Role of ligand substitution in ferrocytochrome c folding

The ligand substitutions that occur during the folding of ferrocytochrome c [Fe(II)cyt c] have been monitored by transient absorption spectroscopy. The folding reaction was triggered by photoinduced electron transfer to unfolded Fe(III)cyt c in guanidine hydrochloride (GuHCl) solutions. Assignments of ligation states were made by reference to the spectra of the imidazole and methionine adducts of N-acetylated microperoxidase 8. At pH 7, the heme in unfolded Fe(II)cyt c is ligated by native His18 and HisX (X = 26, 33) residues. The native Met80 ligand displaces HisX only in the last stages of folding. The ferroheme is predominantly five-coordinate in acidic solution; it remains five-coordinate until the native methionine binds the heme to give the folded protein (the rate of the methionine binding step is 16 +/- 5 s-1 at pH 5, 3.2 M GuHCl). The evidence suggests that the substitution of histidine by methionine is strongly coupled to backbone folding.     

Alkaline isomerization of thermoresistant cytochrome c-552 and horse heart cytochrome c studied by absorption and resonance Raman spectroscopy.

The structure of the thermoresistant cytochrome c (552, Thermus thermophilus) has been investigated at neutral and alkaline pH by absorption and resonance Raman spectroscopy and compared with that of horse heart cytochrome c. The ligands of the ferricytochrome c-552 at neutral pH are considered to be histidine and methionine, whereas the ligands of ferrocytochrome c-552 are histidine and another nitrogen base, histidine or lysine. Ferric cytochrome c-552 undergoes an alkaline isomerization with a pK of 12.3 (25 degrees C), accompanied by a ligand exchange. Horse heart cytochrome c has at least three isomerization states at alkaline pH (pK 9.3, 12.9 and greater than 13.5 at 25 degrees C). The replacement of the sixth ligand may not be involved in the second isomerization. The thermodynamic parameters for the isomerization were also estimated. The entropy change upon isomerization of cytochrome c-552 is negative, whereas for that of horse heart cytochrome c the entropy change is positive.     

Characterization of pH-dependent conformational heterogeneity in Rhodospirillum rubrum cytochrome c2 using 15N and 1H NMR

The 15N-enriched ferricytochrome c2 from Rhodospirillum rubrum has been studied by 15N and 1H NMR spectroscopy as a function of pH. The 15N resonances of the heme and ligand tau nitrogen are broadened beyond detection because of paramagnetic relaxation. The 15N resonance of the ligand histidine phi nitrogen was unambiguously identified at 184 ppm (pH 5.6). The 15N resonances of the single nonligand histidine are observed only at low pH, as in the ferrocytochrome because of the severe broadening caused by tautomerization. The dependence of the 15N and 1H spectra of the ferricytochrome on pH indicated that the ligand histidine tau NH does not dissociate in the neutral pH range and is involved in a hydrogen bond, similar to that in the reduced state. Because neither deprotonated nor non-hydrogen-bonded forms of the ligand histidine are observed in the spectra of either oxidation state, the participation of such forms in producing heterogeneous populations having different electronic g tensors is ruled out. Transitions having pKa's of 6.2, 8.6, and 9.2 are observed in the ferricytochrome. The localized conformational change around the omega loops is observed in the neutral pH range, as in the ferrocytochrome. Structural heterogeneity leads to multiple resonances of the heme ring methyl at position 8. The exchange rate between the conformations is temperature dependent. The transition with a pKa of 6.2 is assigned to the His-42 imidazole group. The displacement of the ligand methionine, which occurs with a pKa of 9.2, causes gross conformational change near the heme center.(ABSTRACT TRUNCATED AT 250 WORDS)     

Magnetic circular dichroism studies of the active site structure of hemoprotein H-450: comparison to cytochrome P-450 and sensitivity to pH effects

Ferric, ferrous and ferrous-CO hemoprotein H-450 from rat liver have been examined with magnetic circular dichroism spectroscopy under alkaline (pH 8.0) and acidic (pH 6.0) conditions. The spectral properties of these species require that one of the axial heme iron ligands in the alkaline ferric and ferrous states must be a thiolate sulfur, presumably from cysteine. The data are most consistent with the ligand trans to thiolate being either histidine or methionine. The reversible pH effects on the spectral properties of the ferrous protein, but not of the ferric protein, appear to involve protonation or displacement of the thiolate. As treatment of the ferrous protein with CO does not yield a thiolate-ligated ferrous-CO adduct, CO either displaces the thiolate or its addition is accompanied by protonation of the thiolate.     

