Photoperturbation of the heme a3-CuB binuclear center of cytochrome c oxidase CO complex observed by Fourier transform infrared spectroscopy

The catalytic reaction of cytochrome c oxidase probed by in situ gas titrations and FTIR difference spectroscopy

Cytochrome c oxidase (CcO) is a transmembrane heme‑copper metalloenzyme that catalyzes the reduction of O2 to H2O at the reducing end of the respiratory electron transport chain. To understand this reaction, we followed the conversion of CcO from Rhodobacter sphaeroides between several active-ready and carbon monoxide-inhibited states via attenuated total reflection Fourier-transform infrared (ATR FTIR) difference spectroscopy. Utilizing a novel gas titration setup, we prepared the mixed-valence, CO-inhibited R2CO state as well as the fully-reduced R4 and R4CO states and induced the "active ready" oxidized state OH. These experiments are performed in the dark yielding FTIR difference spectra exclusively triggered by exposure to O2, the natural substrate of CcO. Our data demonstrate that the presence of CO at heme a3 does not impair the catalytic oxidation of CcO when the cycle starts from the fully-reduced states. Interestingly, when starting from the R2CO state, the release of the CO ligand upon purging with inert gas yield a product that is indistinguishable from photolysis-induced states. The observed changes at heme a3 in the catalytic binuclear center (BNC) result from the loss of CO and are unrelated to electronic excitation upon illumination. Based on our experiments, we re-evaluate the assignment of marker bands that appear in time-resolved photolysis and perfusion-induced experiments on CcO.

Mitochondria and light: An overview of the pathways triggered in skin and retina with incident infrared radiation

Slightly more than half of the solar radiation that passes through the atmosphere and reaches the Earth's surface is infrared. Over the past few years, many papers have been published on the possible positive effects of receiving this part of the electromagnetic spectrum. In this article we analyse the role of mitochondria in the supposed effects of infrared light based on the published literature. It is claimed that ATP synthesis is stimulated, which has a positive effect on the skin by increasing fibroblast proliferation, anchorage and production of collagen fibres, procollagen, and various cytokines responsible for the wound healing process, such as keratinocyte growth factor. Currently there are infrared light emitting equipment whose manufacturers and the centres where this service or treatment is offered claim that they are used for skin rejuvenation among other positive effects. Based on the literature review, it is necessary to deepen the scientific study of the mechanism of absorption of infrared radiation through the skin to better understand its possible positive effects, the risks of overexposure and to improve consumer health protection.

Diurnal Changes in Transcript and Metabolite Levels during the Iron Deficiency Response of Rice

BACKGROUND

Rice (Oryza sativa L.) is highly susceptible to iron (Fe) deficiency due to low secretion levels of the mugineic acid (MA) family phytosiderophore (PS) 2'-deoxymugineic acid (DMA) into the rhizosphere. The low levels of DMA secreted by rice have proved challenging to measure and, therefore, the pattern of DMA secretion under Fe deficiency has been less extensively studied relative to other graminaceous monocot species that secrete high levels of PS, such as barley (Hordeum vulgare L.).

RESULTS

Gene expression and metabolite analyses were used to characterise diurnal changes occurring during the Fe deficiency response of rice. Iron deficiency inducible genes involved in root DMA biosynthesis and secretion followed a diurnal pattern with peak induction occurring 3-5 h after the onset of light; a result consistent with that of other Strategy II plant species such as barley and wheat. Furthermore, triple quadrupole mass spectrometry identified 3-5 h after the onset of light as peak time of DMA secretion from Fe-deficient rice roots. Metabolite profiling identified accumulation of amines associated with metal chelation, metal translocation and plant oxidative stress responses occurring with peak induction 10-12 h after the onset of light.

CONCLUSION

The results of this study confirmed that rice shares a similar peak time of Fe deficiency associated induction of DMA secretion compared to other Strategy II plant species but has less prominent daily fluctuations of DMA secretion. It also revealed metabolic changes associated with the remediation of Fe deficiency and mitigation of damage from resulting stress in rice roots. This study complements previous studies on the genetic changes in response to Fe deficiency in rice and constitutes an important advance towards our understanding of the molecular mechanisms underlying the rice Fe deficiency response.

CO-dynamics in the active site of cytochrome c oxidase

The transfer of CO from heme a3 to the Cu(B) site in Cytochrome c oxidase (CcO) after photolysis is studied using molecular dynamics simulations using an explicitly reactive, parametrized potential energy surface based on density functional theory calculations. After photodissociation from the heme-Fe, the CO ligand rebinds to the Cu(B) site on the sub-picosecond time scale. Depending on the simulation protocol the characteristic time ranges from 260 fs to 380 fs which compares with an estimated 450 fs from experiment based on the analysis of the spectral changes as a function of time delay after the photodissociating pulse. Following photoexcitation ≈90% of the ligands are found to rebind to either the Cu(B) (major component, 85%) or the heme-Fe (minor component, 2%) whereas about 10% remain in an unbound state. The infrared spectra of unbound CO in the active site is broad and featureless and no appreciable shift relative to gas-phase CO is found, which is in contrast to the situation in myoglobin. These observations explain why experimentally, unbound CO in the binuclear site of CcO has not been found as yet.

Assignment of the CO-sensitive carboxyl group in mitochondrial forms of cytochrome c oxidase using yeast mutants

Point mutations of E243D and I67N were introduced into subunit I of a 6histidine-tagged (6H-WT) form of yeast Saccharomyces cerevisiae mitochondrial cytochrome c oxidase. The two mutants (6H-E243D(I) and 6H-I67N(I)) were purified and showed ≈50 and 10% of the 6H-WT turnover number. Light-induced CO photolysis FTIR difference spectra of the 6H-WT showed a peak/trough at 1749/1740cm(-1), as seen in bovine CcO, which downshifted by 7cm(-1) in D(2)O. The bands shifted to 1736/1762cm(-1) in 6H-E243D(I), establishing that the carboxyl group affected by CO binding in mitochondrial CcOs is E243. In 6H-I67N(I), the trough at 1740cm(-1) was shifted to 1743cm(-1) and its accompanying peak intensity was greatly reduced. This confirms that the I67N mutation interferes with conformational alterations around E243. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

Probing the Environment of CuB in Heme−Copper Oxidases

Time-resolved step-scan FTIR (TRS2-FTIR) and density functional theory have been applied to probe the structural dynamics of CuB in heme-copper oxidases at room temperature. The TRS2-FTIR data of cbb3 from Pseudomonas stutzeri indicate a small variation in the frequency of the transient CO bound to CuB in the pH/pD 7-9 range. This observation in conjunction with density functional theory calculations, in which significant frequency shifts of the nu(CO) are observed upon deprotonation and/or detachment of the CuB ligands, demonstrates that the properties of the CuB ligands including the cross-linked tyrosine, in contrast to previous reports, remain unchanged in the pH 7-9 range. We attribute the small variations in the nu(CO) of CuB to protein conformational changes in the vicinity of CuB. Consequently, the split of the heme Fe-CO vibrations (alpha-, beta-, and gamma-forms) is not due to changes in the ligation and/or protonation states of the CuB ligands or to the presence of one or more ionizable groups, as previously suggested, but the result of global protein conformational changes in the vicinity of CuB which, in turn, affect the position of CuB with respect to the heme Fe.

