CytochromecOxidase as a Calcium Binding Protein. Studies on the Role of a Conserved Aspartate in Helices XI−XII Cytoplasmic Loop in Cation Binding†

Calcium-bound structure of bovine cytochrome c oxidase

The crystal structure of bovine cytochrome c oxidase (CcO) shows a sodium ion (Na⁺) bound to the surface of subunit I. Changes in the absorption spectrum of heme a caused by calcium ions (Ca²⁺) are detected as small red shifts, and inhibition of enzymatic activity under low turnover conditions is observed by addition of Ca²⁺ in a competitive manner with Na⁺. In this study, we determined the crystal structure of Ca²⁺-bound bovine CcO in the oxidized and reduced states at 1.7 Å resolution. Although Ca²⁺ and Na⁺ bound to the same site of oxidized and reduced CcO, they led to different coordination geometries. Replacement of Na⁺ with Ca²⁺ caused a small structural change in the loop segments near the heme a propionate and formyl groups, resulting in spectral changes in heme a. Redox-coupled structural changes observed in the Ca²⁺-bound form were the same as those previously observed in the Na⁺-bound form, suggesting that binding of Ca²⁺ does not severely affect enzymatic function, which depends on these structural changes. The relation between the Ca²⁺ binding and the inhibitory effect during slow turnover, as well as the possible role of bound Ca²⁺ are discussed.

Spectral components of detergent-solubilized bovine cytochrome oxidase

Cytochrome oxidase is the terminal oxidase of the mitochondrial electron transport chain and pumps 4 protons per oxygen reduced to water. Spectral shifts in the α-band of heme a have been observed in multiple studies and these shifts have the potential to shed light on the proton pumping intermediates. Previously we found that heme a had two spectral components in the α-band during redox titrations in living RAW 264.7 mouse macrophage cells, the classical 605 nm form and a blue-shifted 602 nm form. To confirm these spectral changes were not an artifact due to the complex milieu of the living cell, redox titrations were performed in the isolated detergent-solubilized bovine enzyme from both the Soret- and α-band using precise multiwavelength spectroscopy. This data verified the presence of the 602 nm form in the α-band, revealed a similar shift of heme a in the Soret-band and ruled out the reversal of calcium binding as the origin of the blue shift. The 602 nm form was found to be stabilized at high pH or by binding of azide, which is known to blue shift the α-band of heme a. Azide also stabilized the 602 nm form in the living cells. It is concluded there is a form of cytochrome oxidase in which heme a undergoes a blue shift to a 602 nm form and that redox titrations can be successfully performed in living cells where the oxidase operates in its authentic environment and in the presence of a proton motive force.

Effect of Ca2+ on the redox potential of heme a in cytochrome c oxidase

Subunit I of cytochrome c oxidase (CcO) from mitochondria and many bacteria contains a cation binding site (CBS) located at the outer positively charged aqueous phase not far from heme a. Binding of Ca²⁺ with the CBS in bovine CcO inhibits activity of the enzyme 2-3 -fold [Vygodina, T., Kirichenko, A. & Konstantinov A.A. (2013) Direct Regulation of Cytochrome c Oxidase by Calcium Ions, PLoS One.8 e74436]. Here we show that binding of Ca²⁺ at CBS of bovine CcO shifts E_m of heme a to the positive by 15-20 mV. Na⁺ ions that bind to the same site and compete with Ca²⁺ do not affect E_m of heme a and also prevent and reverse the effect of Ca²⁺. No effect of Ca²⁺ or EGTA is observed on E_m of heme a with the wild type bacterial oxidases from R.sphaeroides or P.denitrificans that contain tightly-bound calcium at the site. In the D477A mutant CcO from P. denitrificans that binds Ca²⁺ reversibly like the mitochondrial CcO, calcium shifts redox titration curve of heme a to the positive by ∼35-50 mV that is in good agreement with the results of electrostatic calculations; however, as shown earlier, it does not inhibit CcO activity of the mutant enzyme. Therefore the data do not support the proposal that the inhibitory effect of Ca²⁺ on CcO activity may be explained by the Ca²⁺-induced shift of E_m of heme a. Rather, Ca²⁺ retards electron transfer by inhibition of charge dislocation in the exit part of the proton channel H in mammalian CcO, that is absent in the bacterial oxidases.

