Methanol dissimilation in Xanthobacter H4-14: activities, induction and comparison to Pseudomonas AM1 and Paracoccus denitrificans

Methanol dissimilatory enzymes detected in the methanol autotroph Xanthobacter H4-14 were a typical phenazine methosulphate-linked methanol dehydrogenase, a NAD+-linked formate dehydrogenase, and a dye-linked formaldehyde dehydrogenase that could be assayed only by activity stains of polyacrylamide gels. This same methanol dehydrogenase activity was found in ethanol-grown cells and was apparently utilized for ethanol oxidation. Formaldehyde dehydrogenase activities were investigated in Paracoccus denitrificans, Xanthobacter H4-14, and Pseudomonas AM1. P. denitrificans contained a previously reported NAD+-linked, GSH-dependent activity, but both Xanthobacter H4-14 and Pseudomonas AM1 contained numerous activities detected by activity stains of polyacrylamide gels. Induction studies showed that in Xanthobacter H4-14, a 10 kDal polypeptide, probably a dehydrogenase-associated cytochrome c, was co-induced with methanol dehydrogenase, but the formaldehyde and formate dehydrogenases were not co-regulated. Analogous induction experiments revealed similar patterns in P. denitrificans, but no evidence for co-regulation of dissimilatory activities in Pseudomonas AM1.     

Acetyl-coenzyme A deacylase activity in liver is not an artifact. Subcellular distribution and substrate specificity of acetyl-coenzyme A deacylase activities in rat liver

The stoicheiometry of proton translocation, the amounts of cytochromes firmly bound to membranes, and cell yields with respect to succinate and O(2) have been measured in the facultative methylotroph Pseudomonas AM1 and in the mutant lacking cytochrome c (mutant PCT76) during carbon-limited growth and carbon-excess growth. -->H(+)/O ratios during endogenous respiration of about 4 were measured in wild-type bacteria grown in carbon-excess conditions, and in the mutant in all growth conditions. During methanol- or succinate-limited growth of wild-type bacteria the -->H(+)/O ratio increased to about 6. Cell yields with respect to succinate and O(2) were higher in wild-type than in the mutant lacking cytochrome c by an amount suggesting loss in the mutant of 30% of the ATP-generating capacity of wild-type bacteria. During carbon-limited growth on methanol or succinate some cytochrome c was tightly bound to bacterial membranes, whereas none was tightly bound in bacteria grown in batch-culture or in NH(4) (+)-limited conditions. It is proposed that the role of cytochrome c in Pseudomonas AM1 depends on growth conditions and hence on the ;needs' of the bacteria. During growth in carbon-excess conditions it is only required for methanol oxidation, mediating between methanol dehydrogenase and cytochrome a/a(3). In these conditions oxidation of NADH and succinate by way of cytochrome b and cytochrome a/a(3) occurs without the mediation of cytochrome c. This is the only route for oxidation of NADH and succinate in the cytochrome c-deficient mutant in all growth conditions. During carbon-limited growth the cytochrome c becomes bound to the membrane in such a way that it can mediate between cytochromes b and a/a(3), hence becoming involved in proton translocation and ATP synthesis during NADH and succinate oxidation. An alternative possibility is that in wild-type bacteria the cytochrome c is always involved in electron transport, but that its involvement in measurable proton translocation only occurs in carbon-limited conditions.     

The microbial metabolism of C1 compounds. The electron-transport chain of Pseudomonas AM1

Pseudomonas AM1, Hyphomicrobium X and Pseudomonas MS all contain cytochrome a/a(3) and a b-type cytochrome able to react with CO. Pseudomonas AM1 and Hyphomicrobium X also have a CO-binding cytochrome c. The purified cytochrome c (redox potential 0.26V) of Pseudomonas AM1 was not susceptible to oxidation by molecular oxygen. CO reacted slowly with the reduced form giving a CO difference spectrum with a peak at 412nm and troughs at 420nm and 550nm. Similar results were obtained with the cytochrome c of Hyphomicrobium (aerobically grown or anaerobically grown with nitrate) and with that of Pseudomonas extorquens. The results given in the present paper are incompatible with an oxygenase or oxidase function for the soluble cytochrome c of methylotrophs. Studies with whole cells of Pseudomonas AM1 and a cytochrome c-deficient mutant have demonstrated that cytochrome b (redox potential 0.009V) is the first cytochrome in the electron-transport chain for oxidation of all substrates except methanol (and ethanol) whose oxidation does not involve this cytochrome. All substrates are usually oxidized by way of cytochrome c and cytochrome oxidase (cytochrome a/a(3)), but there is an alternative route for the reduction of cytochrome a/a(3) in the mutant lacking cytochrome c. Results of experiments on cyanide inhibition of respiration and cytochrome oxidation support the suggestion that the susceptibility of cytochrome b to oxidation by molecular oxygen (reflected in its ability to react with CO) is probably irrelevant to the normal physiology of Pseudomonas AM1.     