An IR study of protonation changes associated with heme-heme electron transfer in bovine cytochrome c oxidase

IR changes caused by photolysis of CO from the mixed valence form of bovine cytochrome c oxidase have been investigated over the pH/pD range 6-9.8. Band assignments were based on effects of H2O/D2O exchange and by comparisons with published IR data and crystallographic data. Changes arise both from CO photolysis and from subsequent reversed electron transfer from heme a3 to heme a. This reversed electron transfer is known to have pH-independent and, above pH 8, pH-dependent components. The pH-independent component is associated with a trough around the 1742 cm(-1) band attributable to one or more protonated carboxylic acids. Its peak position, but not extent, is pH-dependent, indicative of a titratable group with a pK of 8.2 whose acid form causes increased hydrogen bonding to the IR-detectable carboxylic group. A different protonatable group with pK above 9 controls the extent of the pH-dependent component. This phase is associated with perturbation of an arginine guanidinium that is most clearly observed as a trough at 1592 cm(-1) after H/D exchange. It is suggested that this group, probably Arg-438 that is in close contact with propionate groups of both hemes and already proposed to be of functional significance, lowers the energy of the transient charge-uncompensated electron-transfer intermediate by changing the charge distribution in response to heme-heme electron transfer. No other IR signature of a titratable group that controls the extent of the pH-dependent phase is present, and it most likely arises from a nonphysiological deprotonation of the proximal water ligand of ferric heme a3 at high pH that has been reported to exhibit a similar pK.

Structural and Chemical Changes of the PMIntermediate ofParacoccus denitrificansCytochromecOxidase Revealed by IR Spectroscopy with Labeled Tyrosines and Histidine†

Structural and chemical changes in the P(M) intermediate of Paracoccus denitrificans cytochrome c oxidase have been investigated by attenuated total reflection-Fourier transform infrared spectroscopy. Prior studies of P(M) minus oxidized (O) IR difference spectra of unlabeled, universally (15)N-labeled and ring-d(4)-tyrosine-labeled proteins (Iwaki, M., Puustinen, A., Wikström, M., and Rich, P. R. (2004) Biochemistry 43, 14370-14378). provided a basis for band assignments to changes in metal centers and the covalently linked His-Tyr ligand of Cu(B) and highlighted a structural alteration of the protonated Glu278 in the P(M) intermediate. This work has been extended to equivalent measurements on enzymes with (13)C(9)(15)N-labeled and ring-(13)C(6)-labeled tyrosine and with (13)C(6)(15)N(3)-labeled histidine. Histidine labeling allows the assignment of troughs at 1104 and 973 cm(-1) in reduced minus O spectra to histidine changes, whereas tyrosine labeling moves otherwise obscured tyrosine bandshifts to 1454-1437 and 1287-1284 cm(-1). P(M) minus O spectra reveal bands at 1506, 1311, and 1094 cm(-1) in the oxidized state that are replaced by a band at 1519 cm(-1) in P(M). These bands shift with both tyrosine- and histidine-labeling, providing evidence for their assignment to the covalent His-Tyr and for its chemical change in P(M). Comparisons of isotope effects on the amide I regions in P(M) minus O spectra demonstrate that amide carbonyl bonds of tyrosine and histidine are major contributors. This suggests a structural alteration in P(M) that is centered on the His276-Pro277-Glu278-Val279-Tyr280 pentapeptide formed by the His-Tyr covalent linkage. This structural change is proposed to mediate the perturbation of the IR band of the protonated Glu278 headgroup.

Ligand, cofactor, and residue vibrations in the catalytic site of endothelial nitric oxide synthase

A study of bovine endothelial nitric oxide synthase by Fourier transform infrared (FTIR) spectroscopy in the 1000-2500 cm(-)(1) range is reported. Binding of CO to the reduced enzyme gives two heme(II)-CO nu(C)(-)(O) stretches (1927 and 1904 cm(-)(1)) which appear to be in rapid equilibrium. Photolysis of this heme(II)-CO compound is accompanied by perturbation of the local fine structure around the catalytic site giving vibrational changes of protein backbone, substrate, amino acid residues, and cofactors, to which heme, substrate arginine, and catalytic site residues contribute. Possible assignments of vibrations to heme, substrate arginine, and catalytic site residues are discussed. The discussion of assignments is informed by known structures, absorbance frequencies, and extinction coefficients of residues and cofactors, analysis of H(2)O-D(2)O exchange effects, analysis of substrate (14)N-(15)N (guanidinium)-arginine exchange effects, and comparison with the nNOS isoform (which differs in the replacement of asparagine 368 with an aspartate within the substrate binding site). The FTIR data can be modeled on the known structure of the catalytic site and indicate the extent of modulation of vibrational modes upon photolysis of the CO compound.