Mitochondrial cytochrome c oxidase is inhibited by ATP only at very high ATP/ADP ratios

In the past, divergent results have been reported based on different methods and conditions used for enzymatic activity measurements of cytochrome c oxidase (CytOx). Here, we analyze in detail and show comparable and reproducible polarographic activity measurements of ATP-dependent inhibition of CytOx kinetics in intact and non-intact rat heart mitochondria and mitoplasts. We found that this mechanism is always present in isolated rat heart mitochondria and mitoplasts; however, it is measurable only at high ATP/ADP ratios using optimal protein concentrations. In the kinetics assay, measurement of this mechanism is independent of presence or absence of Tween-20 and the composition of measuring buffer. Furthermore, the effect of atractyloside on intact rat heart mitochondria confirms that (i) ATP inhibition occurs under uncoupled conditions [in the presence of carbonly cyanide m-chlorophenyl hydrazone (CCCP)] when the classical respiratory control is absent and (ii) high ATP/ADP ratios in the matrix as well as in the cytosolic space are required for full ATP inhibition of CytOx. Additionally, ATP inhibition measured in intact mitochondria extends in the presence of oligomycin, thus indicating further that the problem to measure the inhibitory effect of ATP on CytOx is apparently due to the lack of very high ATP/ADP ratios in isolated mitochondria.

Calcium ions inhibit reduction of heme a in bovine cytochrome c oxidase

The effect of Ca(2+) on the rate of heme a reduction by dithionite and hexaammineruthenium (RuAm) was studied in the cyanide-complexed bovine cytochrome oxidase (CcO). The rate of heme a reduction is proportional to RuAm concentration below 300 μM with kv of 0.53×10(6) M(-1) s(-1). Ca(2+) inhibits the rate of heme a reduction by dithionite by ∼25%. As the reaction speeds up with increased concentrations of RuAm, the inhibition by Ca(2+) disappears. The inhibition of heme a reduction may contribute to recently described partial inhibition of CcO by Ca(2+) in the enzymatic assays. The inhibitory effect of Ca(2+) on heme a reduction indicates that ET through heme a may be coupled to proton movement in the exit part of the proton channel H.

Reconstruction of absolute absorption spectrum of reduced heme a in cytochrome C oxidase from bovine heart

This paper presents a new experimental approach for determining the individual optical characteristics of reduced heme a in bovine heart cytochrome c oxidase starting from a small selective shift of the heme a absorption spectrum induced by calcium ions. The difference spectrum induced by Ca2+ corresponds actually to a first derivative (differential) of the heme a(2+) absolute absorption spectrum. Such an absolute spectrum was obtained for the mixed-valence cyanide complex of cytochrome oxidase (a(2+)a3(3+)-CN) and was subsequently used as a basis spectrum for further procession and modeling. The individual absorption spectrum of the reduced heme a in the Soret region was reconstructed as the integral of the difference spectrum induced by addition of Ca2+. The spectrum of heme a(2+) in the Soret region obtained in this way is characterized by a peak with a maximum at 447 nm and half-width of 17 nm and can be decomposed into two Gaussians with maxima at 442 and 451 nm and half-widths of ~10 nm (589 cm(-1)) corresponding to the perpendicularly oriented electronic π→π* transitions B0x and B0y in the porphyrin ring. The reconstructed spectrum in the Soret band differs significantly from the "classical" absorption spectrum of heme a(2+) originally described by Vanneste (Vanneste, W. H. (1966) Biochemistry, 65, 838-848). The differences indicate that the overall γ-band of heme a(2+) in cytochrome oxidase contains in addition to the B0x and B0y transitions extra components that are not sensitive to calcium ions, or, alternatively, that the Vanneste's spectrum of heme a(2+) contains significant contribution from heme a3(2+). The reconstructed absorption band of heme a(2+) in the α-band with maximum at 605 nm and half-width of 18 nm (850 cm(-1)) corresponds most likely to the individual Q0y transition of heme a, whereas the Q0x transition contributes only weakly to the spectrum.

