Transport of anions and protons by the mitochondrial uncoupling protein and its regulation by nucleotides and fatty acids. A new look at old hypotheses

Coenzyme Q is an obligatory cofactor for uncoupling protein function

Uncoupling proteins (UCPs) are thought to be intricately controlled uncouplers that are responsible for the futile dissipation of mitochondrial chemiosmotic gradients, producing heat rather than ATP. They occur in many animal and plant cells and form a subfamily of the mitochondrial carrier family. Physiological uncoupling of oxidative phosphorylation must be strongly regulated to avoid deterioration of the energy supply and cell death, which is caused by toxic uncouplers. However, an H+ transporting uncoupling function is well established only for UCP1 from brown adipose tissue, and the regulation of UCP1 by fatty acids, nucleotides and pH remains controversial. The failure of UCP1 expressed in Escherichia coli inclusion bodies to carry out fatty-acid-dependent H+ transport activity inclusion bodies made us seek a native UCP cofactor. Here we report the identification of coenzyme Q (ubiquinone) as such a cofactor. On addition of CoQ10 to reconstituted UCP1 from inclusion bodies, fatty-acid-dependent H+ transport reached the same rate as with native UCP1. The H+ transport was highly sensitive to purine nucleotides, and activated only by oxidized but not reduced CoQ. H+ transport of native UCP1 correlated with the endogenous CoQ content.

How do uncoupling proteins uncouple?

According to the proton buffering model, introduced by Klingenberg, UCP1 conducts protons through a hydrophilic pathway lined with fatty acid head groups that buffer the protons as they move across the membrane. According to the fatty acid protonophore model, introduced by Garlid, UCPs do not conduct protons at all. Rather, like all members of this gene family, they are anion carriers. A variety of anions are transported, but the physiological substrates are fatty acid (FA) anions. Because the carboxylate head group is translocated by UCP, and because the protonated FA rapidly diffuses across the membrane, this mechanism permits FA to behave as regulated cycling protonophores. Favoring the latter mechanism is the fact that the head group of long-chain alkylsulfonates, strong acid analogues of FA, is also translocated by UCP.

A functional-phylogenetic classification system for transmembrane solute transporters

A comprehensive classification system for transmembrane molecular transporters has been developed and recently approved by the transport panel of the nomenclature committee of the International Union of Biochemistry and Molecular Biology. This system is based on (i) transporter class and subclass (mode of transport and energy coupling mechanism), (ii) protein phylogenetic family and subfamily, and (iii) substrate specificity. Almost all of the more than 250 identified families of transporters include members that function exclusively in transport. Channels (115 families), secondary active transporters (uniporters, symporters, and antiporters) (78 families), primary active transporters (23 families), group translocators (6 families), and transport proteins of ill-defined function or of unknown mechanism (51 families) constitute distinct categories. Transport mode and energy coupling prove to be relatively immutable characteristics and therefore provide primary bases for classification. Phylogenetic grouping reflects structure, function, mechanism, and often substrate specificity and therefore provides a reliable secondary basis for classification. Substrate specificity and polarity of transport prove to be more readily altered during evolutionary history and therefore provide a tertiary basis for classification. With very few exceptions, a phylogenetic family of transporters includes members that function by a single transport mode and energy coupling mechanism, although a variety of substrates may be transported, sometimes with either inwardly or outwardly directed polarity. In this review, I provide cross-referencing of well-characterized constituent transporters according to (i) transport mode, (ii) energy coupling mechanism, (iii) phylogenetic grouping, and (iv) substrates transported. The structural features and distribution of recognized family members throughout the living world are also evaluated. The tabulations should facilitate familial and functional assignments of newly sequenced transport proteins that will result from future genome sequencing projects.

