pH dependence of the reduction of dioxygen to water by cytochrome c oxidase. 2. Branched electron transfer pathways linked by proton transfer

The Reactions of O2 and NO with Mixed-Valence ba3 Cytochrome c Oxidase from Thermus thermophilus

Earlier CO flow-flash experiments on the fully reduced Thermus thermophilus ba₃ (Tt ba₃) cytochrome oxidase revealed that O₂ binding was slowed down by a factor of 10 in the presence of CO (Szundi et al., 2010, PNAS 107, 21010-21015). The goal of the current study is to explore whether the long apparent lifetime (∼50 ms) of the Cu_B⁺-CO complex generated upon photolysis of the CO-bound mixed-valence Tt ba₃ (Koutsoupakis et al., 2019, Acc. Chem. Res. 52, 1380-1390) affects O₂ and NO binding and the ability of Cu_B to act as an electron donor during O-O bond splitting. The CO recombination, NO binding, and the reaction of mixed-valence Tt ba₃ with O₂ were investigated by time-resolved optical absorption spectroscopy using the CO flow-flash approach and photolabile O₂ and NO carriers. No electron backflow was detected after photolysis of the mixed-valence CO-bound Tt ba₃. The rate of O₂ and NO binding was two times slower than in the fully reduced enzyme in the presence of CO and 20 times slower than in the absence of CO. The purported long-lived Cu_B⁺-CO complex did not prevent O-O bond splitting and the resulting P_M formation, which was significantly faster (5-10 times) than in the bovine heart enzyme. We propose that O₂ binding to heme a₃ in Tt ba₃ causes CO to dissociate from Cu_B⁺ in a concerted manner through steric and/or electronic effects, thus allowing Cu_B⁺ to act as an electron donor in the mixed-valence enzyme. The significantly faster O₂ binding and O-O bond cleavage in Tt ba₃ compared to analogous steps in the aa₃ oxidases could reflect evolutionary adaptation of the enzyme to the microaerobic conditions of the T. thermophilus HB8 species.

The CO Photodissociation and Recombination Dynamics of the W172Y/F282T Ligand Channel Mutant of Rhodobacter sphaeroides aa3 Cytochrome c Oxidase

In the ligand channel of the cytochrome c oxidase from Rhodobacter sphaeroides (Rs aa3 ) W172 and F282 have been proposed to generate a constriction that may slow ligand access to and from the active site. To explore this issue, the tryptophan and phenylalanine residues in Rs aa3 were mutated to the less bulky tyrosine and threonine residues, respectively, which occupy these sites in Thermus thermophilus (Tt) ba3 cytochrome oxidase. The CO photolysis and recombination dynamics of the reduced wild-type Rs aa3 and the W172Y/F282T mutant were investigated using time-resolved optical absorption spectroscopy. The spectral changes associated with the multiple processes are attributed to different conformers. The major CO recombination process (44 μs) in the W172Y/F282T mutant is ~500 times faster than the predominant CO recombination process in the wild-type enzyme (~23 ms). Classical dynamic simulations of the wild-type enzyme and double mutant showed significant structural changes at the active site in the mutant, including movement of the heme a3 ring-D propionate toward CuB and reduced binuclear center cavity volume. These structural changes effectively close the ligand exit pathway from the binuclear center, providing a basis for the faster CO recombination in the double mutant.

Conserved glycine 232 in the ligand channel of ba3 cytochrome oxidase from Thermus thermophilus

Knowing how the protein environment modulates ligand pathways and redox centers in the respiratory heme-copper oxidases is fundamental for understanding the relationship between the structure and function of these enzymes. In this study, we investigated the reactions of O2 and NO with the fully reduced G232V mutant of ba3 cytochrome c oxidase from Thermus thermophilus (Tt ba3) in which a conserved glycine residue in the O2 channel of the enzyme was replaced with a bulkier valine residue. Previous studies of the homologous mutant of Rhodobacter sphaeroides aa3 cytochrome c oxidase suggested that the valine completely blocked the access of O2 to the active site [Salomonsson, L., et al. (2004) Proc. Natl. Acad. Sci. U.S.A. 101, 11617-11621]. Using photolabile O2 and NO carriers, we find by using time-resolved optical absorption spectroscopy that the rates of O2 and NO binding are not significantly affected in the Tt ba3 G232V mutant. Classical molecular dynamics simulations of diffusion of O2 to the active site in the wild-type enzyme and G232V mutant show that the insertion of the larger valine residue in place of the glycine appears to open up other O2 and NO exit/entrance pathways that allow these ligands unhindered access to the active site, thus compensating for the larger valine residue.