A spin label study of conformational changes in cytochrome c

Spin-labeled pig heart cytochromes c singly modified at Met-65, Tyr-74 and at one of the lysine residues, Lys-72 or Lys-73, were investigated by the ESR method under conditions of different ligand and redox states of the heme and at various pH values. Replacement of Met-80 by the external ligand, cyanide, was shown to produce a sharp increase in the mobility of all the three bound labels while reduction of the spin-labeled ferricytochromes c did not cause any marked changes in their ESR spectra. In the pH range 6-13, two conformational transitions in ferricytochrome c were observed which preceded its alkaline denaturation: the first with pK 9.3 registered by the spin label at the Met-65 position, and the second with pK 11.1 registered by the labels bound to Tyr-74 and Lys-72(73). The conformational changes in the 'left-hand part' of ferricytochrome c are most probably induced in both cases by the exchange of internal protein ligands at the sixth coordination site of the heme.     

1H NMR spectroscopy of Paracoccus denitrificans cytochrome c-550

The 1H NMR spectra of ferri- and ferro-cytochrome c-550 from Paracoccus denitrificans (ATCC 13543) have been investigated at 300 MHz. The ferri-cytochrome c-550 shows hyperfine-shifted heme methyl resonances at 29.90, 29.10, 16.70, and 12.95 ppm and a ligand methionyl methyl resonance at -15.80 ppm (pH 8 and 23 degrees C). Four pH-linked structural transitions were detected in spectra taken as a function of pH. The transitions have been interpreted as loss of the histidine heme ligand (pK less than or equal to 3), ionization of a buried heme propionate (pK = 6.3 +/- 0.2), displacement of the methionine heme ligand by a lysyl amino group (pK congruent to 10.5), and loss of the lysyl ligand (pK greater than or equal to 11.3). The temperature behavior of hyperfine-shifted resonances was determined. Two heme methyl resonances (at 16.70 and 12.95 ppm) showed downfield hyperfine shifts with increasing temperature. The cyanoferricytochrome had methyl resonances at 23.3, 20.1, and 19.4 ppm. NMR spectroscopy did not detect the formation of a complex with azide. The second-order rate constant for electron transfer between ferric and ferrous forms was determined to be 1.6 X 10(4) M-1 s-1. Heme proton resonances were assigned in both oxidation states by cross-saturation and nuclear Overhauser enhancement experiments. Spin-coupling patterns in the aromatic region of the ferro-cytochrome spectrum were investigated.     

Alkaline isomerization of ferricytochrome C from Euglena gracilis

$\text{Euglena gracilis}$ ferricytochrome $\text{c}$ has a small absorption maximum at about 700 nm having an extinction of 850 ± 10 M^?1cm^?1. This absorption band is analogous to the more commonly found maximum at 695 nm which is observed in ferricytochromes from other sources and which is characteristic of ligation of methionine 80 with the heme ion. The 700 nm band disappears upon raising the pH to 11 giving a transition involving a single proton having an apparent pK of about 10. These results demonstrate that the phenolic ionization of tyrosine 67 is not required to trigger the alkaline isomerization of ferricytochromes $\text{c}$ since Euglena cytochrome has a phenylalanine residue at position 67.     

Correlation of acid-induced conformational transition of ferricytochrome <Emphasis Type="Italic">c</Emphasis> with cyanide binding kinetics

A relation between pH-induced conformational transitions of horse heart ferricytochrome c and the kinetics of external ligand coordination to heme iron was investigated by optical spectroscopy, circular dichroism and viscometry. The dependencies of both the association, k (a), and dissociation rate constants of cyanide binding on pH were determined from kinetic measurements. The association rate constant exhibits a bell-shaped form of dependence on pH in the region where this protein unfolds. The maximum of the dependence of k (a) on pH is found to be coincident with the pK values of conformational transitions of ferricytochrome c in solutions with both low and high ionic strengths. This observation is explained in terms of ferricytochrome c unfolding, which is characterized by two processes: the gradual opening of the heme crevice accompanied by the detachment of the axial Met80 and its replacement with a water molecule. The former process enhances the rate, whereas the latter results in the inhibition of the rate of cyanide binding.     