ATR-FTIR Spectroscopy and Isotope Labeling of the PM Intermediate of Paracoccus denitrificans Cytochrome c Oxidase

The structure of the P(M) intermediate of Paracoccus denitrificans cytochrome c oxidase was investigated by perfusion-induced attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. Transitions from the oxidized to P(M) state were initiated by perfusion with CO/oxygen buffer, and the extent of conversion was quantitated by simultaneously monitoring visible absorption changes. In prior work, tentative assignments of bands were proposed for heme a(3), a change in the environment of the protonated state of a carboxylic acid, and a covalently linked histidine-tyrosine ligand to Cu(B) that has been found in the catalytic site. In this work, reduced minus oxidized difference spectra at pH 6.5 and 9.0 and P(M) minus oxidized difference spectra at pH 9.0 were compared in unlabeled, universally (15)N-labeled, and tyrosine-ring-d(4)-labeled proteins to improve these assignments. In the reduced minus oxidized difference spectrum, (15)N labeling resulted in large changes in the amide II region and a 9 cm(-1) downshift in a 1105 cm(-1) trough that is attributed to histidine. In contrast, changes induced by tyrosine-ring-d(4) labeling were barely detectable where the isotope-sensitive bands are expected. Both isotope substitutions had large effects on P(M) minus oxidized difference spectra. A prominent trough at 1542 cm(-1) was shifted to 1527 cm(-1) with (15)N labeling, and its magnitude was diminished with the appearance of a 1438 cm(-1) trough with tyrosine-ring-d(4) labeling. Both isotope substitutions also had large effects on a 1314 cm(-1) trough in the same spectra. These shifts indicate that the bands are linked to both a nitrogenous compound and a tyrosine, the most obvious candidate being the covalent histidine-tyrosine ligand of Cu(B). Comparison with model material data suggests that the tyrosine hydroxyl group is protonated when the binuclear center is oxidized but deprotonated in the P(M) intermediate. Positive bands at 1519 and 1570 cm(-1) were replaced with bands at 1504 and 1556 cm(-1), respectively, with tyrosine-ring-d(4) labeling, are characteristic of upsilon(7a)(C-O) and upsilon(C-C) bands of neutral phenolic radicals, and most likely reflect the formation of the neutral radical state of the histidine-tyrosine ligand in P(M).

Synthesis, characterization, and laser flash photolysis reactivity of a carbonmonoxy heme complex

We present here the synthesis, characterization, and flash photolysis study of [(F(8)TPP)Fe(II)(CO)(THF)] (1) [F(8)TPP = tetrakis(2,6-difluorophenyl)porphyrinate(2-)]. Complex 1 crystallizes from THF/heptane solvent system as a tris-THF solvate, [(F(8)TPP)Fe(II)(CO)(THF)].3THF (1.3THF), with ferrous ion in the porphyrin plane (C(61)H(52)F(8)FeN(4)O(5); a = 11.7908(2) A, b = 20.4453(2) A, c = 39.9423(3), alpha = 90 degrees, beta = 90 degrees, gamma = 90 degrees; orthorhombic, P2(1)2(1)2(1), Z = 8; Fe-N(4)(av) = 2.00 A; N-Fe-N (all) = 90.0 degrees ). This complex (as 1.THF) has also been characterized by (1)H NMR [six-coordinate, low-spin heme; CD(3)CN, RT, delta 8.82 (s, pyrrole-H, 8H), 7.89 (s, para-phenyl-H, 8H), 7.46 (s, meta-phenyl-H, 4H), 3.58 (s, THF, 8H), 1.73 (s, THF, 8H)], (2)H NMR (pyrrole-deuterated analogue) [(F(8)TPP-d(8))Fe(II)(CO)(THF)] [THF, RT, delta 8.78 ppm (s, pyrrole-D)], (13)C NMR (on (13)CO-enriched adduct) [THF-d(8), RT, delta 206.5 ppm; CD(2)Cl(2), RT, delta 206.1 ppm], UV-vis [THF, RT, lambda(max), 411 (Soret), 525 nm], and IR [293 K, solution, nu(CO) 1979 cm(-)(1) (THF), 1976 cm(-)(1) (acetone), 1982 cm(-)(1) (CH(3)CN)] spectroscopies. In order to more fully understand the intricacies of solvent-ligand binding (as compared to CO rebinding to the photolyzed heme), we have also synthesized the bis-THF adduct [(F(8)TPP)Fe(II)(THF)(2)]. Complex 2 also crystallizes from THF/heptane solvent system as a bis-THF solvate, [(F(8)TPP)Fe(II)(THF)(2)].2THF (2.2THF), with ferrous iron in the porphyrin plane (C(60)H(52)F(8)FeN(4)O(4); a = 21.3216(3) A, b = 12.1191(2) A, c = 21.0125(2) A, alpha = 90 degrees, beta = 105.3658(5) degrees, gamma = 90 degrees; monoclinic, C2/c, Z = 4; Fe-N(4)(av) = 2.07 A; N-Fe-N (all) = 90.0 degrees ). Further characterization of 2 includes UV-vis [THF, lambda(max), 421 (Soret), 542 nm] and (1)H NMR [six-coordinate, high spin heme; THF-d(8), RT, delta 56.7 (s, pyrrole-H, 8H), 8.38 (s, para-phenyl-H, 8H), 7.15 (s, meta-phenyl-H, 4H)] spectroscopies. Flash photolysis studies employing 1 were able to resolve the CO rebinding kinetics in both THF and cyclohexane solvents. In CO saturated THF [[CO] approximately 5 mM] and at [1] congruent with 5 microM, the conversion of [(F(8)TPP)Fe(II)(THF)(2)] (produced after photolytic displacement of CO) to [(F(8)TPP)Fe(II)(CO)(THF)] was monoexponential, with k(obs) = 1.6 (+/-0.2) x 10(4) s(-)(1). Reduction in [CO] by vigorous Ar purging gave k(obs) congruent with 10(3) s(-)(1) in cyclohexane. The study presented in this report lays the foundation for applying fast-time scale studies based on CO flash photolysis to the more complicated heterobimetallic heme/Cu systems.

ATR-FTIR spectroscopy of the P(M) and F intermediates of bovine and Paracoccus denitrificans cytochrome c oxidase

The structures of P(M) and F intermediates of bovine and Paracoccus denitrificans cytochrome c oxidase were investigated by perfusion-induced attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. Transitions from the "fast" oxidized state to the P(M) or F states were initiated by perfusion with buffer containing either CO/oxygen or H(2)O(2). Intermediates were quantitated by simultaneous monitoring of visible absorption changes in the protein film. For both bovine and P. denitrificans oxidase, the major features of the IR difference spectrum of P(M) were similar when produced by CO/oxygen or by H(2)O(2) treatments. These IR difference spectra were distinctly different from the IR difference spectrum of F that formed with extended treatment with H(2)O(2). Some IR bands could be assigned tentatively to perturbations of heme a(3) ring modes and substituents, and these perturbations were greater in P(M) than in F. Other bands could be assigned to surrounding protein changes. Strong perturbation of the environment of a carboxylic acid, most likely E-242 (bovine numbering), occurred in P(M) and relaxed back in F. A second redox-sensitive carboxylic acid was also perturbed in the bovine P(M) intermediate. Further consistent signatures of P(M) in both oxidases that were absent in F were strong negative bands at 1547 and 1313 cm(-1) in bovine oxidase (1542 and 1314 cm(-1) in P. denitrificans) and a positive band at approximately 1519 cm(-1). From comparison with available IR data on model compounds, it is suggested that these reflect changes in the covalent tyrosine-histidine ligand to Cu(B). These findings are discussed in relation to the oxidase catalytic cycle.