The microbial metabolism of C1 compounds. The stoicheiometry of respiration-driven proton translocation in Pseudomonas AM1 and in a mutant lacking cytochrome c

This paper clarifies the role of cytochrome c in Pseudomonas AM1 by measuring the stoicheiometry of proton translocation driven by respiration of endogenous or added substrates in wild-type bacteria and in a mutant lacking cytochrome c (mutant PCT76). The maximum -->H(+)/O ratio (protons translocated out of the bacteria per atom of oxygen consumed during respiration) was about 4 and, except when respiration was markedly affected, this ratio was similar in mutant and wild-type bacteria. The -->H(+)/O ratios were unaltered when the usual oxidase (cytochrome a(3)) was inhibited by 300mum-KCN and respiration involved the single cytochrome b functioning as an alternative oxidase. Ratios measured in cells respiring endogenous substrate and in cells loaded with malate or 3-hydroxybutyrate suggest that there are two proton-translocating segments operating during the oxidation of NADH. By contrast, during oxidation of formaldehyde or methylamine only one pair of protons is translocated. Proton translocation could not be measured with methanol as substrate, because its oxidation was inhibited (90-95%) by 5mm-KSCN. It is tentatively proposed that the electron-transport chain for NADH oxidation in Pseudomonas AM1 is arranged such that the NADH-ubiquinone oxidoreductase forms one proton-translocating segment and the second segment consists of ubiquinone and cytochromes b and a/a(3). The cytochrome c appears to be essential only for respiration and proton translocation from methanol (and possibly from methylamine); there is no conclusive evidence that cytochrome c ever mediates between cytochromes b and a/a(3) in Pseudomonas AM1.     

The microbial metabolism of C1 compounds. The cytochromes of Pseudomaonas AM1.

Pseudomonas AM1 contains cytochromes a, b and c and more than one CO-binding pigment (cytochrome a3, cytochrome c and possibly a cytochrome o). The soluble cytochrome c has been purified; its isoelectric point is low and its molecular weight is 20000. This cytochrome is reduced in whole bacteria by all oxidizable substrates at rates determined by the primary dehydrogenases. A mutant lacking cytochrome c oxidizes all substrates except methanol, ethanol and methylamine; these no longer support growth. The role of cytochrome c in electron transport in Pseudomonas AM1 is discussed.     

A comparative survey of several bacterial aa3-type cytochrome c oxidases

The aa3-type cytochrome c oxidases purified from Nitrobacter agilis, Thiobacillus novellus, Nitrosomonas europaea, and Pseudomonas AM 1 were compared. They have haem a and copper atom as the prosthertic groups and show alpha and gamma absorption peaks at around 600 and 440 nm, respectively. Each oxidase molecule is composed of two kinds of subunits. The N. agilis oxidase has 2 moles of haem a and 2 atoms of copper in the minimal structural unit composed of one molecule each of the two kinds of subunits, while the T. novellus enzyme seems to contain one molecule of the haem and one atom of the metal in the unit. The N. europaea oxidase shows very low affinity for carbon monoxide. Each oxidase reacts rapidly with some eukaryotic cytochromes c as well as with its native cytochrome c. The cytochrome c oxidase activity of the N. agilis oxidase is 50% inhibited by 1 microM KCN, while 50% inhibition of the activity requires 100 microM KCN in the case of the N. europaea enzyme.     

The nucleotide sequence and deduced amino acid sequence of the cytochrome cL gene of Methylobacterium extorquens AM1, a novel class of c-type cytochrome.

The nucleotide sequence and deduced amino acid sequence of the cytochrome cL of Methylobacterium extorquens (Pseudomonas AM1; Methylobacterium AM1) shows that this cytochrome c is completely different, except for its haem-binding site, from all other cytochromes.     