Uncoupling proteins 1 and 3 are regulated differently

Using a heterologous yeast expression system, we have previously found a marked discordance between the effects of uncoupling protein (UCP) 1 and UCP3L on basal O(2) consumption in whole yeast versus isolated mitochondria. In whole yeast, UCP3L produces a greater stimulation of basal O(2) consumption, while in isolated mitochondria, UCP1 produces a much greater effect. As shown previously and in this report, UCP3L, in contrast to UCP1, is not inhibited by purine nucleotides. In the present study, we addressed two hypothetical mechanisms that could account for the observed discordance: (i) in whole yeast, purine nucleotides inhibit UCP1 but not UCP3L and (ii) preparations of isolated mitochondria lack an activator of UCP3L that is normally present in vivo. By use of a mutant of UCP1 that lacks purine nucleotide inhibition, it is demonstrated that cytosolic concentrations of purine nucleotides present in yeast effectively inhibit UCP1 activity. This suggests that the lower activity of UCP1 compared to UCP3L in whole yeast is due to purine nucleotide inhibition of UCP1 but not UCP3L. As potential activators of UCP3L we tested free fatty acids in whole yeast and isolated mitochondria. While UCP1 was strongly activated by free fatty acids, no stimulatory effect on UCP3L was observed. In summary, this study indicates that UCP1 and UCP3L differ in their regulation by purine nucleotides and free fatty acids. This different regulation may be related to different physiological functions of the two proteins.

Alternative mitochondrial functions in cell physiopathology: beyond ATP production

It is well known that mitochondria are the main site for ATP generation within most tissues. However, mitochondria also participate in a surprising number of alternative activities, including intracellular Ca2+ regulation, thermogenesis and the control of apoptosis. In addition, mitochondria are the main cellular generators of reactive oxygen species, and may trigger necrotic cell death under conditions of oxidative stress. This review concentrates on these alternative mitochondrial functions, and their role in cell physiopathology.

The bioenergetics of brown fat mitochondria from UCP1-ablated mice. Ucp1 is not involved in fatty acid-induced de-energization ("uncoupling")

The bioenergetics of brown fat mitochondria isolated from UCP1-ablated mice were investigated. The mitochondria had lost the high GDP-binding capacity normally found in brown fat mitochondria, and they were innately in an energized state, in contrast to wild-type mitochondria. GDP, which led to energization of wild-type mitochondria, was without effect on the brown fat mitochondria from UCP1-ablated mice. The absence of thermogenic function did not result in reintroduction of high ATP synthase activity. Remarkably and unexpectedly, the mitochondria from UCP1-ablated mice were as sensitive to the de-energizing ("uncoupling") effect of free fatty acids as were UCP1-containing mitochondria. Therefore, the de-energizing effect of free fatty acids does not appear to be mediated via UCP1, and free fatty acids would not seem to be the intracellular physiological activator involved in mediation of the thermogenic signal from the adrenergic receptor to UCP1. In the UCP1-ablated mice, Ucp2 mRNA levels in brown adipose tissue were 14-fold higher and Ucp3 mRNA levels were marginally lower than in wild-type. The Ucp2 and Ucp3 mRNA levels were therefore among the highest found in any tissue. These high mRNA levels did not confer on the isolated mitochondria any properties associated with de-energization. Thus, the mere observation of a high level of Ucp2 or Ucp3 mRNA in a tissue cannot be taken as an indication that mitochondria isolated from that tissue will display innate de-energization or thermogenesis.

Transport function and regulation of mitochondrial uncoupling proteins 2 and 3

Uncoupling protein 1 (UCP1) dissipates energy and generates heat by catalyzing back-flux of protons into the mitochondrial matrix, probably by a fatty acid cycling mechanism. If the newly discovered UCP2 and UCP3 function similarly, they will enhance peripheral energy expenditure and are potential molecular targets for the treatment of obesity. We expressed UCP2 and UCP3 in Escherichia coli and reconstituted the detergent-extracted proteins into liposomes. Ion flux studies show that purified UCP2 and UCP3 behave identically to UCP1. They catalyze electrophoretic flux of protons and alkylsulfonates, and proton flux exhibits an obligatory requirement for fatty acids. Proton flux is inhibited by purine nucleotides but with much lower affinity than observed with UCP1. These findings are consistent with the hypothesis that UCP2 and UCP3 behave as uncoupling proteins in the cell.