ATR-FTIR difference spectroscopy of the P(M) intermediate of bovine cytochrome c oxidase

Perfusion-induced attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy was used to investigate changes induced in protein and cofactors of bovine cytochrome c oxidase when it was converted from the oxidised state to the catalytic P(M) intermediate. The transition was induced in a film of detergent-depleted 'fast' oxidase with a buffer containing CO and O(2). The extent of formation of the P(M) state was quantitated simultaneously by monitoring formation of its characteristic 607-nm band with a scanned visible beam reflected off the top surface of the prism. The P(M) minus O FTIR difference spectrum is distinctly different from the redox spectra reported to date and includes features that can be assigned to changes of haem a(3) and surrounding protein. Tentative assignments are made based on vibrational data of related proteins and model compounds.

Observation of the equilibrium CuB-CO complex and functional implications of the transient heme a3 propionates in cytochrome ba3-CO from Thermus thermophilus. Fourier transform infrared (FTIR) and time-resolved step-scan FTIR studies

We report the first evidence for the existence of the equilibrium Cu(B)1+-CO species of CO-bound reduced cytochrome ba(3) from Thermus thermophilus at room temperature. The frequency of the C-O stretching mode of Cu(B)1+-CO is located at 2053 cm(-1) and remains unchanged in H(2)O/D(2)O exchanges and, between pD 5.5 and 9.7, indicating that the chemical environment does not alter the protonation state of the Cu(B) histidine ligands. The data and conclusions reported here are in contrast to the changes in protonation state of Cu(B)-His-290, reported recently (Das, T. K., Tomson, F. K., Gennis, R. B., Gordon, M., and Rousseau, D. L. (2001) Biophys. J. 80, 2039-2045 and Das, T. P., Gomes, C. M., Teixeira, M., and Rousseau, D. L. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 9591-9596). The time-resolved step-scan FTIR difference spectra indicate that the rate of decay of the transient Cu(B)1+-CO complex is 34.5 s(-1) and rebinding to heme a(3) occurs with k(2) = 28.6 s(-1). The rate of decay of the transient Cu(B)1+-CO complex displays a similar time constant as the absorption changes at 1694(+)/1706(-), attributed to perturbation of the heme a(3) propionates (COOH). The nu(C-O) of the transient Cu(B)1+-CO species is the same as that of the equilibrium Cu(B)1+-CO species and remains unchanged in the pD range 5.5-9.7 indicating that no structural change takes place at Cu(B) between these states. The implications of these results with respect to proton pathways in heme-copper oxidases are discussed.

Fourier transform infrared (FTIR) and step-scan time-resolved FTIR spectroscopies reveal a unique active site in cytochrome caa3 oxidase from Thermus thermophilus

Fourier transform infrared (FTIR) and step-scan time-resolved FTIR difference spectra are reported for the [carbonmonoxy]cytochrome caa(3) from Thermus thermophilus. A major C-O mode of heme a(3) at 1958 cm(-1) and two minor modes at 1967 and 1975 cm(-1) (7:1:1) have been identified at room temperature and remained unchanged in H(2)O/D(2)O exchange. The observed C-O frequencies are 10 cm(-1) higher than those obtained previously at 21 K (Einarsdóttir, O., Killough, P. M., Fee, J. A., and Woodruff, W. H. (1989) J. Biol. Chem. 264, 2405-2408). The time-resolved FTIR data indicate that the transient Cu(B)(1+)-CO complex is formed at room temperature as revealed by the CO stretching mode at 2062 cm(-1). Therefore, the caa(3) enzyme is the only documented member of the heme-copper superfamily whose binuclear center consists of an a(3)-type heme of a beta-form and a Cu(B) atom of an alpha-form. These results illustrate that the properties of the binuclear center in other oxidases resulting in the alpha-form are not required for enzymatic activity. Dissociation of the transient Cu(B)(1+)-CO complex is biphasic. The rate of decay is 2.3 x 10(4) s(-1) (fast phase, 35%) and 36.3 s(-1) (slow phase, 65%). The observed rate of rebinding to heme a(3) is 34.1 s(-1). The implications of these results with respect to the molecular motions that are general to the photodynamics of the binuclear center in heme-copper oxidases are discussed.

Mutual control of axial and equatorial ligands: model studies with [Ni]-bacteriochlorophyll-a

Modification of the metal's electronic environment by ligand association and dissociation in metalloenzymes is considered cardinal to their catalytic activity. We have recently presented a novel system that utilizes the bacteriochlorophyll (BChl) macrocycle as a ligand and reporter. This system allows for charge mobilization in the equatorial plane and experimental estimate of changes in the electronic charge density around the metal with no modification of the metal's chemical environment. The unique spectroscopy, electrochemistry and coordination chemistry of [Ni]-bacteriochlorophyll ([Ni]-BChl) enable us to follow directly fine details and steps involved in the function of the metal redox center. This approach is utilized here whereby electro-chemical reduction of [Ni]-BChl to the monoanion [Ni]-BChl(-) results in reversible dissociation of biologically relevant axial ligands. Similar ligand dissociation was previously detected upon photoexcitation of [Ni]-BChl (Musewald, C.; Hartwich, G.; Lossau, H.; Gilch, P.; Pollinger-Dammer, F.; Scheer, H.; Michel-Beyerle, M. E. J. Phys. Chem. B 1999, 103, 7055-7060 and Noy, D.; Yerushalmi, R.; Brumfeld, V.; Ashur, I.; Baldridge, K. K.; Scheer, H.; Scherz, A. J. Am. Chem. Soc. 2000, 122, 3937-3944). The electrochemical measurements and quantum mechanical (QM) calculations performed here for the neutral, singly reduced, monoligated, and singly reduced, monoligated [Ni]-BChl suggest the following: (a) Electroreduction, although resulting in a pi anion [Ni]-BChl(-) radical, causes electron density migration to the [Ni]-BChl core. (b) Reduction of nonligated [Ni]-BChl does not change the macrocycle conformation, whereas axial ligation results in a dramatic expansion of the metal core and a flattening of the highly ruffled macrocycle conformation. (c) In both the monoanion and singly excited [Ni]-BChl ([Ni]-BChl*), the frontier singly occupied molecular orbital (SOMO) has a small but nonnegligible metal character. Finally, (d) computationally, we found that a reduction of [Ni]-BChl*imidazole results in a weaker metal-axial ligand bond. Yet, it remains weakly bound in the gas phase. The experimentally observed ligand dissociation is accounted for computationally when solvation is considered. On the basis of the experimental observations and QM calculations, we propose a mechanism whereby alterations in the equatorial pi system and modulation of sigma bonding between the axial ligands and the metal core are mutually correlated. Such a mechanism highlights the dynamic role of axial ligands in regulating the activity of metal centers such as factor F430 (F430), a nickel-based coenzyme that is essential in methanogenic archea.

