/mol) for [GlyE7]MbCO as compared with wild-type MbCO. These results reinforce the notion that the dominant control of the binding step at the heme iron comes from the proximal side through the protein structure.

Dehydration effects on the heme environment of nitric oxide hemoglobin

Our aim is to study the binding of nitric oxide to human hemoglobin with different water contents. The binding is investigated using electron paramagnetic resonance (EPR), considering the paramagnetic character of the nitric oxide hemoglobin derivative. The EPR spectra obtained are made up of two types of spectrum whose proportions vary with the decrease in hydration down to about 0.25 g H2O per g protein, and are constant below this value. The two types of spectrum are interpreted as two possible configurations of the heme pocket in different populations of nitric oxide hemoglobin.

Effect of unfolding on the tryptophanyl fluorescence lifetime distribution in apomyoglobin

Proteins exhibit, even in their native state, a large number of conformations differing in small details (substates). The fluorescence lifetime of tryptophanyl residues can reflect the microenvironmental characteristics of these subconformations. We have analyzed the lifetime distribution of the unique indole residue of tuna apomyoglobin (Trp A-12) during the unfolding induced by temperature or guanidine hydrochloride. The results show that the increase of the temperature from 10 to 30 degrees C causes a sharpening of the lifetime distribution. This is mainly due to the higher rate of interconversion among the conformational substates in the native state. A further temperature increase produces partially or fully unfolded states, resulting in a broadening of the tryptophanyl lifetime distribution. The data relative to the guanidine-induced unfolding show a sigmoidal increase of the distribution width, which is due to the transition of the protein structure from the native to the random-coiled state. The broadening of the lifetime distribution indicates that, even in the fully unfolded protein, the lifetime of the tryptophanyl residues is influenced by the protein matrix, which generates very heterogeneous microenvironments.

5 K extended X-ray absorption fine structure and 40 K 10-s resolved extended X-ray absorption fine structure studies of photolyzed carboxymyoglobin

A previous extended X-ray absorption fine structure (EXAFS) study of photolyzed carboxymyoglobin (MbCO) [Chance, B., Fischetti, R., & Powers, L. (1983) Biochemistry 22, 3820-3829; Powers, L., Sessler, J. L., Woolery, G. L., & Chance, B. (1984) Biochemistry 23, 5519-5523] has provoked much discussion on the heme structure of the photoproduct (MbCO). The EXAFS interpretation that the Fe-CO distance increases by no more than 0.05 A following photodissociation has been regarded as inconsistent with optical, infrared, and magnetic susceptibility studies [Fiamingo, F. G., & Alben, J. O. (1985) Biochemistry 24, 7964-7970; Sassaroli, M., & Rousseau, D. L. (1986) J. Biol. Chem. 261, 16292-16294]. The present experiment was performed with well-characterized dry film samples in which MbCO molecules were embedded in a poly(vinyl alcohol) matrix [Teng, T. Y., & Huang, H. W. (1986) Biochim. Biophys. Acta 874, 13-18]. The sample had a high protein concentration (12 mM) to yield adequate EXAFS signals but was very thin (40 micron) so that complete photolysis could be easily achieved by a single flash from a xenon lamp. Although the electronic state of MbCO resembles that of deoxymyoglobin (deoxy-Mb), direct comparison of EXAFS spectra indicates that structurally MbCO is much closer to MbCO than to deoxy-Mb.(ABSTRACT TRUNCATED AT 250 WORDS)

Linkage of functional and structural heterogeneity in proteins: dynamic hole burning in carboxymyoglobin

Inhomogeneous broadening of the 760-nanometer photoproduct band of carboxymyoglobin at cryogenic temperatures has been demonstrated with a dynamic hole burning technique. Line-shape changes and frequency shifts in this spectral band are generated by ligand recombination and are shown not to be the result of structural relaxation below 60 K. The observation of dynamic hole burning exposes the relation between the structural disorder responsible for the inhomogeneous broadening and the well-known distributed ligand rebinding kinetics. The findings provide direct evidence for the functional relevance of conformational substrates in myoglobin rebinding. In addition, a general protocol for evaluating the relative contributions of structural relaxation and hole burning to the spectral changes accompanying rebinding in hemeproteins is presented.