D-lactate oxidation and generation of the proton electrochemical gradient in membrane vesicles from Escherichia coli GR19N and in proteoliposomes reconstituted with purified D-lactate dehydrogenase and cytochrome o oxidase

The respiratory chain in the cytochrome d deficient mutant Escherichia coli GR19N is a relatively simple, linear system consisting of primary dehydrogenases, ubiquinone 8, cytochrome b-556, and cytochrome o oxidase. By use of right-side-out and inside-out membrane vesicles from this strain, various oxidase activities and the generation of the H+ electrochemical gradient were studied. Oxidation of ubiquinol 1 or N,N,-N',N'-tetramethyl-p-phenylenediamine, which donate electrons directly to the terminal oxidase, generates a H+ electrochemical gradient comparable to that observed during D-lactate oxidation. In contrast, D-lactate/ubiquinone 1 or D-lactate/ferricyanide oxidoreductase activity does not appear to generate a membrane potential, suggesting that electron flow from D-lactate dehydrogenase to ubiquinone is not electrogenic. Moreover, proteoliposomes reconstituted with purified D-lactate dehydrogenase, ubiquinone 8, and purified cytochrome o catalyze D-lactate and ubiquinol 1 oxidation and generate a H+ electrochemical gradient similar to that observed in membrane vesicles. Strikingly, in inside-out vesicles, NADH oxidation generates a H+ electrochemical gradient that is very significantly greater than that produced by either D-lactate or ubiquinol 1; furthermore, NADH/ubiquinone 1 and NADH/ferricyanide oxidoreductase activities are electrogenic. It is suggested that the only component between D-lactate dehydrogenase or ubiquinol and oxygen in GR19N membranes that is directly involved in the generation of the H+ electrochemical gradient is cytochrome o, which functions as a "half-loop" (i.e., the oxidase catalyzes the scalar release of 2 H+ from ubiquinol on the outer surface of the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)     

Measurement of proton pump activity of the thermophilic bacterium PS3 and Nitrobacter agilis at the cytochrome oxidase level using total membrane and heptyl thioglucoside

It is possible to prepare liposomal vesicles by solubilization of total bacterial membranes with n-heptyl beta-D-thioglucoside followed by reconstitution into proteoliposomes by a freeze-thaw-sonication procedure with soybean phospholipids. The resulting proteoliposomes from total membrane fraction of sufficiently aerated cells of the thermophilic bacterium PS3 containing cytochrome aa3 showed a reasonable H+ pumping activity upon addition of reduced cytochrome c. On the other hand, the proteoliposomes reconstituted from air-limited PS3 cells containing cytochrome o and those from Nitrobacter agilis cells containing cytochrome aa3 did not show H+ pumping upon addition of reduced cytochrome c, although the vesicles showed "respiratory control"; 3-4-fold stimulation of oxygen consumption took place upon addition of an uncoupler. In proteoliposomes prepared from PS3 membranes by this method, H+-translocating ATPase (F0 X F1) was successfully reconstituted as well, suggesting that this method has wide applicability for investigation of enzymes catalyzing transmembrane processes.     

One of two copper atoms is not necessary for the cytochrome c oxidase activity of Pseudomonas AM 1 cytochrome aa3

From Pseudomonas AM 1 grown in a medium deficient in Cu, aa3-type cytochrome c oxidase was purified which contained 2 molecules of haem a and one atom of Cu per molecule. The enzyme showed absorption peaks at 428 and 595 nm in the oxidized form and at 442 and 604 nm in the reduced form, and its CO complex showed peaks at 432 and 602 nm. The enzyme in the oxidized state showed an obscure absorption peak around 800 nm instead of a peak at 820 nm. One mol of the enzyme oxidized maximally 76, 75, and 98 mol of the ferrocytochromes c of Candida krusei, horse and Pseudomonas AM 1 per sec, respectively. These reactions were 50% inhibited by 7 microM KCN. The product of reduction of O2 catalyzed by the enzyme was concluded to be H2O on the basis of the ratio of ferrocytochrome c oxidized to O2 consumed.     

