Time-resolved resonance Raman investigation of oxygen reduction mechanism of bovine cytochrome c oxidase

Structural and functional mechanisms of cytochrome c oxidase

Cytochrome c oxidase (CcO) is the terminal enzyme in the electron transfer chain in mitochondria. It catalyzes the four-electron reduction of O2 to H2O and harnesses the redox energy to drive unidirectional proton translocation against a proton electrochemical gradient. A great deal of research has been conducted to comprehend the molecular properties of CcO. However, the mechanism by which the oxygen reduction reaction is coupled to proton translocation remains poorly understood. Here, we review the chemical properties of a variety of key oxygen intermediates of bovine CcO (bCcO) revealed by time-resolved resonance Raman spectroscopy and the structural features of the enzyme uncovered by serial femtosecond crystallography, an innovative technique that allows structural determination at room temperature without radiation damage. The implications of these data on the proton translocation mechanism are discussed.

Detection of a Geminate Photoproduct of Bovine Cytochrome c Oxidase by Time-Resolved Serial Femtosecond Crystallography

Cytochrome c oxidase (CcO) is a large membrane-bound hemeprotein that catalyzes the reduction of dioxygen to water. Unlike classical dioxygen binding hemeproteins with a heme b group in their active sites, CcO has a unique binuclear center (BNC) composed of a copper atom (CuB) and a heme a3 iron, where O2 binds and is reduced to water. CO is a versatile O2 surrogate in ligand binding and escape reactions. Previous time-resolved spectroscopic studies of the CO complexes of bovine CcO (bCcO) revealed that photolyzing CO from the heme a3 iron leads to a metastable intermediate (CuB-CO), where CO is bound to CuB, before it escapes out of the BNC. Here, with a pump-probe based time-resolved serial femtosecond X-ray crystallography, we detected a geminate photoproduct of the bCcO-CO complex, where CO is dissociated from the heme a3 iron and moved to a temporary binding site midway between the CuB and the heme a3 iron, while the locations of the two metal centers and the conformation of Helix-X, housing the proximal histidine ligand of the heme a3 iron, remain in the CO complex state. This new structure, combined with other reported structures of bCcO, allows for a clearer definition of the ligand dissociation trajectory as well as the associated protein dynamics.

Structural insights into functional properties of the oxidized form of cytochrome c oxidase

Cytochrome c oxidase (CcO) is an essential enzyme in mitochondrial and bacterial respiration. It catalyzes the four-electron reduction of molecular oxygen to water and harnesses the chemical energy to translocate four protons across biological membranes. The turnover of the CcO reaction involves an oxidative phase, in which the reduced enzyme (R) is oxidized to the metastable OH state, and a reductive phase, in which OH is reduced back to the R state. During each phase, two protons are translocated across the membrane. However, if OH is allowed to relax to the resting oxidized state (O), a redox equivalent to OH, its subsequent reduction to R is incapable of driving proton translocation. Here, with resonance Raman spectroscopy and serial femtosecond X-ray crystallography (SFX), we show that the heme a3 iron and CuB in the active site of the O state, like those in the OH state, are coordinated by a hydroxide ion and a water molecule, respectively. However, Y244, critical for the oxygen reduction chemistry, is in the neutral protonated form, which distinguishes O from OH, where Y244 is in the deprotonated tyrosinate form. These structural characteristics of O provide insights into the proton translocation mechanism of CcO.

Detection of a geminate photoproduct of bovine cytochrome c oxidase by time-resolved serial femtosecond crystallography