Neuronal nitric oxide synthase ligand and protein vibrations at the substrate binding site. A study by FTIR

Improvements in sensitivity and data processing of Fourier transform infrared (FTIR) spectroscopy enable it to be used to detect changes in protein structure at the atomic level. This paper reports a study of neuronal nitric oxide synthase (nNOS) by FTIR difference spectroscopy in the 1000-2500 cm(-1) range where vibrational bands of ligands, prosthetic groups, and protein and amino acid side chains are found. We have exploited the photolyzable CO compound of the ferrous heme of nNOS to produce light-induced CO photolysis difference spectra and to compare spectra after hydrogen/deuterium exchange. In (reduced) minus (reduced plus CO) difference spectra, negative bands at 1931 and 1907 cm(-1) are observed due to photolysis of multiple forms of ferrous heme-ligated CO, similar to those observed by resonance Raman spectroscopy [Wang et al. (1997) Biochemistry 36, 4595-4606]. Photolysis of the ferrous heme CO compound is accompanied by hitherto unreported changes in the 1000-2000 cm(-1) region that arise from changes of protein backbone, substrate, amino acid side chain, and cofactor vibrations. Preliminary assignments of vibrations are made on the basis of frequencies and the effects of hydrogen/deuterium exchange, and in the light of known atomic structures.

Attenuated total reflection Fourier transform infrared spectroscopy of redox transitions in photosynthetic reaction centers: comparison of perfusion- and light-induced difference spectra

Chemically induced Fourier transform infrared difference spectra associated with redox transitions of several primary electron donors and acceptors in photosynthetic reaction centers (RCs) have been compared with the light-induced FTIR difference spectra involving the same cofactors. The RCs are deposited on an attenuated total reflection (ATR) prism and form a film that is enclosed in a flow cell. Redox transitions in the film of RCs can be repetitively induced either by perfusion of buffers poised at different redox potentials or by illumination. The perfusion-induced ATR-FTIR difference spectra for the oxidation of the primary electron donor P in the RCs of the purple bacteria Rb. sphaeroides and Rp. viridis and P700 in the photosystem 1 of Synechocystis 6803, as well as the Q(A)/Q(A) transition of the quinone acceptor (Q(A)) in Rb. sphaeroides RCs are reported for the first time. They are compared with the light-induced ATR-FTIR difference spectra P+Q(A)/PQ(A) for the RCs of Rb. sphaeroides and P700+/P700 for photosystem 1. It is shown that the perfusion-induced and light-induced ATR-FTIR difference spectra recorded on the same RC film display identical signal to noise ratios when they are measured under comparable conditions. The ATR-FTIR difference spectra are very similar to the equivalent FTIR difference spectra previously recorded upon photochemical or electrochemical excitation of these RCs in the more conventional transmission mode. The ATR-FTIR technique requires a smaller amount of sample compared with transmission FTIR and allows precise control of the aqueous environment of the RC films.

The role of the cross-link His-Tyr in the functional properties of the binuclear center in cytochrome c oxidase

Resonance Raman and Fourier transform infrared spectroscopies have been used to study the aa(3)-type cytochrome c oxidase and the Y280H mutant from Paracoccus denitrificans. The stability of the binuclear center in the absence of the Tyr(280)-His(276) cross-link is not compromised since heme a(3) retains the same proximal environment, spin, and coordination state as in the wild type enzyme in both the oxidized and reduced states. We observe two C-O modes in the Y280H mutant at 1966 and 1975 cm(-1). The 1975 cm(-1) mode is assigned to a gamma-form and represents a structure of the active site in which Cu(B) exerts a steric effect on the heme a(3)-bound CO. Therefore, the role of the cross-link is to fix Cu(B) in a certain configuration and distance from heme a(3), and not to allow histidine ligands to coordinate to Cu(B) rather than to heme a(3), rendering the enzyme inactive, as proposed recently (Das, T. K., Pecoraro, C., Tomson, F. L., Gennis, R. B., and Rousseau, D. L. (1998) Biochemistry 37, 14471-14476). The results provide solid evidence that in the Y280H mutant the catalytic site retains its active configuration that allows O(2) binding to heme a(3). Oxygenated intermediates are formed by mixing oxygen with the CO-bound mixed-valence wild type and Y280H enzymes with similar Soret maxima at 438 nm.

Decay of the transient Cu(B)-CO complex is accompanied by formation of the heme Fe-CO complex of cytochrome cbb(3)-CO at ambient temperature: evidence from time-resolved Fourier transform infrared spectroscopy

Time-resolved step-scan Fourier infrared spectroscopy has been used to study the CO-bound cbb(3)-type cytochrome c oxidase from Pseudomonas stutzeri at room temperature. We observe a single band in the FTIR spectrum at 1956 cm(-1) (beta-form). The time-resolved data indicate that upon photolysis, CO is transferred from heme b(3) (nu(CO) = 1956 cm(-1)) to CuB (nu(CO) = 2064 cm(-1)). The decay of the 2065 cm(-1) peak (t(1/2) = 120 +/- 16 ms) and the development of the 1956 cm(-1) peak (t(1/2) = 144 +/- 8 ms ) suggest that formation of the Fe-CO complex is concurrent with the decay of the CuB-CO complex. The intensity ratio of the Fe-CO/CuB-CO (2.15) remains constant for all data points, and thus we conclude that no fraction of CO escapes the binuclear center at 293 K.