Overexpression of muscle uncoupling protein 2 content in human obesity associates with reduced skeletal muscle lipid utilization

Uncoupling proteins (UCP) may influence thermogenesis. Since skeletal muscle plays an important role in energy homeostasis and substrate oxidation, this study was undertaken to test the hypotheses that skeletal muscle UCP2 content is altered in obesity and could be linked to basal energy expenditure, insulin sensitivity, or substrate oxidation within skeletal muscle under postabsorptive (fasting) conditions. To examine these possibilities, limb basal energy expenditure and respiratory quotient (bRQ) were measured in 18 obese nondiabetic (Ob) and lean individuals (L). Total body fat (%) ranged from 11% to 46%. In addition, insulin-stimulated rates of glucose disposal (Rd) were measured under euglycemic hyperinsulinemic conditions. Biopsy of vastus lateralis muscle was used to measure cytochrome c oxidase (COX) enzyme activity and UCP2 content. Whereas low muscle COX activity was found in the Ob compared to L (6.9+/-1.6 vs. 9.6+/-1.2 U/g; P<0.001), skeletal muscle UCP2 content in Ob was significantly higher than in L (48+/-9 vs. 33+/-12 arbitrary units/g; P<0.05). Moreover, UCP2 content was positively correlated with percent of total body fat (r=0.57; P<0. 05) and bRQ (r=0.59; P<0.01), but not with visceral fat (r=0.17; P=0. 49), basal energy expenditure (r=0.07; P=0.79) or Rd (r=-0.23; P=0. 34). In summary, these results indicate that if development of obesity in humans is mediated by defective expression of UCP2 within skeletal muscle, then this effect is not observed in people with established obesity. The present study also suggests that skeletal muscle UCP2 content is not related to basal energy expenditure or insulin sensitivity in humans. However, the increased content of UCP2 within skeletal muscle in obesity appears to coincide with a reduced postabsorptive lipid utilization by muscle.

The guanosine monophosphate reductase gene is conserved in rats and its expression increases rapidly in brown adipose tissue during cold exposure

Non-shivering thermogenesis is required for survival of rodents during cold stress. Uncoupling protein-1 acts in brown adipose tissue (BAT) to transport protons, thus dissipating the proton gradient across the inner mitochondrial membrane. This permits respiration uncoupled from ATP synthesis. UCP-1 function is inhibited by the binding of purine nucleotides, with GTP/GDP being more potent than ATP/ADP. We used a cDNA subtraction analysis to identify cDNAs rapidly induced by cold exposure. One of these encodes rat guanosine monophosphate reductase (GMP-r). This was surprising in that previous data had suggested that this enzyme was absent in rodents. Rat GMP-r is 96% identical to human GMP-r, and its mRNA is increased 30-fold in BAT within 6 h of cold exposure. The gene is also expressed (but not cold-responsive) in muscle and kidney, but not in white fat. We speculate that the physiological function of the marked increase in BAT GMP-r during cold stress may be to deplete the brown adipocyte of guanine nucleotides, converting them to IMP, thus permitting enhanced UCP-1 function. This is a previously unrecognized regulatory aspect of thermogenesis, an essential physiological response of rodents to cold.

The mechanism of proton transport mediated by mitochondrial uncoupling proteins

The effort to understand the mechanism of uncoupling by UCP has devolved into two models - the fatty acid protonophore model and the proton buffering model. Evidence for each hypothesis is summarized and evaluated. We also evaluate the obligatory requirement for fatty acids in UCP1-mediated uncoupling and the question of fatty acid affinity for UCP1. The structural bases of UCP transport function and nucleotide inhibition are discussed in light of recent mutagenesis studies and in relationship to the sequences of newly discovered UCPs.