Cytochrome c oxidase (C c O) is a large membrane-bound hemeprotein that catalyzes the reduction of dioxygen to water. Unlike classical dioxygen binding hemeproteins with a heme b group in their active sites, C c O has a unique binuclear center (BNC) comprised of a copper atom (Cu B ) and a heme a 3 iron, where O 2 binds and is reduced to water. CO is a versatile O 2 surrogate in ligand binding and escape reactions. Previous time-resolved spectroscopic studies of the CO complexes of bovine C c O (bC c O) revealed that photolyzing CO from the heme a 3 iron leads to a metastable intermediate (Cu B -CO), where CO is bound to Cu B , before it escapes out of the BNC. Here, with a time-resolved serial femtosecond X-ray crystallography-based pump-probe method, we detected a geminate photoproduct of the bC c O-CO complex, where CO is dissociated from the heme a 3 iron and moved to a temporary binding site midway between the Cu B and the heme a 3 iron, while the locations of the two metal centers and the conformation of the Helix-X, housing the proximal histidine ligand of the heme a 3 iron, remain in the CO complex state. This new structure, combined with other reported structures of bC c O, allows the full definition of the ligand dissociation trajectory, as well as the associated protein dynamics.

Structural basis for functional properties of cytochrome c oxidase

Cytochrome c oxidase (CcO) is an essential enzyme in mitochondrial and bacterial respiration. It catalyzes the four-electron reduction of molecular oxygen to water and harnesses the chemical energy to translocate four protons across biological membranes, thereby establishing the proton gradient required for ATP synthesis1. The full turnover of the CcO reaction involves an oxidative phase, in which the reduced enzyme (R) is oxidized by molecular oxygen to the metastable oxidized OH state, and a reductive phase, in which OH is reduced back to the R state. During each of the two phases, two protons are translocated across the membranes2. However, if OH is allowed to relax to the resting oxidized state (O), a redox equivalent to OH, its subsequent reduction to R is incapable of driving proton translocation2,3. How the O state structurally differs from OH remains an enigma in modern bioenergetics. Here, with resonance Raman spectroscopy and serial femtosecond X-ray crystallography (SFX)4, we show that the heme a3 iron and CuB in the active site of the O state, like those in the OH state5,6, are coordinated by a hydroxide ion and a water molecule, respectively. However, Y244, a residue covalently linked to one of the three CuB ligands and critical for the oxygen reduction chemistry, is in the neutral protonated form, which distinguishes O from OH, where Y244 is in the deprotonated tyrosinate form. These structural characteristics of O provide new insights into the proton translocation mechanism of CcO.

Temperature-dependent structural transition following X-ray-induced metal center reduction in oxidized cytochrome c oxidase

Cytochrome c oxidase (CcO) is the terminal enzyme in the electron transfer chain in the inner membrane of mitochondria. It contains four metal redox centers, two of which, Cu_B and heme a₃, form the binuclear center (BNC), where dioxygen is reduced to water. Crystal structures of CcO in various forms have been reported, from which ligand-binding states of the BNC and conformations of the protein matrix surrounding it have been deduced to elucidate the mechanism by which the oxygen reduction chemistry is coupled to proton translocation. However, metal centers in proteins can be susceptible to X-ray-induced radiation damage, raising questions about the reliability of conclusions drawn from these studies. Here, we used microspectroscopy-coupled X-ray crystallography to interrogate how the structural integrity of bovine CcO in the fully oxidized state (O) is modulated by synchrotron radiation. Spectroscopic data showed that, upon X-ray exposure, O was converted to a hybrid O∗ state where all the four metal centers were reduced, but the protein matrix was trapped in the genuine O conformation and the ligands in the BNC remained intact. Annealing the O∗ crystal above the glass transition temperature induced relaxation of the O∗ structure to a new R∗ structure, wherein the protein matrix converted to the fully reduced R conformation with the exception of helix X, which partly remained in the O conformation because of incomplete dissociation of the ligands from the BNC. We conclude from these data that reevaluation of reported CcO structures obtained with synchrotron light sources is merited.

Control of transmembrane charge transfer in cytochrome c oxidase by the membrane potential

The respiratory chain in mitochondria is composed of membrane-bound proteins that couple electron transfer to proton translocation across the inner membrane. These charge-transfer reactions are regulated by the proton electrochemical gradient that is generated and maintained by the transmembrane charge transfer. Here, we investigate this feedback mechanism in cytochrome c oxidase in intact inner mitochondrial membranes upon generation of an electrochemical potential by hydrolysis of ATP. The data indicate that a reaction step that involves proton uptake to the catalytic site and presumably proton translocation is impaired by the potential, but electron transfer is not affected. These results define the order of electron and proton-transfer reactions and suggest that the proton pump is regulated by the transmembrane electrochemical gradient through control of internal proton transfer rather than by control of electron transfer.