Time-resolved step-scan Fourier transform infrared spectroscopy of the CO adducts of bovine cytochrome c oxidase and of cytochrome bo(3) from Escherichia coli

We have used cryogenic difference FTIR and time-resolved step-scan Fourier transform infrared (TR-FTIR) spectroscopies to explore the redox-linked proton-pumping mechanism of heme-copper respiratory oxidases. These techniques are used to probe the structure and dynamics of the heme a(3)-Cu(B) binuclear center and the coupled protein structures in response to the photodissociation of CO from heme Fe and its subsequent binding to and dissociation from Cu(B). Previous cryogenic (80 K) FTIR CO photodissociation difference results were obtained for cytochrome bo(3), the ubiquinol oxidase of Escherichia coli [Puustinen, A., et al. (1997) Biochemistry 36, 13195-13200]. These data revealed a connectivity between Cu(B) and glutamic acid E286, a residue which has been implicated in proton pumping. In the current work, the same phenomenon is observed using the CO adduct of bovine cytochrome aa(3) under cryogenic conditions, showing a perturbation of the equivalent residue (E242) to that in bo(3). Furthermore, using time-resolved (5 micros resolution) step-scan FTIR spectroscopy at room temperature, we observe the same spectroscopic perturbation in both cytochromes aa(3) and bo(3). In addition, we observe evidence for perturbation of a second carboxylic acid side chain, at higher frequency in both enzymes at room temperature. The high-frequency feature does not appear in the cryogenic difference spectra, indicating that the perturbation is an activated process. We postulate that the high-frequency IR feature is due to the perturbation of E62 (E89 in bo(3)), a residue near the opening of the proton K-channel and required for enzyme function. The implications of these results with respect to the proton-pumping mechanism are discussed. Finally, a fast loss of over 60% of the Cu(B)-CO signal in bo(3) is observed and ascribed to one or more additional conformations of the enzyme. This fast conformer is proposed to account for the uninhibited reaction with O(2) in flow-flash experiments.

Attenuated total reflection Fourier transform infrared studies of redox changes in bovine cytochrome c oxidase: resolution of the redox Fourier transform infrared difference spectrum of heme a(3)

Attenuated total reflection Fourier transform infrared (ATR-FTIR) difference spectroscopy has been performed on samples of bovine cytochrome c oxidase that have been deposited as a thin film on the surface of a silicon microprism. The technique has several advantages over transmission methods in terms of amount of material required, the time required to reach sufficient optical stability, and the range of reactants that can be repetitively added and removed. The ATR-FTIR method has been used to record redox difference spectra of cytochrome c oxidase in the unligated and cyanide-ligated states. By subtraction of the spectra, the redox FTIR difference spectrum of heme a(3) can be resolved from those of the other metal centers. This difference spectrum is compared with available vibrational and Raman data on homologous oxidases and on heme A model compounds.

Transient binding of CO to Cu(B) in cytochrome c oxidase is dynamically linked to structural changes around a carboxyl group: a time-resolved step-scan Fourier transform infrared investigation

The redox-driven proton pump cytochrome c oxidase is that enzymatic machinery of the respiratory chain that transfers electrons from cytochrome c to molecular oxygen and thereby splits molecular oxygen to form water. To investigate the reaction mechanism of cytochrome c oxidase on the single vibrational level, we used time-resolved step-scan Fourier transform infrared spectroscopy and studied the dynamics of the reduced enzyme after photodissociation of bound carbon monoxide across the mid-infrared range (2300-950 cm(-1)). Difference spectra of the bovine complex were obtained at -20 degrees C with 5 micros time resolution. The data demonstrate a dynamic link between the transient binding of CO to Cu(B) and changes in hydrogen bonding at the functionally important residue E(I-286). Variation of the pH revealed that the pK(a) of E(I-286) is >9.3 in the fully reduced CO-bound oxidase. Difference spectra of cytochrome c oxidase from beef heart are compared with those of the oxidase isolated from Rhodobacter sphaeroides. The bacterial enzyme does not show the environmental change in the vicinity of E(I-286) upon CO dissociation. The characteristic band shape appears, however, in redox-induced difference spectra of the bacterial enzyme but is absent in redox-induced difference spectra of mammalian enzyme. In conclusion, it is demonstrated that the dynamics of a large protein complex such as cytochrome c oxidase can be resolved on the single vibrational level with microsecond Fourier transform infrared spectroscopy. The applied methodology provides the basis for future investigations of the physiological reaction steps of this important enzyme.

FTIR studies of the CO and cyanide adducts of fully reduced bovine cytochrome c oxidase

Photolysis spectra of the CO and cyanide adducts of reduced bovine cytochrome c oxidase have been studied by FTIR difference spectroscopy. Bound CO is predominantly in a single 1963 cm(-1) form whereas cyanide is bound in at least two forms (2058/2045 cm(-1)). These forms are pH-independent between pH 6.5 and 8.5, indicating that there is no titratable protonatable group that influences significantly their binding in this pH range. Photolysis spectra of the cyanide adduct have a positive band around 2090 cm(-1) in H(2)O due at least in part to free HCN and at 1880 cm(-1) in D(2)O due to free DCN. The frequency of the positive band around 2090 cm(-1), and its persistence in D(2)O media, raises the possibility that a transient cyanide-Cu(B) adduct also contributes to this signal, equivalent to the CO-Cu(B) species that is formed when CO is photolyzed. Photolysis produces changes throughout the 1000-1800 cm(-1) region. Reduced minus (reduced + CO) photolysis spectra in H(2)O exhibit a pH-independent and symmetrical peak/trough at 1749/1741 cm(-1). A related feature in homologous oxidases has been suggested to arise from a conserved glutamic acid. However, only around one-third of the feature is shifted to lower frequencies by incubation in D(2)O media, and an additional fraction is shifted if catalytic turnover occurs in D(2)O. Reduced minus (reduced + cyanide) photolysis spectra exhibit multiple features in H(2)O in this region with peaks at 1752, 1725, and 1708 cm(-1) and troughs at 1740, 1715, and 1698 cm(-1). Again, only a part of these features shift in D(2)O, even with catalytic turnover. A variety of additional H/D-sensitive features in the 1700-1000 cm(-1) region of the spectra can be discerned, one of which in cyanide photolysis spectra is tentatively assigned to a conserved tyrosine, Y244. Data are discussed in relation to the structure of the binuclear center and protonatable groups in its vicinity.