Spectroscopic and oxidation-reduction properties of Rhodobacter capsulatus cytochrome c1 and its M183K and M183H variants

Two variants of the cytochrome c1 component of the Rhodobacter capsulatus cytochrome bc1 complex, in which Met183 (an axial heme ligand) was replaced by lysine (M183K) or histidine (M183H), have been analyzed. Electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectra of the intact complex indicate that the histidine/methionine heme ligation of the wild-type cytochrome is replaced by histidine/lysine ligation in M183K and histidine/histidine ligation in M183H. Variable amounts of histidine/histidine axial heme ligation were also detected in purified wild-type cytochrome c1 and its M183K variant, suggesting that a histidine outside the CSACH heme-binding domain can be recruited as an alternative ligand. Oxidation-reduction titrations of the heme in purified cytochrome c1 revealed multiple redox forms. Titrations of the purified cytochrome carried out in the oxidative or reductive direction differ. In contrast, titrations of cytochrome c1 in the intact bc1 complex and in a subcomplex missing the Rieske iron-sulfur protein were fully reversible. An Em7 value of -330 mV was measured for the single disulfide bond in cytochrome c1. The origins of heme redox heterogeneity, and of the differences between reductive and oxidative heme titrations, are discussed in terms of conformational changes and the role of the disulfide in maintaining the native structure of cytochrome c1.     

Proton-NMR studies of the effects of ionic strength and pH on the hyperfine-shifted resonances and phenylalanine-82 environment of three species of mitochondrial ferricytochrome c.

Ferricytochromes c from three species (horse, tuna, yeast) display sensitivity to variations in solution ionic strength or pH that is manifested in significant changes in the proton NMR spectra of these proteins. Irradiation of the heme 3-CH3 resonances in the proton NMR spectra of tuna, horse and yeast iso-1 ferricytochromes c is shown to give NOE connectivities to the phenyl ring protons of Phe82 as well as to the beta-CH2 protons of this residue. This method was used to probe selectively the Phe82 spin systems of the three cytochromes c under a variety of solution conditions. This phenylalanine residue has previously been shown to be invariant in all mitochondrial cytochromes c, located near the exposed heme edge in proximity to the heme 3-CH3, and may function as a mediator in electron transfer reactions [Louie, G. V., Pielak, G. J., Smith, M. & Brayer, G. D. (1988) Biochemistry 27, 7870-7876]. Ferricytochromes c from all three species undergo a small but specific structural rearrangement in the environment around the heme 3-CH3 group upon changing the solution conditions from low to high ionic strength. This structural change involves a decrease in the distance between the Phe82 beta-CH2 group and the heme 3-CH3 substituent. In addition, studies of the effect of pH on the 1H-NMR spectrum of yeast iso-1 ferricytochrome c show that the heme 3-CH3 proton resonance exhibits a pH-dependent shift with an apparent pK in the range of 6.0-7.0. The chemical shift change of the yeast iso-1 ferricytochrome c heme 3-CH3 resonance is not accompanied by an increase in the linewidth as previously described for horse ferricytochrome c [Burns, P. D. & La Mar, G. N. (1981) J. Biol. Chem. 256, 4934-4939]. These spectral changes are interpreted as arising from an ionization of His33 near the C-terminus. In general, the larger spectral changes observed for the resonances in the vicinity of the heme 3-CH3 group in yeast iso-1 ferricytochrome c with changes in solution conditions, relative to the tuna and horse proteins, suggest that the region around Phe82 is more open and that movement of the Phe82 residue is less constrained in yeast ferricytochrome c. Finally, it is demonstrated here that both the heme 8-CH3 and the 7 alpha-CH resonances of yeast ferricytochrome c titrate with p2H and exhibit apparent pK values of approximately 7.0. The titrating group responsible for these spectral changes is proposed to be His39.     

Activation of the cytochrome cd1 nitrite reductase from Paracoccus pantotrophus. Reaction of oxidized enzyme with substrate drives a ligand switch at heme c

Cytochromes cd(1) are dimeric bacterial nitrite reductases, which contain two hemes per monomer. On reduction of both hemes, the distal ligand of heme d(1) dissociates, creating a vacant coordination site accessible to substrate. Heme c, which transfers electrons from donor proteins into the active site, has histidine/methionine ligands except in the oxidized enzyme from Paracoccus pantotrophus where both ligands are histidine. During reduction of this enzyme, Tyr(25) dissociates from the distal side of heme d(1), and one heme c ligand is replaced by methionine. Activity is associated with histidine/methionine coordination at heme c, and it is believed that P. pantotrophus cytochrome cd(1) is unreactive toward substrate without reductive activation. However, we report here that the oxidized enzyme will react with nitrite to yield a novel species in which heme d(1) is EPR-silent. Magnetic circular dichroism studies indicate that heme d(1) is low-spin Fe(III) but EPR-silent as a result of spin coupling to a radical species formed during the reaction with nitrite. This reaction drives the switch to histidine/methionine ligation at Fe(III) heme c. Thus the enzyme is activated by exposure to its physiological substrate without the necessity of passing through the reduced state. This reactivity toward nitrite is also observed for oxidized cytochrome cd(1) from Pseudomonas stutzeri suggesting a more general involvement of the EPR-silent Fe(III) heme d(1) species in nitrite reduction.     