The pathway of O₂to the active site in heme-copper oxidases

The route of O₂to and from the high-spin heme in heme-copper oxidases has generally been believed to emulate that of carbon monoxide (CO). Time-resolved and stationary infrared experiments in our laboratories of the fully reduced CO-bound enzymes, as well as transient optical absorption saturation kinetics studies as a function of CO pressure, have provided strong support for CO binding to CuB⁺ on the pathway to and from the high-spin heme. The presence of CO on CuB⁺ suggests that O₂binding may be compromised in CO flow-flash experiments. Time-resolved optical absorption studies show that the rate of O₂and NO binding in the bovine enzyme (1 × 10⁸M⁻¹s⁻¹) is unaffected by the presence of CO, which is consistent with the rapid dissociation (t½ = 1.5μs) of CO from CuB⁺. In contrast, in Thermus thermophilus (Tt) cytochrome ba3 the O₂and NO binding to heme a3 slows by an order of magnitude in the presence of CO (from 1 × 10⁹ to 1 × 10⁸M⁻¹s⁻¹), but is still considerably faster (~10μs at 1atm O₂) than the CO off-rate from CuB in the absence of O₂(milliseconds). These results show that traditional CO flow-flash experiments do not give accurate results for the physiological binding of O₂and NO in Tt ba3, namely, in the absence of CO. They also raise the question whether in CO flow-flash experiments on Tt ba3 the presence of CO on CuB⁺ impedes the binding of O₂to CuB⁺ or, if O₂does not bind to CuB⁺ prior to heme a3, whether the CuB⁺-CO complex sterically restricts access of O₂to the heme. Both possibilities are discussed, and we argue that O₂binds directly to heme a3 in Tt ba3, causing CO to dissociate from CuB⁺ in a concerted manner through steric and/or electronic effects. This would allow CuB⁺ to function as an electron donor during the fast (5μs) breaking of the OO bond. These results suggest that the binding of CO to CuB⁺ on the path to and from heme a3 may not be applicable to O₂and NO in all heme-copper oxidases. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

Ligand access to the active site in Thermus thermophilus ba(3) and bovine heart aa(3) cytochrome oxidases

Knowledge of the structure and dynamics of the ligand channel(s) in heme-copper oxidases is critical for understanding how the protein environment modulates the functions of these enzymes. Using photolabile NO and O(2) carriers, we recently found that NO and O(2) binding in Thermus thermophilus (Tt) ba(3) is ~10 times faster than in the bovine enzyme, indicating that inherent structural differences affect ligand access in these enzymes. Using X-ray crystallography, time-resolved optical absorption measurements, and theoretical calculations, we investigated ligand access in wild-type Tt ba(3) and the mutants, Y133W, T231F, and Y133W/T231F, in which tyrosine and threonine in the O(2) channel of Tt ba(3) are replaced by the corresponding bulkier tryptophan and phenylalanine, respectively, present in the aa(3) enzymes. NO binding in Y133W and Y133W/T231F was found to be 5 times slower than in wild-type ba(3) and the T231F mutant. The results show that the Tt ba(3) Y133W mutation and the bovine W126 residue physically impede NO access to the binuclear center. In the bovine enzyme, there is a hydrophobic "way station", which may further slow ligand access to the active site. Classical simulations of diffusion of Xe to the active sites in ba(3) and bovine aa(3) show conformational freedom of the bovine F238 and the F231 side chain of the Tt ba(3) Y133W/T231F mutant, with both residues rotating out of the ligand channel, resulting in no effect on ligand access in either enzyme.