Redox dependent conformational changes in the mixed valence form of the cytochrome c oxidase from p. The reorganization of glutamic acid 278 is coupled to the electron transfer from/to heme a and the binuclear center. denitrificans

In this work we present the separation of FTIR difference signals induced by electron transfer to/from the redox centers of the cytochrome c oxidase from P. denitrificans and compare electrochemically induced FTIR difference spectra with those induced by CO photolysis. FTIR difference spectra of rebinding of CO to the half reduced (mixed valence) form of the cytochrome c oxidase after photolysis reflect the conformational changes induced by the rebinding of CO and by electron transfer reactions from heme a3 to heme a and further on to CUA. During this process, heme a3 (and CUB) are oxidized, whereas heme a and CuA are reduced. By subtracting these difference spectra from an electrochemically induced FTIR difference spectrum, where all four cofactors are reduced, the contributions for heme a3 (and CuB) could be separated. Correspondingly, the spectral contributions of heme a and CuA have been separated. The comparison of these spectra with the spectra calculated for the hemes on the basis of their redox dependent changes previously published in Hellwig et al., (Biochemistry 38, (1999) 1685-1694) show a high degree of similarity, except for additional signals coupled to the reorganization of the binuclear center upon CO rebinding. The separated spectra clearly show that the signals attributed to Glu278, an amino acid discussed to be crucial for proton pumping, is coupled to electron transfer to/from heme a and the binuclear heme a3-CuB center.

Protonation reactions in relation to the coupling mechanism of bovine cytochrome c oxidase

Identification of the locations of protonatable sites in cytochrome c oxidase that are influenced by reactions in the binuclear centre is critical to assessment of proposed coupling mechanisms, and to controversies on where the pumping steps occur. One such protonation site is that which governs interconversion of the isoelectronic 607 nm 'P(M)' and 580 nm 'F' forms of the two-electron-reduced oxygen intermediate. Low pH favours protonation of a site that is close to an electron paramagnetic resonance (EPR)-silent radical species in P(M), and this induces a partial electronic redistribution to form an EPR-detectable tryptophan radical in F. A further protonatable group that must be close to the binuclear centre has been detected in bacterial oxidases by Fourier transform infrared spectroscopy from pH-dependent changes in the haem-bound CO vibration frequency at low temperatures. However, in bovine cytochrome c oxidase under similar conditions of measurement, haem-bound CO remains predominantly in a single 1963 cm(-1) form between pH 6.5 and 8.5, indicating that this group is not present. Lack of pH dependence extends to the protein region of the CO photolysis spectra and suggests that both the reduced and the reduced/CO states do not have titratable groups that affect the binuclear centre strongly in the pH range 6.5-8.5. This includes the conserved glutamic acid residue E242 whose pK appears to be above 8.5 even in the fully oxidised enzyme. The results are discussed in relation to recent ideas on coupling mechanism.

Binding of O(2) and its reduction are both retarded by replacement of valine 279 by isoleucine in cytochrome c oxidase from Paracoccus denitrificans

The crystal structure of the heme-copper oxidases suggested a putative channel of oxygen entry into the heme-copper site of O(2) reduction. Changing a conserved valine near this center in cytochrome bo(3) of Escherichia coli to isoleucine caused a significant increase in the apparent K(M) for oxygen with little or no change in V(max), suggesting that oxygen diffusion had been partially blocked [Riistama, S., Puustinen, A., García-Horsman, A., Iwata, S., Michel, H., and Wikström, M. (1996) Biochim. Biophys. Acta 1275, 1-4]. To study this phenotype further using rapid kinetic methods, the corresponding change (V279I) has been made in cytochrome aa(3) from Paracoccus denitrificans. In this mutant, the apparent K(M) for oxygen is 8 times higher than in the wild-type enzyme, whereas V(max) is decreased only to approximately half of the wild-type value. Flow-flash kinetic measurements show that the initial binding of oxygen to the heme of the binuclear site is indeed much slower in the mutant than in the wild-type enzyme. However, the subsequent phases of the reaction with O(2) are also slow although the pure heme-to-heme electron transfer process is essentially unperturbed. It is suggested that the mutation sterically hinders O(2) entry into the binuclear site and that it may also perturb the structure of local water molecules involved in proton transfer to this site.

Electrochemical, FTIR, and UV/VIS spectroscopic properties of the ba(3) oxidase from Thermus thermophilus

The ba3 cytochrome c oxidase from Thermus thermophilus has been studied with a combined electrochemical, UV/VIS, and FTIR spectroscopic approach. Oxidative electrochemical redox titrations yielded midpoint potentials of Em1= -0.02 +/- 0.01 V and Em2 = 0.16 +/- 0.04 V for heme b and Em1 = 0.13 +/- 0.04 V and Em2 = 0.22 +/- 0.03 V for heme a(3) (vs Ag/AgCl/3 M KCl). Fully reversible electrochemically induced UV/VIS and FTIR difference spectra were obtained for the full potential step from -0. 5 to 0.5 V as well as for the critical potential steps from -0.5 to 0.1 V (heme b is fully oxidized and heme a3 remains essentially reduced) and from 0.1 to 0.5 V (heme b remains oxidized and heme a3 becomes oxidized). The difference spectra thus allow to us distinguish modes coupled to heme b and heme a3. Analogous difference spectra were obtained for the enzyme in D2O buffer for additional assignments. The FTIR difference spectra reveal the reorganization of the polypeptide backbone, perturbations of single amino acids and of hemes b and a3 upon electron transfer to/from the four redox-active centers heme b and a3, as well as CuB and CuA. Proton transfer coupled to redox transitions can be expected to manifest in the spectra. Tentative assignments of heme vibrational modes, of individual amino acids, and of secondary structure elements are presented. Aspects of the uncommon electrochemical and spectroscopic properties of the ba3 oxidase from T. thermophilus are discussed.

Time-resolved FT-IR studies on the CO adduct of Paracoccus denitrificans cytochrome c oxidase: comparison of the fully reduced and the mixed valence form

The rebinding of CO to cytochrome c oxidase from Paracoccus denitrificans in the fully reduced and in the half-reduced (mixed valence) form as a function of temperature was investigated using time-resolved rapid-scan FT-IR spectroscopy in the mid-IR (1200-2100 cm-1). For the fully reduced enzyme, rebinding was complete in approximately 2 s at 268 K and showed a biphasic reaction. At 84 K, nonreversible transfer of CO from heme a3 to CuB was observed. Both photolysis at 84 K and photolysis at 268 K result in FT-IR difference spectra which show similarities in the amide I, amide II, and heme modes. Both processes, however, differ in spectral features characteristic for amino acid side chain modes and may thus be indicative for the motional constraint of CO at low temperature. Rebinding of photodissociated CO for the mixed-valence enzyme at 268 K is also biphasic, but much slower as compared to the fully reduced enzyme. FT-IR difference spectra show band features similar to those for the fully reduced enzyme. Additional strong bands in the amide I and amide II range indicate local conformational changes induced by electron and coupled proton transfer. These signals disappear when the temperature is lowered to 84 K. At 268 K, a difference signal at 1746 cm-1 is observed which is shifted by 6 cm-1 to 1740 cm-1 in 2H2O. The absence of this signal for the mutant Glu 278 Gln allows assignment to the COOH stretching mode of Glu 278, and indicates changes of the conformation, proton position, or protonation of this residue upon electron transfer.