The nitrite oxidizing system of Nitrobacter winogradskyi

Cytochrome components which participate in the oxidation of nitrite in Nitrobacter winogradskyi have been highly purified and their properties studied in detail. Cytochrome a1c1 is an iron-sulphur molybdoenzyme which has haems a and c and acts as a nitrite-cytochrome c oxidoreductase. Cytochrome c-550 is homologous to eukaryotic cytochrome c and acts as the electron mediator between cytochrome a1c1 and aa3-type cytochrome c oxidase. The oxidase is composed of two kinds of subunits, has two molecules of haem a and two atoms of copper in the molecule, and oxidizes actively eukaryotic ferrocytochrome c as well as its own ferrocytochrome c-550. Further, a flavoenzyme has been obtained which has transhydrogenase activity and catalyses reduction of NADP+ with benzylviologen radical. This enzyme may be responsible for production of NADPH in N. winogradskyi. The electron transfer against redox potential from NO2- to cytochrome c could be pushed through prompt removal by cytochrome aa3 of H+ formed by the dehydrogenation of NO2- + H2O. As cytochrome c in anaerobically kept cell-free extracts is rapidly reduced on addition of NO2-, a membrane potential does not seem necessary for the reduction of cytochrome c by cytochrome a1c1 with NO2- in vivo.     

The methanol-oxidizing system of Methylobacterium extorquens AM1 reconstituted with purified constituents

The electron transport system (with cytochrome aa3) coupled to the oxidation of methanol in Methylobacterium extorquens AM1 (former Pseudomonas AM1) was reconstituted with highly purified constituents of the system. A mixture of 2.7 microM methanol dehydrogenase, 3.2 microM cytochrome cH, and 71 nM cytochrome c oxidase (= cytochrome aa3) consumed oxygen at a lower rate in the presence of methanol, while its activity was enhanced 3-fold by the addition of 1.4 microM cytochrome cL (74 mol of O2 consumed/mol of heme a of cytochrome c oxidase per min). Further addition of amicyanin to the above mixture did not affect the activity. Although ammonium ion greatly activated the activity of methanol dehydrogenase, the ion had little effect on the oxygen consumption activity of the above mixture. On the basis of the results obtained in the present study, an electron transport system is proposed for the oxidation of methanol in M. extorquens AM1.     

Assignment of the 1H and 15N NMR spectra of Rhodobacter capsulatus ferrocytochrome c2

The peptide resonances of the 1H and 15N nuclear magnetic resonance spectra of ferrocytochrome c2 from Rhodobacter capsulatus are sequentially assigned by a combination of 2D 1H-1H and 1H-15N spectroscopy, the latter performed on 15N-enriched protein. Short-range nuclear Overhauser effect (NOE) data show alpha-helices from residues 3-17, 55-65, 69-88, and 103-115. Within the latter two alpha-helices, there are three single 3(10) turns, 70-72, 76-78, and 107-109. In addition alpha H-NHi+1 and alpha H-NHi+2 NOEs indicate that the N-terminal helix (3-17) is distorted. Compared to horse or tuna cytochrome c and cytochrome c2 of Rhodospirillium rubrum, there is a 6-residue insertion at residues 23-29 in R. capsulatus cytochrome c2. The NOE data show that this insertion forms a loop, probably an omega loop. 1H-15N heteronuclear multiple quantum correlation experiments are used to follow NH exchange over a period of 40 h. As the 2D spectra are acquired in short time periods (30 min), rates for intermediate exchanging protons can be measured. Comparison of the NH exchange data for the N-terminal helix of cytochrome c2 of R. capsulatus with the highly homologous horse heart cytochrome c [Wand, A. J., Roder, H., & Englander, S. W. (1986) Biochemistry 25, 1107-1114] shows that this helix is less stable in cytochrome c2.     

Hydrogen-oxidizing electron transport components in nitrogen-fixing Azotobacter vinelandii.

Membranes from N2-fixing Azotobacter vinelandii were isolated to identify electron transport components involved in H2 oxidation. We found direct evidence for the involvement of cytochromes b, c, and d in H2 oxidation by the use of H2-reduced minus O2-oxidized absorption difference spectra. Carbon monoxide spectra showed that H2 reduced cytochrome d but not cytochrome o. Inhibition of H2 oxidation by cyanide was monophasic with a high Ki (135 microM); this was attributed to cytochrome d. Cyanide inhibition of malate oxidation showed the presence of an additional, low Ki (0.1 microM cyanide) component in the membranes; this was attributed to cytochrome o. However, H2 oxidation was not sensitive to this cyanide concentration. Chlorpromazine (at 160 microM) markedly inhibited malate oxidation, but it did not greatly inhibit H2 oxidation. Irradiation of membranes with UV light inhibited H2 oxidation. Adding A. vinelandii Q8 to the UV-damaged membranes partially restored H2 oxidation activity, whereas addition of UV-treated Q8 did not increase the activity. 2-n-Heptyl-4-hydroxyquinoline-N-oxide inhibited both H2 and malate oxidation.     