Splitting of the O-O bond at the heme-copper catalytic site of respiratory oxidases

Heme-copper oxidases catalyze the four-electron reduction of O₂ to H₂O at a catalytic site that is composed of a heme group, a copper ion (Cu_B), and a tyrosine residue. Results from earlier experimental studies have shown that the O-O bond is cleaved simultaneously with electron transfer from a low-spin heme (heme a/b), forming a ferryl state (P_R ; Fe⁴⁺=O^2-, Cu_B²⁺-OH^-). We show that with the Thermus thermophilus ba₃ oxidase, at low temperature (10°C, pH 7), electron transfer from the low-spin heme b to the catalytic site is faster by a factor of ~10 (τ ≅ 11 μs) than the formation of the P_R ferryl (τ ≅110 μs), which indicates that O₂ is reduced before the splitting of the O-O bond. Application of density functional theory indicates that the electron acceptor at the catalytic site is a high-energy peroxy state [Fe³⁺-O^--O^-(H⁺)], which is formed before the P_R ferryl. The rates of heme b oxidation and P_R ferryl formation were more similar at pH 10, indicating that the formation of the high-energy peroxy state involves proton transfer within the catalytic site, consistent with theory. The combined experimental and theoretical data suggest a general mechanism for O₂ reduction by heme-copper oxidases.

The unusual redox properties of C-type oxidases

Cytochrome cbb₃ (also known as C-type) oxidases belong to the family of heme-copper terminal oxidases which couple at the end of the respiratory chain the reduction of molecular oxygen into water and the pumping of protons across the membrane. They are expressed most often at low pressure of O₂ and they exhibit a low homology of sequence with the cytochrome aa₃ (A-type) oxidases found in mitochondria. Their binuclear active site comprises a high-spin heme b₃ associated with a Cu_B center. The protein also contains one low-spin heme b and 3 hemes c. We address here the redox properties of cbb₃ oxidases from three organisms, Rhodobacter sphaeroides, Vibrio cholerae and Pseudomonas stutzeri by means of electrochemical and spectroscopic techniques. We show that the redox potential of the heme b₃ exhibits a relatively low midpoint potential, as in related cytochrome c-dependent nitric oxide reductases. Potential implications for the coupled electron transfer and proton uptake mechanism of C-type oxidases are discussed.

Proton translocation in cytochrome c oxidase: insights from proton exchange kinetics and vibrational spectroscopy

Cytochrome c oxidase is the terminal enzyme in the electron transfer chain. It reduces oxygen to water and harnesses the released energy to translocate protons across the inner mitochondrial membrane. The mechanism by which the oxygen chemistry is coupled to proton translocation is not yet resolved owing to the difficulty of monitoring dynamic proton transfer events. Here we summarize several postulated mechanisms for proton translocation, which have been supported by a variety of vibrational spectroscopic studies. We recently proposed a proton translocation model involving proton accessibility to the regions near the propionate groups of the heme a and heme a3 redox centers of the enzyme based by hydrogen/deuterium (H/D) exchange Raman scattering studies (Egawa et al., PLoS ONE 2013). To advance our understanding of this model and to refine the proton accessibility to the hemes, the H/D exchange dependence of the heme propionate group vibrational modes on temperature and pH was measured. The H/D exchange detected at the propionate groups of heme a3 takes place within a few seconds under all conditions. In contrast, that detected at the heme a propionates occurs in the oxidized but not the reduced enzyme and the H/D exchange is pH-dependent with a pKa of ~8.0 (faster at high pH). Analysis of the thermodynamic parameters revealed that, as the pH is varied, entropy/enthalpy compensation held the free energy of activation in a narrow range. The redox dependence of the possible proton pathways to the heme groups is discussed. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