Electrochemical and ultraviolet/visible/infrared spectroscopic analysis of heme a and a3 redox reactions in the cytochrome c oxidase from Paracoccus denitrificans: separation of heme a and a3 contributions and assignment of vibrational modes

Cytochrome c oxidase from Paracoccus denitrificans was studied with a combined electrochemical and ultraviolet/visible/infrared (UV/vis/IR) spectroscopic approach. Global fit analysis of oxidative electrochemical redox titrations was used to separate the spectral contributions coupled to heme a and a3 redox transitions, respectively. Simultaneous adjustment of the midpoint potentials and of the amplitudes for a user-defined number of redox components (here heme a and a3) at all wavelengths in the UV/vis (900-400 nm) and at all wavenumbers in the infrared (1800-1250 cm-1) yielded difference spectra for the number of redox potentials selected. With an assumption of two redox components, two spectra for the redox potential at -0.03 +/- 0.01 V and 0.22 +/- 0.04 V (quoted vs Ag/AgCl) were obtained. The method used here allows the separation of the heme signals from the electrochemically induced visible difference spectra of native cytochrome c oxidase without the addition of any inhibitors. The separated heme a and a3 UV/vis difference spectra essentially correspond to spectra obtained for high/low-spin and 5/6-coordinated heme a/a3 model compounds presented by Babcock [(1988) in Biological Applications of Resonance Raman Spectroscopy (Spiro, T., Ed.) Wiley and Sons, New York]. Single-component Fourier transform infrared (FTIR) difference spectra were calculated for both hemes on the basis of these fits, thus revealing contributions from the reorganization of the polypeptide backbone, from the hemes, and from single amino acids upon electron transfer of the cofactors (heme a/a3, CuA, and CuB), as well from coupled processes such as proton transfer. A tentative assignment of heme vibrational modes is presented and the assignment of the signals to the reorganization of the polypeptide backbone and to perturbations of single amino acids, in particular Asp, Glu, Arg, or Tyr, is discussed.

Involvement of glutamic acid 278 in the redox reaction of the cytochrome c oxidase from Paracoccus denitrificans investigated by FTIR spectroscopy

The molecular processes concomitant with the redox reactions of wild-type and mutant cytochrome c oxidase from Paracoccus denitrificans were analyzed by a combination of protein electrochemistry and Fourier transform infrared (FTIR) difference spectroscopy. Oxidized-minus-reduced FTIR difference spectra in the mid-infrared (4000-1000 cm-1) reflecting full or stepwise oxidation and reduction of the respective cofactor(s) were obtained. In the 1800-1000 cm-1 range, these FTIR difference spectra reflect changes of the polypeptide backbone geometry in the amide I (ca. 1620-1680 cm-1) and amide II (ca. 1560-1540 cm-1) region in response to the redox transition of the cofactor(s). In addition, several modes in the 1600-1200 cm-1 range can be tentatively attributed to heme modes. A peak at 1746 cm-1 associated with the oxidized form and a peak at 1734 cm-1 associated with the reduced form were previously discussed by us as proton transfer between Asp or Glu side chain modes in the course of the redox reaction of the enzyme [Hellwig, P., Rost, B., Kaiser, U., Ostermeier, C., Michel, H., and Mäntele, W. (1996) FEBS Lett. 385, 53-57]. These signals were resolved into several components associated with the oxidation of different cofactors. For a stepwise potential titration from the fully reduced state (-0.5 V) to the fully oxidized state (+0.5 V), a small component at 1738 cm-1 develops in the potential range of approximately +0.15 V and disappears at more positive potentials while the main component at 1746 cm-1 appears in the range of approximately +0.20 V (all potentials quoted vs Ag/AgCl/3 M KCl). This observation clearly indicates two different ionizable residues involved in redox-induced proton transfer. The major component at 1746 cm-1 is completely lost in the FTIR difference spectra of the Glu 278 Gln mutant enzyme. In the spectrum of the subunit I Glu 278 Asp mutant enzyme, the major component of the discussed difference band is lost. In contrast, the complete difference signal of the wild-type enzyme is preserved in the Asp 124 Asn, Asp 124 Ser, and Asp 399 Asn variants, which are critical residues in the discussed proton pump channel as suggested from structure and mutagenesis experiments. On the basis of these difference spectra of mutants, we present further evidence that glutamic acid 278 in subunit I is a crucial residue for the redox reaction. Potential titrations performed simultaneously for the IR and for the UV/VIS indicate that the signal related to Glu 278 is coupled to the electron transfer to/from heme a; however, additional involvement of CuB electron transfer cannot be excluded.

Fourier transform infrared evidence for connectivity between CuB and glutamic acid 286 in cytochrome bo3 from Escherichia coli

Photodissociation of fully reduced, carbonmonoxy cytochrome bo3 causes ultrafast transfer of carbon monoxide (C triple bond O) from heme iron to CuB in the binuclear site. At low temperatures, the C triple bond O remains bound to CuB for extended times. Here, we show that the binding of C triple bond O to CuB perturbs the IR stretch of an un-ionized carboxylic acid residue, which is identified as Glu286 by mutation to Asp or to Cys. Before photodissociation, the carbonyl (C=O)-stretching frequency of this carboxylic acid residue is 1726 cm-1 for Glu286 and 1759 cm-1 for Glu286Asp. These frequencies are definitive evidence for un-ionized R-COOH and suggest that the carboxylic acids are hydrogen-bonded, though more extensively in Glu286. In Glu286Cys, this IR feature is lost altogether. We ascribe the frequency shifts in the C=O IR absorptions to the effects of binding photodissociated C triple bond O to CuB, which are relay ed to the 286 locus. Conversely, the 2065 cm-1 C triple bond O stretch of CuB-CO is markedly affected by both mutations. These effects are ascribed to changes in the Lewis acidity of CuB, or to displacement of a CuB histidine ligand by C triple bond O. C triple bond O binding to CuB also induces a downshift of an IR band which can be attributed to an aromatic C-H stretch, possibly of histidine imidazole, at about 3140 cm-1. The results suggest an easily polarizable, through-bond connectivity between one of the histidine CuB ligands and the carboxylic group of Glu286. A chain of bound water molecules may provide such a connection, which is of interest in the context of the proton pump mechanism of the heme-copper oxidases.