The methylamine oxidizing system of Pseudomonas AM1 reconstituted with purified components

The electron transport system coupled to the oxidation of methylamine in Pseudomonas AM1 was investigated by reconstituting it from the highly purified components. A mixture of methylamine dehydrogenase, cytochrome cH and cytochrome c oxidase (= cytochrome aa3) actively oxidized methylamine (161 mol of O2 consumed/mol of heme a of cytochrome c oxidase X min). In this system, addition of amicyanin did not affect the oxygen consumption rate. The oxygen consumption rate of the cell-free extract prepared from the cells cultivated in a copper-deficient medium was directly proportional to the amount of amicyanin added, and extrapolation to zero copper concentration gave a value of 28 mol of O2 consumed/mol of heme a of cytochrome c oxidase X min. These results suggest that methylamine oxidation in the bacterium can occur at least to some extent without participation of amicyanin.     

The role of cytochrome c4 in bacterial respiration. Cellular location and selective removal from membranes.

The cellular location of cytochrome c4 in Pseudomonas stutzeri and Azotobacter vinelandii was investigated by the production of spheroplasts. Soluble cytochrome c4 was found to be located in the periplasm in both organisms. The remaining cytochrome c4 was membrane-bound. The orientation of this membrane-bound cytochrome c4 fraction was investigated by proteolysis of the cytochrome on intact spheroplasts. In P. stutzeri, 78% of the membrane-bound cytochrome c4 could be proteolysed, whilst 82% of the spheroplasts remained intact, suggesting that the membrane-bound cytochrome c4 is on the periplasmic face of the membrane in this organism. Cytochrome c4 was not susceptible to proteolysis on A. vinelandii spheroplasts, in spite of being digestible in the purified state. Cytochrome c5 was shown to have a similar cellular distribution to cytochrome c4. Selective removal of cytochrome c4 from membranes of P. stutzeri was accomplished by the use of sodium iodide and propan-2-ol, with the retention of most of the ascorbate-TMPD (NNN'N'-tetramethylbenzene-1,4-diamine) oxidase activity associated with the membrane. Sodium iodide removed most of the cytochrome c4 from A. vinelandii membranes with retention of 62% of the ascorbate-TMPD oxidase activity. Cytochrome c4 could be returned to the washed membranes, but with no recovery of this enzyme activity. We conclude that cytochrome c4 is not involved in the ascorbate-TMPD oxidase activity associated with the membranes of these two organisms.     

Cytochrome c oxidase of Pseudomonas AM 1: purification, and molecular and enzymatic properties

Cytochrome c oxidase (cytochrome aa3-type) [EC 1.9.3.1] was purified from Pseudomonas AM 1 to an electrophoretically homogeneous state and some of its properties were studied. The oxidase showed absorption peaks at 428 and 598 nm in the oxidized form, and at 442 and 604 nm in the reduced form. The CO compound of the reduced enzyme showed peaks at 432 and 602 nm. The enzyme molecule was composed of two kinds of subunits with molecular weights of 50,000 and 30,000 and it contained equimolar amounts of heme a and copper atom. The enzyme rapidly oxidized Candida krusei and horse ferrocytochromes c as well as Pseudomonas AM 1 ferrocytochrome c. The reactions catalyzed by the enzyme were strongly inhibited by KCN.     