The uncoupling protein UCP1 does not increase the proton conductance of the inner mitochondrial membrane by functioning as a fatty acid anion transporter

The activity of the brown fat uncoupling protein (UCP1) is regulated by purine nucleotides and fatty acids. Although the inhibition by nucleotides is well established, the activation by fatty acids is still controversial. It has been reported that the ADP/ATP carrier, and possibly other members of the mitochondrial carrier family, mediate fatty acid uncoupling of mitochondria from a variety of sources by facilitating the transbilayer movement of the fatty acid anion. Brown fat mitochondria are known to be more sensitive to fatty acid uncoupling, a property that has been assigned to the presence of UCP1. We have analyzed the transport properties of UCP1 and conclude that fatty acids are not essential for UCP1 function, although they increase its uncoupling activity. In order to establish the difference between the proposed carrier-mediated uncoupling and that exerted through UCP1, we have studied the facility with which fatty acids uncouple respiration in mitochondria from control yeast and strains expressing UCP1 or the mutant Cys-304 --> Gly. The concentration of free palmitate required for half-maximal activation of respiration in UCP1-expressing mitochondria is 80 or 40 nM for the mutant protein. These concentrations have virtually no effect on the respiration of mitochondria from control yeast and are nearly 3 orders of magnitude lower than those reported for carrier-mediated uncoupling. We propose that there exist two modes of fatty acid-mediated uncoupling; nanomolar concentrations activate proton transport through UCP1, but only if their concentrations rise to the micromolar range do they become substrates for nonspecific carrier-mediated uncoupling.

Fatty acid cycling mechanism and mitochondrial uncoupling proteins

We hypothesize that fatty acid-induced uncoupling serves in bioenergetic systems to set the optimum efficiency and tune the degree of coupling of oxidative phosphorylation. Uncoupling results from fatty acid cycling, enabled by several phylogenetically specialized proteins and, to a lesser extent, by other mitochondrial carriers. It is suggested that the regulated uncoupling in mammalian mitochondria is provided by uncoupling proteins UCP-1, UCP-2 and UCP-3, whereas in plant mitochondria by PUMP and StUCP, all belonging to the gene family of mitochondrial carriers. UCP-1, and hypothetically UCP-3, serve mostly to provide nonshivering thermogenesis in brown adipose tissue and skeletal muscle, respectively. Fatty acid cycling was documented for UCP-1, PUMP and ADP/ATP carrier, and is predicted also for UCP-2 and UCP-3. UCP-1 mediates a purine nucleotide-sensitive uniport of monovalent unipolar anions, including anionic fatty acids. The return of protonated fatty acid leads to H+ uniport and uncoupling. UCP-2 is probably involved in the regulation of body weight and energy balance, in fever, and defense against generation of reactive oxygen species. PUMP has been discovered in potato tubers and immunologically detected in fruits and corn, whereas StUCP has been cloned and sequenced froma a potato gene library. PUMP is supposed to act in the termination of synthetic processes in mature fruits and during the climacteric respiratory rise.

The structure and function of the brown fat uncoupling protein UCP1: current status

The uncoupling protein of brown adipose tissue (UCP1) is a transporter that allows the dissipation as heat of the proton gradient generated by the respiratory chain. The discovery of new UCPs in other mammalian tissues and even in plants suggests that the proton permeability of the mitochondrial inner membrane can be regulated and its control is exerted by specialised proteins. The UCP1 is regulated both at the gene and the mitochondrial level to ensure a high thermogenic capacity to the tissue. The members of the mitochondrial transporter family, which includes the UCPs, present two behaviours with carrier and channel transport modes. It has been proposed that this property reflects a functional organization in two domains: a channel and a gating domain. Mounting evidence suggest that the matrix loops contribute to the formation of the gating domain and thus they are determinants to the control of transport activity.