Spectral identification of intermediates generated during the reaction of dioxygen with the wild-type and EQ(I-286) mutant of Rhodobacter sphaeroides cytochrome c oxidase

Cytochrome c oxidase from Rhodobacter sphaeroides is frequently used to model the more complex mitochondrial enzyme. The O(2) reduction in both enzymes is generally described by a unidirectional mechanism involving the sequential formation of the ferrous-oxy complex (compound A), the P(R) state, the oxyferryl F form, and the oxidized state. In this study we investigated the reaction of dioxygen with the wild-type reduced R. sphaeroides cytochrome oxidase and the EQ(I-286) mutant using the CO flow-flash technique. Singular value decomposition and multiexponential fitting of the time-resolved optical absorption difference spectra showed that three apparent lifetimes, 18 μs, 53 μs, and 1.3 ms, are sufficient to fit the kinetics of the O(2) reaction of the wild-type enzyme. A comparison of the experimental intermediate spectra with the corresponding intermediate spectra of the bovine enzyme revealed that P(R) is not present in the reaction mechanism of the wild-type R. sphaeroides aa(3). Transient absorbance changes at 440 and 610 nm support this conclusion. For the EQ(I-286) mutant, in which a key glutamic residue in the D proton pathway is replaced by glutamine, two lifetimes, 16 and 108 μs, were observed. A spectral analysis of the intermediates shows that the O(2) reaction in the EQ(I-286) mutant terminates at the P(R) state, with 70% of heme a becoming oxidized. These results indicate significant differences in the kinetics of O(2) reduction between the bovine and wild-type R. sphaeroides aa(3) oxidases, which may arise from differences in the relative rates of internal electron and proton movements in the two enzymes.

Kinetic studies of the reactions of O(2) and NO with reduced Thermus thermophilus ba(3) and bovine aa(3) using photolabile carriers

The reactions of molecular oxygen (O(2)) and nitric oxide (NO) with reduced Thermus thermophilus (Tt) ba(3) and bovine heart aa(3) were investigated by time-resolved optical absorption spectroscopy to establish possible relationships between the structural diversity of these enzymes and their reaction dynamics. To determine whether the photodissociated carbon monoxide (CO) in the CO flow-flash experiment affects the ligand binding dynamics, we monitored the reactions in the absence and presence of CO using photolabile O(2) and NO complexes. The binding of O(2)/NO to reduced ba(3) in the absence of CO occurs with a second-order rate constant of 1×10(9)M(-1)s(-1). This rate is 10-times faster than for the mammalian enzyme, and which is attributed to structural differences in the ligand channels of the two enzymes. Moreover, the O(2)/NO binding in ba(3) is 10-times slower in the presence of the photodissociated CO while the rates are the same for the bovine enzyme. This indicates that the photodissociated CO directly or indirectly impedes O(2) and NO access to the active site in Tt ba(3), and that traditional CO flow-flash experiments do not accurately reflect the O(2) and NO binding kinetics in ba(3). We suggest that in ba(3) the binding of O(2) (NO) to heme a(3)(2+) causes rapid dissociation of CO from Cu(B)(+) through steric or electronic effects or, alternatively, that the photodissociated CO does not bind to Cu(B)(+). These findings indicate that structural differences between Tt ba(3) and the bovine aa(3) enzyme are tightly linked to mechanistic differences in the functions of these enzymes. This article is part of a Special Issue entitled: Respiratory Oxidases.

CO impedes superfast O2 binding in ba3 cytochrome oxidase from Thermus thermophilus

Kinetic studies of heme-copper terminal oxidases using the CO flow-flash method are potentially compromised by the fate of the photodissociated CO. In this time-resolved optical absorption study, we compared the kinetics of dioxygen reduction by ba(3) cytochrome c oxidase from Thermus thermophilus in the absence and presence of CO using a photolabile O(2)-carrier. A novel double-laser excitation is introduced in which dioxygen is generated by photolyzing the O(2)-carrier with a 355 nm laser pulse and the fully reduced CO-bound ba(3) simultaneously with a second 532-nm laser pulse. A kinetic analysis reveals a sequential mechanism in which O(2) binding to heme a(3) at 90 μM O(2) occurs with lifetimes of 9.3 and 110 μs in the absence and presence of CO, respectively, followed by a faster cleavage of the dioxygen bond (4.8 μs), which generates the P intermediate with the concomitant oxidation of heme b. The second-order rate constant of 1 × 10(9) M(-1) s(-1) for O(2) binding to ba(3) in the absence of CO is 10 times greater than observed in the presence of CO as well as for the bovine heart enzyme. The O(2) bond cleavage in ba(3) of 4.8 μs is also approximately 10 times faster than in the bovine enzyme. These results suggest important structural differences between the accessibility of O(2) to the active site in ba(3) and the bovine enzyme, and they demonstrate that the photodissociated CO impedes access of dioxygen to the heme a(3) site in ba(3), making the CO flow-flash method inapplicable.