Radical formation in cytochrome c oxidase

The formation of radicals in bovine cytochrome c oxidase (bCcO), during the O(2) redox chemistry and proton translocation, is an unresolved controversial issue. To determine if radicals are formed in the catalytic reaction of bCcO under single turnover conditions, the reaction of O(2) with the enzyme, reduced by either ascorbate or dithionite, was initiated in a custom-built rapid freeze quenching (RFQ) device and the products were trapped at 77K at reaction times ranging from 50μs to 6ms. Additional samples were hand mixed to attain multiple turnover conditions and quenched with a reaction time of minutes. X-band (9GHz) continuous wave electron paramagnetic resonance (CW-EPR) spectra of the reaction products revealed the formation of a narrow radical with both reductants. D-band (130GHz) pulsed EPR spectra allowed for the determination of the g-tensor principal values and revealed that when ascorbate was used as the reductant the dominant radical species was localized on the ascorbyl moiety, and when dithionite was used as the reductant the radical was the SO(2)(-) ion. When the contributions from the reductants are subtracted from the spectra, no evidence for a protein-based radical could be found in the reaction of O(2) with reduced bCcO. As a surrogate for radicals formed on reaction intermediates, the reaction of hydrogen peroxide (H(2)O(2)) with oxidized bCcO was studied at pH 6 and pH 8 by trapping the products at 50μs with the RFQ device to determine the initial reaction events. For comparison, radicals formed after several minutes of incubation were also examined, and X-band and D-band analysis led to the identification of radicals on Tyr-244 and Tyr-129. In the RFQ measurements, a peroxyl (ROO) species was formed, presumably by the reaction between O(2) and an amino acid-based radical. It is postulated that Tyr-129 may play a central role as a proton loading site during proton translocation by ejecting a proton upon formation of the radical species and then becoming reprotonated during its reduction via a chain of three water molecules originating from the region of the propionate groups of heme a(3). This article is part of a Special Issue entitled: "Allosteric cooperativity in respiratory proteins".

Vibrational resonances and CuB displacement controlled by proton motion in cytochrome c oxidase

Cytochrome c oxidase (CcO), found in the inner mitochondrial membranes or in many bacteria, catalyzes the four-electron reduction of molecular oxygen to water. Four protons are pumped across the inner mitochondrial membrane through CcO. In this study, quantum mechanics/molecular mechanics and molecular dynamics calculations are used to probe the spectroscopic characteristics of the ferryl intermediates in the aa(3) CcO/O(2) reaction. These highly elaborate calculations, supported by several calculations on smaller model systems, demonstrate the sensitivity of vibrational frequencies on the Coulombic field of heme a(3) and their dependence on the distance of the adjacent Cu(B) to the heme a(3)-Fe atom. This distance seems to be associated with the protonation state of the heme a(3) propionate A, and we propose that it plays a crucial role on the mechanism of action of CcO. In detail, we link proton pumping activity in CcO enzyme (a) to a multiple (1:1:2) resonance among the frequencies of Fe(IV)=O bond stretching, the breathing mode of Histidine 411, and a bending mode of the His411-Fe(IV)=O species (aa(3) from Paracoccus denitrificans numbering) and (b) to Cu(B) displacement by electrostatic interactions toward the heme a(3) iron. We find that the vibrations of the His411-Fe(IV)=O unit become highly coupled depending on the protonation state of the heme a(3) ring A propionate/Asp399 pair, and we propose a mechanism for the resonance Raman enhancement of the bending mode delta(His411-Fe(IV)=O). Calculations on model systems demonstrate that the position of Cu(B) in relation to heme a(3) iron-oxo plays a crucial role in regulating that resonance. We also discuss the origin of the coupling between bending, delta(His411-Fe(IV)=O) and nu(Fe=O) stretching modes, and the role played by such vibrational coupling interactions or Cu(B) position in controlling functional properties of the enzyme, including electron/proton coupling as well as experimental spectra.