Characterization of an alkaline transition intermediate stabilized in the Phe82Trp variant of yeast iso-1-cytochrome c

In general, mutation of the phylogenetically conserved residue Phe82 in yeast iso-1-cytochrome c destabilizes the native conformation of the protein by facilitating the ligand exchange reactions that are associated with the alkaline conformational transitions of the ferricytochrome. Of the Phe82 variants surveyed thus far, Phe82Trp is unique in that it adopts a thermodynamically stable, high-spin conformation at mildly alkaline pH. This species exhibits spectroscopic features that can only be detected transiently in other ferricytochromes c within the first 100 ms immediately after a pH-jump from neutrality to pH >10. Spectroscopic characterization of this high-spin reaction intermediate suggests that in addition to an obligatory pentacoordinate heme iron, a group within the heme pocket coordinates the heme iron but is then replaced either by Met80, to revert to the native conformation, or by Lys73 or Lys79, to yield one of the conventional alkaline conformers. Evidence is presented to suggest that this group is either a hydroxide ion or Tyr67 rather than a loosely bound Met80.     

High-resolution 1H- and 15N-NMR studies of Rhodospirillum rubrum cytochrome c2.

Rhodospirillum rubrum cytochrome c2 was uniformly enriched in 15N and studied by 1H- and 15N-NMR spectroscopy. Relaxation and NOE data allowed determination of the rotational correlation time and indicated more rapid side-chain motion in the native protein and increased segmental motion in the base-denatured protein. The pi nitrogen of the ligand histidine and the indolic nitrogen of the invariant tryptophan both remain protonated and act as proton-donors in hydrogen bonds over a wide pH range and therefore do not contribute to pH-related changes in the midpoint potential. pK values identified by numerous methods in the ferrocytochrome at pH 6.9 and in the ferricytochrome at pH 6.2 arise from His-42. At pH values below the pK, the imidazolium group participates in a salt bridge or in a hydrogen bond with the carboxylate group of the inner propionate of the heme. Loss of the proton causes a local conformational change which alters the midpoint potential. The pK values of the amino terminus and lysines were also determined from pH titrations monitored by 15N-NMR. Similar titrations of the ferricytochrome monitored by 1H-NMR showed structural heterogeneity in that the resonance of heme ring methyl 8 split into a doublet as the pH was raised.     

Proton nuclear magnetic resonance studies of Rhodospirillum rubrum cytochrome.

Rhodospirillum rubrum cytochrome c2 was studied by proton nuclear magnetic resonance at 220 MHz. Assignments were made to the resonances of heme c by double-resonance techniques and by temperature-dependence studies. The aromatic resonances of Trp-62 and Tyr-70 of ferrocytochrome c2 were identified by spin-decoupling experiments. The resonances of the Met-91 methyl group of the ferri- and ferrocytochromes were assigned by saturation-transfer experiments. The assignments are compared to those made for cytochromes c. A pH titration showed that the methionine methyl resonance of ferricytochrome c2 shifted with a pK of 6.25 and disappeared above pH 9. No histidine CH resonances that titrated normally over the neutral pH range were observed in the spectrum of either oxidation state of the protein. The possible origins of the ionizations at pH 6.25 and 9 are discussed.     

A new spatial structure for the axial methionine observed in cytochrome c5 from Pseudomonas mendocina. Correlations with the electronic structure of heme c

Cytochrome c5 from Pseudomonas mendocina has been isolated and the coordination geometry at the heme iron was investigated by 1H nuclear magnetic resonance and circular dichroism spectroscopy. Individual assignments were obtained for heme c and the axial ligands. From studies of nuclear Overhauser enhancements the axial histidine imidazole ring orientation relative to the heme group was found to coincide with that of other c-type cytochromes. In contrast, a new structure was observed for the axial methionine. This includes S chirality at the iron-bound sulfur atom, but compared to cytochromes c-551 from Pseudomonads and Rhodopseudomonas gelatinosa, which also contain S-chiral methionine, the spatial arrangement of the gamma- and beta-methylene groups and the alpha carbon of methionine is markedly different. Analysis of the electron spin density distribution in ferricytochrome c5 in the light of this new coordination geometry provides additional support for the hypothesis that the electronic structure of heme c is primarily governed by the orientation of the sp3 lone-pair orbital of the axial sulfur atom with respect to the heme plane.     