Heme-heme communication during the alkaline-induced structural transition in cytochrome c oxidase

Alkaline-induced conformational changes at pH 12.0 in the oxidized as well as the reduced state of cytochrome c oxidase have been systematically studied with time-resolved optical absorption and resonance Raman spectroscopies. In the reduced state, the heme a(3) first converts from the native five-coordinate configuration to a six-coordinate bis-histidine intermediate as a result of the coordination of one of the Cu(B) ligands, H290 or H291, to the heme iron. The coordination state change in the heme a(3) causes the alteration in the microenvironment of the formyl group of the heme a(3) and the disruption of the H-bond between R38 and the formyl group of the heme a. This structural transition, which occurs within 1min following the initiation of the pH jump, is followed by a slower reaction, in which Schiff base linkages are formed between the formyl groups of the two hemes and their nearby amino acid residues, presumably R38 and R302 for the heme a and a(3), respectively. In the oxidized enzyme, a similar Schiff base modification on heme a and a(3) was observed but it is triggered by the coordination of the H290 or H291 to heme a(3) followed by the breakage of the native proximal H378-iron and H376-iron bonds in heme a and a(3), respectively. In both oxidation states, the synchronous formation of the Schiff base linkages in heme a and a(3) relies on the structural communication between the two hemes via the H-bonding network involving R438 and R439 and the propionate groups of the two hemes as well as the helix X housing the two proximal ligands, H378 and H376, of the hemes. The heme-heme communication mechanism revealed in this work may be important in controlling the coupling of the oxygen and redox chemistry in the heme sites to proton pumping during the enzymatic turnover of CcO.

Plasticity of Proton Pathway Structure and Water Coordination in Cytochrome c Oxidase

Cytochrome c oxidase (CytcO) is a redox-driven, membrane-bound proton pump. One of the proton transfer pathways of the enzyme, the D pathway, used for the transfer of both substrate and pumped protons, accommodates a network of hydrogen-bonded water molecules that span the distance between an aspartate (Asp(132)), near the protein surface, and glutamate Glu(286), which is an internal proton donor to the catalytic site. To investigate how changes in the environment around Glu(286) affect the mechanism of proton transfer through the pathway, we introduced a non-hydrogen-bonding (Ala) or an acidic residue (Asp) at position Ser(197) (S197A or S197D), located approximately 7 A from Glu(286). Although Ser(197) is hydrogen-bonded to a water molecule that is part of the D pathway "proton wire," replacement of the Ser by an Ala did not affect the proton transfer rate. In contrast, the S197D mutant CytcO displayed a turnover activity of approximately 35% of that of the wild-type CytcO, and the O(2) reduction reaction was not linked to proton pumping. Instead, a fraction of the substrate protons was taken from the positive ("incorrect") side of the membrane. Furthermore, the pH dependence of the proton transfer rate was altered in the mutant CytcO. The results indicate that there is plasticity in the water coordination of the proton pathway, but alteration of the electrostatic potential within the pathway results in uncoupling of the proton translocation machinery.

The inside pH determines rates of electron and proton transfer in vesicle-reconstituted cytochrome c oxidase

Cytochrome c oxidase is the terminal enzyme in the respiratory chains of mitochondria and many bacteria where it translocates protons across a membrane thereby maintaining an electrochemical proton gradient. Results from earlier studies on detergent-solubilized cytochrome c oxidase have shown that individual reaction steps associated with proton pumping display pH-dependent kinetics. Here, we investigated the effect of pH on the kinetics of these reaction steps with membrane-reconstituted cytochrome c oxidase such that the pH was adjusted to different values on the inside and outside of the membrane. The results show that the pH on the inside of the membrane fully determines the kinetics of internal electron transfers that are linked to proton pumping. Thus, even though proton release is rate limiting for these reaction steps (Salomonsson et al., Proc. Natl. Acad. Sci. USA, 2005, 102, 17624), the transition kinetics is insensitive to the outside pH (in the range 6-9.5).