Toward a chemical mechanism of proton pumping by the B-type cytochrome c oxidases: application of density functional theory to cytochrome ba3 of Thermus thermophilus

A mechanism for proton pumping by the B-type cytochrome c oxidases is presented in which one proton is pumped in conjunction with the weakly exergonic, two-electron reduction of Fe-bound O 2 to the Fe-Cu bridging peroxodianion and three protons are pumped in conjunction with the highly exergonic, two-electron reduction of Fe(III)- (-)O-O (-)-Cu(II) to form water and the active oxidized enzyme, Fe(III)- (-)OH,Cu(II). The scheme is based on the active-site structure of cytochrome ba 3 from Thermus thermophilus, which is considered to be both necessary and sufficient for coupled O 2 reduction and proton pumping when appropriate gates are in place (not included in the model). Fourteen detailed structures obtained from density functional theory (DFT) geometry optimization are presented that are reasonably thought to occur during the four-electron reduction of O 2. Each proton-pumping step takes place when a proton resides on the imidazole ring of I-His376 and the large active-site cluster has a net charge of +1 due to an uncompensated, positive charge formally associated with Cu B. Four types of DFT were applied to determine the energy of each intermediate, and standard thermochemical approaches were used to obtain the reaction free energies for each step in the catalytic cycle. This application of DFT generally conforms with previously suggested criteria for a valid model (Siegbahn, P. E. M.; Blomberg, M. A. R. Chem. Rev. 2000, 100, 421-437) and shows how the chemistry of O 2 reduction in the heme a 3 -Cu B dinuclear center can be harnessed to generate an electrochemical proton gradient across the lipid bilayer.

Redox-coupled proton translocation in biological systems: proton shuttling in cytochrome c oxidase

In the respiratory chain free energy is conserved by linking the chemical reduction of dioxygen to the electrogenic translocation of protons across a membrane. Cytochrome c oxidase (CcO) is one of the sites where this linkage occurs. Although intensively studied, the molecular mechanism of proton pumping by this enzyme remains unknown. Here, we present data from an investigation of a mutant CcO from Rhodobacter sphaeroides [Asn-139 --> Asp, ND(I-139)] in which proton pumping is completely uncoupled from the catalytic turnover (i.e., reduction of O2). However, in this mutant CcO, the rate by which O2 is reduced to H2O is even slightly higher than that of the wild-type CcO. The data indicate that the disabling of the proton pump is a result of a perturbation of E(I-286), which is located 20 A from N(I-139) and is an internal proton donor to the catalytic site, located in the membrane-spanning part of CcO. The mutation results in raising the effective pKa of E(I-286) by 1.6 pH units. An explanation of how the mutation uncouples catalytic turnover from proton pumping is offered, which suggests a mechanism by which CcO pumps protons.

The X-ray crystal structures of wild-type and EQ(I-286) mutant cytochrome c oxidases from Rhodobacter sphaeroides

The structure of cytochrome c oxidase from Rhodobacter sphaeroides has been solved at 2.3/2.8A (anisotropic resolution). This high-resolution structure revealed atomic details of a bacterial terminal oxidase including water molecule positions and a potential oxygen pathway, which has not been reported in other oxidase structures. A comparative study of the wild-type and the EQ(I-286) mutant enzyme revealed structural rearrangements around E(I-286) that could be crucial for proton transfer in this enzyme. In the structure of the mutant enzyme, EQ(I-286), which cannot transfer protons during oxygen reduction, the side-chain of Q(I-286) does not have the hydrogen bond to the carbonyl oxygen of M(I-107) that is seen in the wild-type structure. Furthermore, the Q(I-286) mutant has a different arrangement of water molecules and residues in the vicinity of the Q side-chain. These differences between the structures could reflect conformational changes that take place upon deprotonation of E(I-286) during turnover of the wild-type enzyme, which could be part of the proton-pumping machinery of the enzyme.