Cation binding site of cytochrome c oxidase: progress report

Cytochrome c oxidase from bovine heart binds Ca(2+) reversibly at a specific Cation Binding Site located near the outer face of the mitochondrial membrane. Ca(2+) shifts the absorption spectrum of heme a, which allowed earlier the determination of the kinetic and equilibrium characteristics of the binding, and, as shown recently, the binding of calcium to the site inhibits cytochrome oxidase activity at low turnover rates of the enzyme [Vygodina, Т., Kirichenko, A., Konstantinov, A.A (2013). Direct Regulation of Cytochrome c Oxidase by Calcium Ions. PloS ONE 8, e74436]. This paper summarizes further progress in the studies of the Cation Binding Site in this group presenting the results to be reported at 18th EBEC Meeting in Lisbon, 2014. The paper revises specificity of the bovine oxidase Cation Binding Site for different cations, describes dependence of the Ca(2+)-induced inhibition on turnover rate of the enzyme and reports very high affinity binding of calcium with the "slow" form of cytochrome oxidase. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference. Guest Editors: Manuela Pereira and Miguel Teixeira.

Direct regulation of cytochrome c oxidase by calcium ions

Cytochrome c oxidase from bovine heart binds Ca²⁺ reversibly at a specific Cation Binding Site located near the outer face of the mitochondrial membrane. Ca²⁺ shifts the absorption spectrum of heme a, which allowed previously to determine the kinetics and equilibrium characteristics of the binding. However, no effect of Ca²⁺ on the functional characteristics of cytochrome oxidase was revealed earlier. Here we report that Ca²⁺ inhibits cytochrome oxidase activity of isolated bovine heart enzyme by 50–60% with K_i of ∼1 µM, close to K_d of calcium binding with the oxidase determined spectrophotometrically. The inhibition is observed only at low, but physiologically relevant, turnover rates of the enzyme (∼10 s⁻¹ or less). No inhibitory effect of Ca²⁺ is observed under conventional conditions of cytochrome c oxidase activity assays (turnover number >100 s⁻¹ at pH 8), which may explain why the effect was not noticed earlier. The inhibition is specific for Ca²⁺ and is reversed by EGTA. Na⁺ ions that compete with Ca²⁺ for binding with the Cation Binding Site, do not affect significantly activity of the enzyme but counteract the inhibitory effect of Ca²⁺. The Ca²⁺-induced inhibition of cytochrome c oxidase is observed also with the uncoupled mitochondria from several rat tissues. At the same time, calcium ions do not inhibit activity of the homologous bacterial cytochrome oxidases. Possible mechanisms of the inhibition are discussed as well as potential physiological role of Ca²⁺ binding with cytochrome oxidase. Ca²⁺- binding at the Cation Binding Site is proposed to inhibit proton-transfer through the exit part of the proton conducting pathway H in the mammalian oxidases.

Functions of the hydrophilic channels in protonmotive cytochrome c oxidase

The structures and functions of hydrophilic channels in electron-transferring membrane proteins are discussed. A distinction is made between proton channels that can conduct protons and dielectric channels that are non-conducting but can dielectrically polarize in response to the introduction of charge changes in buried functional centres. Functions of the K, D and H channels found in A1-type cytochrome c oxidases are reviewed in relation to these ideas. Possible control of function by dielectric channels and their evolutionary relation to proton channels is explored.

Multiple phosphorylations of cytochrome c oxidase and their functions

Cytochrome c oxidase (COX), the terminal enzyme of the mitochondrial electron transport chain, is regulated by isozyme expression, allosteric effectors such as the ATP/ADP ratio, and reversible phosphorylation. Of particular interest is the "allosteric ATP-inhibition," which has been hypothesized to keep the mitochondrial membrane potential at low healthy values (<140 mV), thus preventing the formation of superoxide radical anions, which have been implicated in multiple degenerative diseases. It has been proposed that the "allosteric ATP-inhibition" is switched on by the protein kinase A-dependent phosphorylation of COX. The goal of this study was to identify the phosphorylation site(s) involved in the "allosteric ATP-inhibition" of COX. We report the mass spectrometric identification of four new phosphorylation sites in bovine heart COX. The identified phosphorylation sites include Tyr-218 in subunit II, Ser-1 in subunit Va, Ser-2 in subunit Vb, and Ser-1 in subunit VIIc. With the exception of Ser-2 in subunit Vb, the identified phosphorylation sites were found in enzyme samples with and without "allosteric ATP inhibition," making Ser-2 of subunit Vb a candidate site enabling allosteric regulation. We therefore hypothesize that additional phosphorylation(s) may be required for the "allosteric ATP-inhibition," and that these sites may be easily dephosphorylated or difficult to identify by mass spectrometry.