线粒体大麻素受体1在大鼠海马神经元缺氧复氧损伤中对线粒体分裂的影响

目的:探讨线粒体CB1受体(mitochondrial cannabinoid receptor1,mtCB1)在大鼠海马神经元缺氧复氧损伤中对线粒体分裂的影响。方法:原代培养新生的Wistar大鼠海马神经元,将培养至第8天的海马神经元采用随机数字表分为5组(n=60):正常组(N组):正常培养,不做任何处理;缺氧复氧组(H/R组):采用氧糖剥夺法构建海马神经元缺氧复氧损伤模型,缺氧6h,复氧20 h;缺氧复氧组+ACEA+AM251组(H/R+ACEA+AM251组):缺氧6 h结束后立即加入ACEA和AM251,终浓度分别为1μmol/L、10μmol/L,复氧20 h;缺氧复氧+ACEA+Hemopressin(H/R+ACEA+Hemo组):缺氧6h结束后立即加入ACEA和Hemopressin,终浓度分别为1μmol/L、10μmol/L,复氧20 h;缺氧复氧+赋形剂组(H/R+V组):同样于缺氧6h结束后立即加入二甲基亚砜(DMSO),终浓度0.1%,复氧20 h。使用激光共聚焦显微镜检测细胞内Ca~(2+)的浓度,流式细胞仪检测细胞凋亡率,Western blot检测凋亡诱导因子(AIF)、线粒体分裂相关蛋白Drp1、Fis1,细胞凋亡相关蛋白细胞色素C(Cytc)和Rho相关的卷曲蛋白激酶1(ROCK1)的表达。结果:与N组相比,H/R组、H/R+ACEA+AM251组、H/R+ACEA+Hemo组和H/R+V组的细胞内Ca~(2+)浓度、细胞凋亡率、以及AIF、Drp1、Fis1、Cytc、ROCK1蛋白的表达水平均明显增加(P0.05);与H/R组相比,H/R+ACEA+Hem组上述各检测指标明显降低(P0.05),H/R+ACEA+AM251组和H/R+V组各指标比较差异无统计学意义(P0.05)。结论:线粒体CB1受体(mtCB1受体)可能通过降低细胞内ROS的含量来减少细胞内Ca~(2+)浓度和ROCK1的表达,进而抑制线粒体分裂,并最终减轻海马神经元缺氧复氧损伤。     

Electron transfer between azurin from Alcaligenes faecalis and cytochrome c551 from Pseudomonas aeruginosa

The electron transfer equilibrium and kinetics between azurin from Alcaligenes faecalis and cytochrome c551 from Pseudomonas aeruginosa have been studied. The equilibrium constant K = ([Cyt(III)] . [Az(I)])/([Cyt(II)] . [Az(II))]) = 0.5 at 25 degrees C is about seven times smaller than that observed between the cytochrome c551 and the titrations confirmed a 43-mV difference between the mid-point potentials of +266 mV and +309 mV for the Alcaligenes and Pseudomonas azurins respectively. The kinetics of the reaction between Alcaligenes azurin and Pseudomonas cytochrome c551 were investigated by the temperature-jump chemical relaxation method. Only a single relaxation mode was observed throughout the range of concentrations and temperatures examined. Thus, the slow relaxation time observed in the reaction between P. aeruginosa azurin and cytochrome c551 is not observed with the Alcaligenes azurin. The simplest mechanism that can therefore be ascribed to the investigated system is: [formula: see text]. This scheme is similar to that proposed earlier for the reaction between P. aeruginosa azurin and cytochrome c551 but does not involve the conformational transition proposed for azurin. The specific rates for the electron transfer are still fast: 1.8 x 10(6) M-1 . s-1 and 3.0 x 10(6) M-1 . s-1 respectively at 25 degrees C.     

Resonance Raman evidence for low-spin Fe2+ heme a3 in energized cytochrome c oxidase: implications for the inhibition of O2 reduction

Resonance Raman (RR) spectra are reported for reduced submitochondrial particles (SMP) with excitation at 441.6 nm, where Raman bands of the cytochrome c oxidase heme a groups are selectively enhanced. Addition of ATP to energize the membranes induces the formation of a new band at 1644 cm-1 and partial loss of intensity in a band at 1567 cm-1. These changes are modeled by adding cyanide to reduced cytochrome c oxidase and are attributed to partial conversion of cytochrome (cyt) a3 from a high-spin to a low-spin state. This conversion is abolished by addition of excess oligomycin, an ATPase inhibitor, or FCCP, an uncoupler of proton translocation, and is reversed when the ATP is consumed. The observed spin-state conversion is attributed to the binding of an endogenous ligand to the cyt a3 Fe atom. This ligation is suggested to be induced by a local increase in pH and/or by a global conformation change associated with the generation of a transmembrane potential. Since O2 binding requires a vacant coordination site at cyt a3, the ligation of this site must retard O2 reduction and could thus provide a simple mechanism for energy-linked regulation of respiration. No changes in the RR spectrum were observed upon adding Ca2+ or H+ to reduced cytochrome c oxidase. The cyt a3 spin-state change associated with membrane energization is unrelated to the cyt a absorption red shift induced by adding Ca2+ or H+ to cytochrome c oxidase.