Detergent modulation of electron and proton transfer reactions in bovine cytochrome c oxidase

The effect of detergents on electron and proton transfer in bovine cytochrome c oxidase was studied using steady-state and transient-state methods. Cytochrome c oxidase in lauryl maltoside has high maximal turnover (TN(max)=400 s(-1)), whereas activity is low (TN(max)=10 s(-1)) in Triton X-100. Single turnover studies of intramolecular electron transfer show similar rates in either detergent. Transient proton uptake experiments show the oxidase in lauryl maltoside consumes 1.8+/-0.3 H(+)/aa(3) during either partial reduction of the oxidase or reaction of fully reduced enzyme with O(2). However, the oxidase in Triton X-100 consumes 2.6+/-0.4 H(+)/aa(3) during partial reduction and 1.0+/-0.2 H(+)/aa(3) in the O(2) reaction. Absorption spectra recorded during turnover show that the enzyme undergoes activation in lauryl maltoside, but does not activate in Triton X-100. We propose that cytochrome c oxidase in different detergents allows access to different sites of protonation, which in turn influences steady-state activity.

Heme-Cu complexes as oxygen-activating functional models for the active site of cytochrome c oxidase

Tri(2-pyridylmethyl)amineCu complex-linked iron meso-tetraphenylporphyine derivatives were prepared to model the active site of cytochrome c oxidase. Exposure to oxygen converted the reduced forms of the complexes to the corresponding stable mu-peroxo species in spite of the presence of three coordination sites, two on the heme and one on the Cu. The oxy forms were characterized spectroscopically. Kinetic analyses of the oxygenation reactions of the reduced forms suggests that preferential O2 binding occurs at the Cu site over the heme. This mechanism is also supported by examination of the redox potentials of the two metal ions. Since the peroxy complexes of the models exhibit a structure similar to that of the previously reported fully-oxidized form, the relevance of the model chemistry to the enzyme reaction is discussed.

Reaction of nitric oxide with the turnover intermediates of cytochrome c oxidase: reaction pathway and functional effects

The reactions of nitric oxide (NO) with the turnover intermediates of cytochrome c oxidase were investigated by combining amperometric and spectroscopic techniques. We show that the complex of nitrite with the oxidized enzyme (O) is obtained by reaction of both the "peroxy" (P) and "ferryl" (F) intermediates with stoichiometric NO, following a common reaction pathway consistent with P being an oxo-ferryl adduct. Similarly to chloride-free O, NO reacted with P and F more slowly [k approximately (2-8) x 10(4) M(-1) s(-1)] than with the reduced enzyme (k approximately 1 x 10(8) M(-1) s(-1)). Recovery of activity of the nitrite-inhibited oxidase, either during turnover or after a reduction-oxygenation cycle, was much more rapid than nitrite dissociation from the fully oxidized enzyme (t(1/2) approximately 80 min). The anaerobic reduction of nitrite-inhibited oxidase produced the fully reduced but uncomplexed enzyme, suggesting that reversal of inhibition occurs in turnover via nitrite dissociation from the cytochrome a(3)-Cu(B) site: this finding supports the hypothesis that oxidase may have a physiological role in the degradation of NO into nitrite. Kinetic simulations suggest that the probability for NO to be transformed into nitrite is greater at low electron flux through oxidase, while at high flux the fully reduced (photosensitive) NO-bound oxidase is formed; this is fully consistent with our recent finding that light releases the inhibition of oxidase by NO only at higher reductant pressure [Sarti, P., et al. (2000) Biochem. Biophys. Res. Commun. 274, 183].

Formation of the "peroxy" intermediate in cytochrome c oxidase is associated with internal proton/hydrogen transfer