Photoreactions of cytochrome C oxidase

The photoreduction of oxidized bovine heart cytochrome c oxidase (CcO) by visible and UV radiation was investigated in the absence and presence of external reagents. In the former case, the quantum yields for direct photoreduction of heme A (heme a + heme a(3)) were 2.6 +/- 0.5 x 10(-3), 4 +/- 1 x 10(-4), and 4 +/- 2 x 10(-6) with pulsed laser irradiation at 266, 355 and 532 nm, respectively. Within experimental uncertainty, the quantum yields did not depend on pulse energy, implying that the mechanism is monophotonic. Irradiation with 355 nm light resulted in spectral changes similar to those produced independently by reduction with dithionite, whereby the low-spin heme a and Cu(A) are reduced first. Extended illumination at 355 and 532 nm yielded substantial amounts of reduced heme a(3). Heme decomposition was noted with 266 nm light. In the presence of formate and cyanide ions, which bind at the binuclear heme a(3)/copper center in CcO, irradiation at 355 nm caused selective reduction of only the low-spin heme a and Cu(A). The addition of ferrioxalate ion dramatically increased the efficiency of cytochrome c oxidase photoreduction. The quantum efficiency for heme A reduction was found to be near unity, significantly greater than for other known methods of photoreduction. The active reductant is most likely ferrous iron, and its reduction of the enzyme is thermodynamically driven by the reformation of ferrioxalate in the presence of excess oxalate ion. Other metalloenzymes with redox potentials similar to those of cytochrome c oxidase should be amenable to indirect photoreduction by this method.

A New Approach for Studying Fast Biological Reactions Involving Nitric Oxide: Generation of NO Using Photolabile Ruthenium and Manganese NO Donors

Nitric oxide (NO) is recognized as one of the major players in various biochemical processes, including blood pressure, neurotransmission and immune responses. However, experimental studies involving NO are often limited by difficulties associated with the use of NO gas, including its toxicity and precise control over NO concentration. Moreover, the reactions of NO with biological molecules, which frequently occur on time scales of microseconds or faster, are limited by the millisecond time scale of conventional stopped-flow techniques. Here we present a new approach for studying rapid biological reactions involving NO. The method is based on designed ruthenium and manganese nitrosyls, [Ru(PaPy3)(NO)](BF4)2 and [Mn(PaPy3)(NO)](ClO4) (PaPy3H = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide), which upon photolysis produce NO on a fast time scale. The kinetics of the binding of the photogenerated NO to reduced cytochrome c oxidase (CcO) and myoglobin (Mb) was investigated using time-resolved optical absorption spectroscopy. The NO was found to bind to reduced CcO with an apparent lifetime of 77 micros using the [Mn(PaPy3)(NO)]+ complex; the corresponding rate is 10-20 times faster than can be detected by conventional stopped-flow methods. Second-order rate constants of approximately 1 x 10(8) M(-1) s(-1) and approximately 3 x 10(7) M(-1) s(-1) were determined for NO binding to reduced CcO and Mb, respectively. The generation of NO by photolysis of these complexes circumvents the rate limitation of stopped-flow techniques and offers a novel alternative to study other fast biological reactions involving NO.

Heme-copper/dioxygen adduct formation relevant to cytochrome c oxidase: spectroscopic characterization of [(6L)FeIII-(O2(2-))-CuII]+