Phosphorylation of mammalian cytochrome c and cytochrome c oxidase in the regulation of cell destiny: respiration, apoptosis, and human disease

The mitochondrial oxidative phosphorylation (OxPhos) system not only generates the vast majority of cellular energy, but is also involved in the generation of reactive oxygen species (ROS), and apoptosis. Cytochrome c (Cytc) and cytochrome c oxidase (COX) represent the terminal step of the electron transport chain (ETC), the proposed rate-limiting reaction in mammals. Cytc and COX show unique regulatory features including allosteric regulation, isoform expression, and regulation through cell signaling pathways. This chapter focuses on the latter and discusses all mapped phosphorylation sites based on the crystal structures of COX and Cytc. Several signaling pathways have been identified that target COX including protein kinase A and C, receptor tyrosine kinase, and inflammatory signaling. In addition, four phosphorylation sites have been mapped on Cytc with potentially large implications due to its multiple functions including apoptosis, a pathway that is overactive in stressed cells but inactive in cancer. The role of COX and Cytc phosphorylation is reviewed in a human disease context, including cancer, inflammation, sepsis, asthma, and ischemia/reperfusion injury as seen in myocardial infarction and ischemic stroke.

Functional importance of a pair of conserved glutamic acid residues and of Ca(2+) binding in the cbb(3)-type oxygen reductases from Rhodobacter sphaeroides and Vibrio cholerae

The cbb(3)-type cytochrome c oxidases are members of the family of heme-copper proton pumping respiratory oxygen reductases. The structure of the cbb(3)-type oxidase from Pseudomonas stutzeri reveals that, in addition to the six redox-active metal centers (two b-type hemes, three c-type hemes, and Cu(B)), the enzyme also contains at least one Ca(2+). The calcium bridges two propionate carboxyls at the interface between the low-spin heme b and the active-site heme b(3) and, in addition, is ligated to a serine in subunit CcoO and by a glutamate in subunit CcoN. The glutamate that is ligated to Ca(2+) is one of a pair of glutamic acid residues that has previously been suggested to be part of a proton exit pathway for pumped protons. In this work, mutations of these glutamates are investigated in the cbb(3)-type oxidases from Vibrio cholerae and Rhodobacter sphaeroides. Metal analysis shows that each of these wild-type enzymes contains Ca(2+). Mutations of the glutamate expected to ligate the Ca(2+) in each of these enzymes (E126 in V. cholerae and E180 in R. sphaeroides) result in a loss of activity as well as a loss of Ca(2+). Mutations of the nearby glutamate (E129 in V. cholerae and E183 in R. sphaeroides) also resulted in a loss of oxidase activity and a loss of Ca(2+). It is concluded that the Ca(2+) is essential for assembly of the fully functional enzyme and that neither of the glutamates is likely to be part of a pathway for pumped protons within the cbb(3)-type oxygen reductases. A more likely role for these glutamates is the maintenance of the structural integrity of the active conformation of the enzyme.

Regulation of ATP production by mitochondrial Ca(2+)

Stimulation of mitochondrial oxidative metabolism by Ca(2+) is now generally recognised as important for the control of cellular ATP homeostasis. Here, we review the mechanisms through which Ca(2+) regulates mitochondrial ATP synthesis. We focus on cardiac myocytes and pancreatic β-cells, where tight control of this process is likely to play an important role in the response to rapid changes in workload and to nutrient stimulation, respectively. We also describe a novel approach for imaging the Ca(2+)-dependent regulation of ATP levels dynamically in single cells.