When dioxygen is reduced to water by cytochrome c oxidase a sequence of oxygen intermediates are formed at the reaction site. One of these intermediates is called the "peroxy" (P) intermediate. It can be formed by reacting the two-electron reduced (mixed-valence) cytochrome c oxidase with dioxygen (called P(m)), but it is also formed transiently during the reaction of the fully reduced enzyme with oxygen (called P(r)). In recent years, evidence has accumulated to suggest that the O-O bond is cleaved in the P intermediate and that the heme a(3) iron is in the oxo-ferryl state. In this study, we have investigated the kinetic and thermodynamic parameters for formation of P(m) and P(r), respectively, in the Rhodobacter sphaeroides enzyme. The rate constants and activation energies for the formation of the P(r) and P(m) intermediates were 1.4 x 10(4) s(-1) ( approximately 20 kJ/mol) and 3 x 10(3) s(-1) ( approximately 24 kJ/mol), respectively. The formation rates of both P intermediates were independent of pH in the range 6.5-9, and there was no proton uptake from solution during P formation. Nevertheless, formation of both P(m) and P(r) were slowed by a factor of 1.4-1.9 in D(2)O, which suggests that transfer of an internal proton or hydrogen atom is involved in the rate-limiting step of P formation. We discuss the origin of the difference in the formation rates of the P(m) and P(r) intermediates, the formation mechanisms of P(m)/P(r), and the involvement of these intermediates in proton pumping.

Examination of the reaction of fully reduced cytochrome oxidase with hydrogen peroxide by flow-flash spectroscopy

The reaction of cytochrome c oxidase with hydrogen peroxide has been of great value in generating and characterizing oxygenated species of the enzyme that are identical or similar to those formed during turnover of the enzyme with dioxygen. Most previous studies have utilized relatively low peroxide concentrations (millimolar range). In the current work, these studies have been extended to the examination of the kinetics of the single turnover of the fully reduced enzyme using much higher concentrations of peroxide to avoid limitations by the bimolecular reaction. The flow-flash method is used, in which laser photolysis of the CO adduct of the fully reduced enzyme initiates the reaction following rapid mixing of the enzyme with peroxide, and the reaction is monitored by observing the absorbance changes due to the heme components of the enzyme. The following reaction sequence is deduced from the data. (1) The initial product of the reaction appears to be heme a(3) oxoferryl (Fe(4+)=O(2)(-) + H(2)O). Since the conversion of ferrous to ferryl heme a(3) (Fe(2+) to Fe(4+)) is sufficient for this reaction, presumably Cu(B) remains reduced in the product, along with Cu(A) and heme a. (2) The second phase of the reaction is an internal rearrangement of electrons and protons in which the heme a(3) oxoferryl is reduced to ferric hydroxide (Fe(3+)OH(-)). In about 40% of the population, the electron comes from heme a, and in the remaining 60% of the population, Cu(B) is oxidized. This step has a time constant of about 65 micros. (3) The third apparent phase of the reaction includes two parallel reactions. The population of the enzyme with an electron in the binuclear center reacts with a second molecule of peroxide, forming compound F. The population of the enzyme with the two electrons on heme a and Cu(A) must first transfer an electron to the binuclear center, followed by reaction with a second molecule of peroxide, also yielding compound F. In each of these reaction pathways, the reaction time is 100-200 micros, i.e., much faster than the rate of reaction of peroxide with the fully oxidized enzyme. Thus, hydrogen peroxide is an efficient trap for a single electron in the binuclear center. (4) Compound F is then reduced by the final available electron, again from heme a, at the same rate as observed for the reduction of compound F formed during the reaction of the fully reduced oxidase with dioxygen. The product is the fully oxidized enzyme (heme a(3) Fe(3+)OH(-)), which reacts with a third molecule of hydrogen peroxide, forming compound P. The rate of this final reaction step saturates at high concentrations of peroxide (V(max) = 250 s(-)(1), K(m) = 350 mM). The data indicate a reaction mechanism for the steady-state peroxidase activity of the enzyme which, at pH 7.5, proceeds via the single-electron reduction of the binuclear center followed by reaction with peroxide to form compound F directly, without forming compound P. Peroxide is an efficient trap for the one-electron-reduced state of the binuclear center. The results also suggest that the reaction of hydrogen peroxide to the fully oxidized enzyme may be limited by the presence of hydroxide associated with the heme a(3) ferric species. The reaction of hydrogen peroxide with heme a(3) is very substantially accelerated by the availability of an electron on heme a, which is presumably transferred to the binuclear center concomitant with a proton that can convert the hydroxide to water, which is readily displaced.