pH dependence of formation of a partially unfolded state of a Lys 73 --> His variant of iso-1-cytochrome c: implications for the alkaline conformational transition of cytochrome c

The alkaline conformational transition of a lysine 73 --> histidine variant of iso-1-cytochrome c has been studied. The transition has been monitored at 695 nm, a band sensitive to the presence of the heme-methionine 80 bond, at the heme Soret band which is sensitive to the nature of the heme ligand, and by NMR methods. The guanidine hydrochloride dependence of the alkaline conformational transition has also been monitored. The histidine 73 protein has an unusual biphasic alkaline conformational transition at both 695 nm and the heme Soret band, consistent with a three-state process. The conformational transition is fully reversible. An equilibrium model has been developed to account for this behavior. With this model, it has been possible to obtain the acid constant for the trigger group, pK(H), of the low-pH phase from the equilibrium data. A pK(H) value of 6.6 +/- 0.1 in H(2)O was obtained, consistent with a histidine acting as the trigger group. The NMR data for the low-pH phase of the alkaline conformational transition are consistent with an imidazole ligand replacing Met 80. For the high-pH phase of the biphasic alkaline transition, the NMR data are consistent with lysine 79 being the heme ligand. Guanidine hydrochloride m values of 1.67 +/- 0.08 and 1.1 +/- 0.2 kcal mol(-1) M(-1) were obtained for the low- and high-pH phases of the biphasic alkaline transition of the histidine 73 protein, respectively, consistent with a greater structural disruption for the low-pH phase of the transition.     

Characterization of the solution reactivity of a basic heme peroxidase from Cucumis sativus

A basic heme peroxidase has been isolated from cucumber (Cucumis sativus) peelings and characterized through electronic and (1)H NMR spectra from pH 3 to 11. The protein, as isolated, contains a high-spin ferriheme which in the low pH region is sensitive to two acid-base equilibria with apparent pK(a) values of approximately 5 and 3.6, assigned to the distal histidine and to a heme propionate, respectively. At high pH, a new low-spin species develops with an apparent pK(a) of 11, likely due to the binding of an hydroxide ion to the sixth (axial) coordination position of the Fe(III). A number of acid-base equilibria involving heme propionates and residues in the distal cavity also affect the binding of inorganic anions such as cyanide, azide, and fluoride to the ferriheme, as well as the catalytic activity. The reduction potentials of the native protein and of its cyanide derivative, determined through UV-Vis spectroelectrochemistry, result to be -0.320+/-0.015 and -0.412+/-0.010V, respectively. Overall, the reactivity of this protein parallels those of other plant peroxidases, especially horseradish peroxidase. However, some differences exist in the acid-base equilibria affecting its reactivity and in the reduction potential, likely as a result of small structural differences in the heme distal and proximal cavities.     

Multiple low spin forms of the cytochrome c ferrihemochrome. EPR spectra of various eukaryotic and prokaryotic cytochromes c.