Role of mitochondrial Ca2+ in the regulation of cellular energetics

Calcium is an important signaling molecule involved in the regulation of many cellular functions. The large free energy in the Ca(2+) ion membrane gradients makes Ca(2+) signaling inherently sensitive to the available cellular free energy, primarily in the form of ATP. In addition, Ca(2+) regulates many cellular ATP-consuming reactions such as muscle contraction, exocytosis, biosynthesis, and neuronal signaling. Thus, Ca(2+) becomes a logical candidate as a signaling molecule for modulating ATP hydrolysis and synthesis during changes in numerous forms of cellular work. Mitochondria are the primary source of aerobic energy production in mammalian cells and also maintain a large Ca(2+) gradient across their inner membrane, providing a signaling potential for this molecule. The demonstrated link between cytosolic and mitochondrial Ca(2+) concentrations, identification of transport mechanisms, and the proximity of mitochondria to Ca(2+) release sites further supports the notion that Ca(2+) can be an important signaling molecule in the energy metabolism interplay of the cytosol with the mitochondria. Here we review sites within the mitochondria where Ca(2+) plays a role in the regulation of ATP generation and potentially contributes to the orchestration of cellular metabolic homeostasis. Early work on isolated enzymes pointed to several matrix dehydrogenases that are stimulated by Ca(2+), which were confirmed in the intact mitochondrion as well as cellular and in vivo systems. However, studies in these intact systems suggested a more expansive influence of Ca(2+) on mitochondrial energy conversion. Numerous noninvasive approaches monitoring NADH, mitochondrial membrane potential, oxygen consumption, and workloads suggest significant effects of Ca(2+) on other elements of NADH generation as well as downstream elements of oxidative phosphorylation, including the F(1)F(O)-ATPase and the cytochrome chain. These other potential elements of Ca(2+) modification of mitochondrial energy conversion will be the focus of this review. Though most specific molecular mechanisms have yet to be elucidated, it is clear that Ca(2+) provides a balanced activation of mitochondrial energy metabolism that exceeds the alteration of dehydrogenases alone.

Regulation of mitochondrial respiration and apoptosis through cell signaling: cytochrome c oxidase and cytochrome c in ischemia/reperfusion injury and inflammation

Cytochrome c (Cytc) and cytochrome c oxidase (COX) catalyze the terminal reaction of the mitochondrial electron transport chain (ETC), the reduction of oxygen to water. This irreversible step is highly regulated, as indicated by the presence of tissue-specific and developmentally expressed isoforms, allosteric regulation, and reversible phosphorylations, which are found in both Cytc and COX. The crucial role of the ETC in health and disease is obvious since it, together with ATP synthase, provides the vast majority of cellular energy, which drives all cellular processes. However, under conditions of stress, the ETC generates reactive oxygen species (ROS), which cause cell damage and trigger death processes. We here discuss current knowledge of the regulation of Cytc and COX with a focus on cell signaling pathways, including cAMP/protein kinase A and tyrosine kinase signaling. Based on the crystal structures we highlight all identified phosphorylation sites on Cytc and COX, and we present a new phosphorylation site, Ser126 on COX subunit II. We conclude with a model that links cell signaling with the phosphorylation state of Cytc and COX. This in turn regulates their enzymatic activities, the mitochondrial membrane potential, and the production of ATP and ROS. Our model is discussed through two distinct human pathologies, acute inflammation as seen in sepsis, where phosphorylation leads to strong COX inhibition followed by energy depletion, and ischemia/reperfusion injury, where hyperactive ETC complexes generate pathologically high mitochondrial membrane potentials, leading to excessive ROS production. Although operating at opposite poles of the ETC activity spectrum, both conditions can lead to cell death through energy deprivation or ROS-triggered apoptosis.

Cytochrome c oxidase and nitric oxide in action: molecular mechanisms and pathophysiological implications

BACKGROUND

The reactions between Complex IV (cytochrome c oxidase, CcOX) and nitric oxide (NO) were described in the early 60's. The perception, however, that NO could be responsible for physiological or pathological effects, including those on mitochondria, lags behind the 80's, when the identity of the endothelial derived relaxing factor (EDRF) and NO synthesis by the NO synthases were discovered. NO controls mitochondrial respiration, and cytotoxic as well as cytoprotective effects have been described. The depression of OXPHOS ATP synthesis has been observed, attributed to the inhibition of mitochondrial Complex I and IV particularly, found responsible of major effects.

SCOPE OF REVIEW

The review is focused on CcOX and NO with some hints about pathophysiological implications. The reactions of interest are reviewed, with special attention to the molecular mechanisms underlying the effects of NO observed on cytochrome c oxidase, particularly during turnover with oxygen and reductants.