In the further development and understanding of heme-copper dioxygen reactivity relevant to cytochrome c oxidase O(2)-reduction chemistry, we describe a high-spin, five-coordinate dioxygen (peroxo) adduct of an iron(II)-copper(I) complex, [((6)L)Fe(II)Cu(I)](BArF(20)) (1), where (6)L is a tetraarylporphyrinate with a tethered tris(2-pyridylmethyl)amine chelate for copper. Reaction of 1 with O(2) in MeCN affords a remarkably stable [t(1/2) (rt; MeCN) approximately 60 min] adduct, [((6)L)Fe(III)-(O(2) (2-))-Cu(II)](+) (2) [EPR silent; lambda(max)=418 (Soret), 561 nm], formulated as a peroxo complex based on manometry (1:O(2)=1:1; spectrophotometric titration, -40 degrees C, MeCN), mass spectrometry {MALDI-TOF-MS: (16)O(2), m/z 1191 ([((6)L)Fe(III)-((16)O(2) (2-))-Cu(II)](+)); (18)O(2), m/z 1195}, and resonance Raman spectroscopy (nu((O-O))=788 cm(-1); Delta(16)O(2)/(18)O(2)=44 cm(-1); Delta(16)O(2)/(16/18)O(2)=22 cm(-1)). (1)H and (2)H NMR spectroscopy (-40 degrees C, MeCN) reveals that 2 is the first heme-copper peroxo complex which is high-spin, with downfield-shifted pyrrole resonances (delta(pyrrole)=75 ppm, s, br) and upfield shifted peaks at delta= -22, -35, and -40 ppm, similar to the pattern observed for the mu-oxo complex [((6)L)Fe(III)-O-Cu(II)](BAr(F)) (3) (known S=2 system, antiferromagnetically coupled high-spin Fe(III) and Cu(II)). The corresponding magnetic moment measurement (Evans method, CD(3)CN, -40 degrees C) also confirms the S=2 spin state, with mu(B)=4.9. Structural insights were obtained from X-ray absorption spectroscopy, showing Fe-O (1.83 A) and Cu-O (1.882 A) bonds, and an Fe...Cu distance of 3.35(2) A, suggestive of a mu-1,2-peroxo ligand present in 2. The reaction of 2 with cobaltocene gives 3, differing from the observed full reduction seen with other heme-Cu peroxo complexes. Finally, thermal decomposition of 2 yields 3, with concomitant release of 0.5 mol O(2) per mol 2, as confirmed quantitatively by an alkaline pyrogallol dioxygen scavenging solution.

Time-resolved optical absorption studies of cytochrome oxidase dynamics

Time-resolved spectroscopic studies in our laboratory of bovine heart cytochrome c oxidase dynamics are summarized. Intramolecular electron transfer was investigated upon photolysis of CO from the mixed-valence enzyme, by pulse radiolysis, and upon light-induced electron injection into the cytochrome c/cytochrome oxidase complex from a novel photoactivatable dye. The reduction of dioxygen to water was monitored by a gated multichannel analyzer using the CO flow-flash method or a synthetic caged dioxygen carrier. The pH dependence of the intermediate spectra suggests a mechanism of dioxygen reduction more complex than the conventional unidirectional sequential scheme. A branched model is proposed, in which one branch produces the P form and the other branch the F form. The rate of exchange between the two branches is pH-dependent. A cross-linked histidine-phenol was synthesized and characterized to explore the role of the cross-linked His-Tyr cofactor in the function of the enzyme. Time-resolved optical absorption spectra, EPR and FTIR spectra of the compound generated after UV photolysis indicated the presence of a radical residing primarily on the phenoxyl ring. The relevance of these results to cytochrome oxidase function is discussed.

Redox-driven proton pumping by heme-copper oxidases

One of the key problems of molecular bioenergetics is the understanding of the function of redox-driven proton pumps on a molecular level. One such class of proton pumps are the heme-copper oxidases. These enzymes are integral membrane proteins in which proton translocation across the membrane is driven by electron transfer from a low-potential donor, such as, e.g. cytochrome c, to a high-potential acceptor, O(2). Proton pumping is associated with distinct exergonic reaction steps that involve gradual reduction of oxygen to water. During the process of O(2) reduction, unprotonated high pK(a) proton acceptors are created at the catalytic site. Initially, these proton acceptors become protonated as a result of intramolecular proton transfer from a residue(s) located in the membrane-spanning part of the enzyme, but removed from the catalytic site. This residue is then reprotonated from the bulk solution. In cytochrome c oxidase from Rhodobacter sphaeroides, the proton is initially transferred from a glutamate, E(I-286), which has an apparent pK(a) of 9.4. According to a recently published structure of the enzyme, the deprotonation of E(I-286) is likely to result in minor structural changes that propagate to protonatable groups on the proton output (positive) side of the protein. We propose that in this way, the free energy available from the O(2) reduction is conserved during the proton transfer. On the basis of the observation of these structural changes, a possible proton-pumping model is presented in this paper. Initially, the structural changes associated with deprotonation of E(I-286) result in the transfer of a proton to an acceptor for pumped protons from the input (negative) side of the membrane. After reprotonation of E(I-286) this acceptor releases a proton to the output side of the membrane.