1. Despite the same methionine-sulfur:heme-iron:imidazole-nitrogen hemochrome structure observed by x-ray crystallography in four of the seven c-type eukaryotic and prokaryotic cytochromes examined, and the occurrence of the characteristic 695 nm absorption band correlated with the presence of a methionine-sulfur:heme-iron axial ligand in all seven proteins, they fall into two distinct classes on the basis of their EPR and optical spectra. The horse, tuna, and bakers' yeast iso-1 cytochromes c have a predominant neutral pH EPR form with g1=3.06, g2=2.26, and g3=1.25, while the bakers' yeast iso-2 and Euglena cytochromes c, the Rhodospirillum rubrum cytochrome c2, and the Paracoccus denitrificans cytochrome c550 all have a predominant neutral pH EPR form with g1=3.2, g2=2.05, and g3=1.39. The ferricytochromes with g1=3.06 have a B-Q splitting that is approximately 150 cm-1 larger than the ferricytochromes with g1=3.2. 2. Each of the cytochromes displays up to four low spin EPR forms that are in pH-dependent equilibrium and can all be observed at near neutral pH. As the pH is raised the predominant neutral pH form is converted into two forms with g1=3.4 and g1=3.6, identified by comparsion with model compounds and other heme proteins as epsilon-amino:heme-iron:imidazole and bis-epsilon-amino:heme-iron ferrihemochromes, respectively. 3. The pK for the conversion of the predominant neutral pH EPR form into the alkaline pH forms is the same as the pK for the disappearance of the 695 nm absorption band for the cytochromes, even though these pK values range over 2 pH units. This confirms that the g1=3.06 and g1=3.2 forms contain the methionine-sulfur:heme-iron axial ligand while the g1=3.4 and the g1=3.6 forms do not. 4. At extremes of pH, the horse and bakers' yeast iso-1 proteins display several high and low spin forms that are identified, showing that a variety of protein-derived ligands will coordinate to the heme iron including methionine and cysteine sulfur, histidine imidazole, and lysine epsilon-amine. 5. The spectrum of horse cytochrome c with added azide, cyanide, hydroxide, or imidazole as axial ligands has also been examined. 6. From a comparison of the EPR and optical spectral characteristics of these groups of cytochromes with model compounds, it is suggested that the difference between them is due to a change in the hydrogen bonding or perhaps even in the protonation of N-1 of the heme iron-bound histidine imidazole.     

Effect of varying polyglutamate chain length on the structure and stability of ferricytochrome c

The effect of varying polyglutamate chain length on local and global stability of horse heart ferricytochrome c was studied using scanning calorimetry and spectroscopy methods. Spectral data indicate that polyglutamate chain lengths equal or greater than eight monomer units significantly change the apparent pK(a) for the alkaline transition of cytochrome c. The change in pK(a) is comparable to the value when cytochrome c is complexed with cytochrome bc(1). Glutamate and diglutamate do not significantly alter the temperature transition for cleavage of the Met(80)-heme iron bond of cytochrome c. At low ionic strength, polyglutamates consisting of eight or more glutamate monomers increase midpoint of the temperature transition from 57.3+/-0.2 to 66.9+/-0.2 degrees C. On the other hand, the denaturation temperature of cytochrome c decreases from 85.2+/-0.2 to 68.8+/-0.2 degrees C in the presence of polyglutamates with number of glutamate monomers n >or approximately equal 8. The rate constant for cyanide binding to the heme iron of cytochrome c of cytochrome c-polyglutamate complex also decreases by approximately 42.5% with n>or approximately equal 8. The binding constant for the binding of octaglutamate (m.w. approximately 1000) to cyt c was found to be 1.15 x 10(5) M(-1) at pH 8.0 and low ionic strength. The results indicate that the polyglutamate (n>or approximately equal 8) is able to increase the stability of the methionine sulfur-heme iron bond of cytochrome c in spite of structural differences that weaken the overall stability of the cyt c at neutral and slightly alkaline pH.     

A photolysis-triggered heme ligand switch in H93G myoglobin

Resonance Raman spectroscopy and step-scan Fourier transform infrared (FTIR) spectroscopy have been used to identify the ligation state of ferrous heme iron for the H93G proximal cavity mutant of myoglobin in the absence of exogenous ligand on the proximal side. Preparation of the H93G mutant of myoglobin has been previously reported for a variety of axial ligands to the heme iron (e.g., substituted pyridines and imidazoles) [DePillis, G., Decatur, S. M., Barrick, D., and Boxer, S. G. (1994) J. Am. Chem. Soc. 116, 6981-6982]. The present study examines the ligation states of heme in preparations of the H93G myoglobin with no exogenous ligand. In the deoxy form of H93G, resonance Raman spectroscopic evidence shows water to be the axial (fifth) ligand to the deoxy heme iron. Analysis of the infrared C-O and Raman Fe-C stretching frequencies for the CO adduct indicates that it is six-coordinate with a histidine trans ligand. Following photolysis of CO, a time-dependent change in ligation is evident in both step-scan FTIR and saturation resonance Raman spectra, leading to the conclusion that a conformationally driven ligand switch exists in the H93G protein. In the absence of exogenous nitrogenous ligands, the CO trans effect stabilizes endogenous histidine ligation, while conformational strain favors the dissociation of histidine following photolysis of CO. The replacement of histidine by water in the five-coordinate complex is estimated to occur in < 5 micros. The results demonstrate that the H93G myoglobin cavity mutant has potential utility as a model system for studying the conformational energetics of ligand switching in heme proteins such as those observed in nitrite reductase, guanylyl cyclase, and possibly cytochrome c oxidase.