MAJOR CONCLUSIONS AND GENERAL SIGNIFICANCE

The NO inhibition of CcOX is rapid and reversible and may occur in competition with oxygen. Inhibition takes place following two pathways leading to formation of either a relatively stable nitrosyl-derivative (CcOX-NO) of the enzyme reduced, or a more labile nitrite-derivative (CcOX-NO(2)(-)) of the enzyme oxidized, and during turnover. The pathway that prevails depends on the turnover conditions and concentration of NO and physiological substrates, cytochrome c and O(2). All evidence suggests that these parameters are crucial in determining the CcOX vs NO reaction pathway prevailing in vivo, with interesting physiological and pathological consequences for cells.

The role of Ca(2+) signaling in the coordination of mitochondrial ATP production with cardiac work

The heart is capable of balancing the rate of mitochondrial ATP production with utilization continuously over a wide range of activity. This results in a constant phosphorylation potential despite a large change in metabolite turnover. The molecular mechanisms responsible for generating this energy homeostasis are poorly understood. The best candidate for a cytosolic signaling molecule reflecting ATP hydrolysis is Ca(2+). Since Ca(2+) initiates and powers muscle contraction as well as serves as the primary substrate for SERCA, Ca(2+) is an ideal feed-forward signal for priming ATP production. With the sarcoplasmic reticulum to cytosolic Ca(2+) gradient near equilibrium with the free energy of ATP, cytosolic Ca(2+) release is exquisitely sensitive to the cellular energy state providing a feedback signal. Thus, Ca(2+) can serve as a feed-forward and feedback regulator of ATP production. Consistent with this notion is the correlation of cytosolic and mitochondrial Ca(2+) with work in numerous preparations as well as the localization of mitochondria near Ca(2+) release sites. How cytosolic Ca(2+) signaling might regulate oxidative phosphorylation is a focus of this review. The relevant Ca(2+) sensitive sites include several dehydrogenases and substrate transporters together with a post-translational modification of F1-FO-ATPase and cytochrome oxidase. Thus, Ca(2+) apparently activates both the generation of the mitochondrial membrane potential as well as utilization to produce ATP. This balanced activation extends the energy homeostasis observed in the cytosol into the mitochondria matrix in the never resting heart.

Carboxyl group functions in the heme-copper oxidases: information from mid-IR vibrational spectroscopy

Carboxyl groups of possible functional importance in bovine and bacterial cytochrome c oxidases (CcO) are reviewed and assessed. A critical analysis is presented of available mid-infrared vibrational data that pertain to these functional carboxyl groups. These data and their interpretations are discussed in relation to current models of the mechanism of proton and electron coupling in the protonmotive CcO superfamily.

Proton pumping mechanism of bovine heart cytochrome c oxidase

X-ray structures of bovine heart cytochrome c oxidase at 1.8/1.9 A resolution in the oxidized/reduced states exhibit a redox coupled conformational change of an aspartate located near the intermembrane surface of the enzyme. The alteration of the microenvironment of the carboxyl group of this aspartate residue indicates the occurrence of deprotonation upon reduction of the enzyme. The residue is connected with the matrix surface of the enzyme by a hydrogen-bond network that includes heme a via its propionate and formyl groups. These X-ray structures provide evidence that proton pumping occurs through the hydrogen bond network and is driven by the low spin heme. The function of the aspartate is confirmed by mutation of the aspartate to asparagine. Although the amino acid residues of the hydrogen bond network and the structures of the low spin heme peripheral groups are not completely conserved amongst members of the heme-copper terminal oxidase superfamily, the existence of low spin heme and the hydrogen bond network suggests that the low spin heme provides the driving element of the proton-pumping process.

Reaction mechanism of bovine heart cytochrome c oxidase

The 1.9 A resolution X-ray structure of the O2 reduction site of bovine heart cytochrome c oxidase in the fully reduced state indicates trigonal planar coordination of CuB by three histidine residues. One of the three histidine residues has a covalent link to a tyrosine residue to ensure retention of the tyrosine at the O2 reduction site. These moieties facilitate a four electron reduction of O2, and prevent formation of active oxygen species. The combination of a redox-coupled conformational change of an aspartate residue (Asp51) located near the intermembrane surface of the enzyme molecule and the existence of a hydrogen bond network connecting Asp51 to the matrix surface suggest that the proton-pumping process is mediated at Asp51. Mutation analyses using a gene expression system of the Asp51-containing enzyme subunit yield results in support of the proposal that Asp51 plays a critical role in the proton pumping process.