Identification by site-directed mutagenesis of three arginines in uncoupling protein that are essential for nucleotide binding and inhibition

Primary regulation of uncoupling protein is mediated by purine nucleotides, which bind to the protein and allosterically inhibit fatty acid-induced proton transport. To gain increased understanding of nucleotide regulation, we evaluated the role of basic amino acid residues using site-directed mutagenesis. Mutant and wild-type proteins were expressed in yeast, purified, and reconstituted into liposomes. We studied nucleotide binding as well as inhibition of fatty acid-induced proton transport in wild-type and six mutant uncoupling proteins. None of the mutations interfered with proton transport. Two lysine mutants and a histidine mutant had no effect on nucleotide binding or inhibition. Arg83 and Arg182 mutants completely lost both the ability to bind nucleotides and nucleotide inhibition. Surprisingly, the Arg276 mutant exhibited normal nucleotide binding, but completely lost nucleotide inhibition. To account for this dissociation between binding and inhibition, we propose a three-stage binding-conformational change model of nucleotide regulation of uncoupling protein. We have now identified three nucleotides by site-directed mutagenesis that are essential for nucleotide interaction with uncoupling protein.

Reconstituted plant uncoupling mitochondrial protein allows for proton translocation via fatty acid cycling mechanism

Potato and tomato plant uncoupling mitochondrial protein (PUMP) was reconstituted into liposomes, and K+ or H+ fluxes associated with fatty acid (FA)-induced ion movement were measured using fluorescent ion indicators potassium binding benzofuraneisophthalate and 6-methoxy-N-(3-sulfopropyl)-quinolinium. We suggest that PUMP, like its mammalian counterpart, the uncoupling protein of brown adipose tissue mitochondria (Garlid, K. D., Orosz, D. E., Modrianský, M., Vassanelli, S., and Jeek, P. (1996), J. Biol. Chem. 271, 2615-2702), allows for H+ translocation via a FA cycling mechanism. Reconstituted PUMP translocated anionic linoleic and heptylbenzoic acids, undecanesulfonate, and hexanesulfonate, but not phenylvaleric and abscisic acids or Cl-. Transport was inhibited by ATP and GDP. Internal acidification of protein-free liposomes by linoleic or heptylbenzoic acid indicated that H+ translocation occurs by FA flip-flopping across the lipid bilayer. However, addition of valinomycin after FA-initiated GDP-sensitive H+ efflux solely in proteoliposomes, indicating that influx of anionic FA via PUMP precedes a return of protonated FA carrying H+. Phenylvaleric acid, unable to flip-flop, was without effect. Kinetics of FA and undecanesulfonate uniport suggested the existence of an internal anion binding site. Exponential flux-voltage characteristics were also studied. We suggest that regulated uncoupling in plant mitochondria may be important during fruit ripening, senescence, and seed dormancy.

A structure-activity study of fatty acid interaction with mitochondrial uncoupling protein

Fatty acid (FA) uniport via mitochondrial uncoupling protein (UcP) was detected fluorometrically with PBFI, potassium-binding benzofuran phthalate and SPQ, 6-methoxy-N-(3-sulfopropyl)-quinolinium, indicating K+ and H+, respectively. The FA structural patterns required for FA flip-flop, UcP-mediated FA uniport, activation of UcP-mediated H+ transport in proteoliposomes, and inhibition of UcP-mediated Cl- uniport by FA, were identical. Positive responses were found exclusively with FA which were able to flip-flop in a protonated form across the membrane and no responses were found with 'inactive' FA lacking the flip-flop ability. The findings support the existence of FA cycling mechanism.

Brown adipose tissue, beta 3-adrenergic receptors, and obesity

Brown adipose tissue is distinguished by its unique capacity for uncoupled mitochondrial respiration, which is highly regulated by sympathetic nerve activity. Because of this, energy expenditure in brown fat is capable of ranging over many orders of magnitude. The fact that the function of brown adipose tissue is impaired in obese rodents and that transgenic mice with decreased brown fat develop obesity demonstrates the importance of brown fat in maintaining nutritional homeostasis. However, the role of brown fat in humans is less clear. beta 3-Adrenergic receptors are found on brown adipocytes, and treatment with beta 3-selective agonists markedly increases energy expenditure and decreases obesity in rodents. Whether beta 3-selective agonists will be effective anti-obesity agents in humans is presently under investigation.

Evidence for anion-translocating plant uncoupling mitochondrial protein in potato mitochondria

Transport properties of plant mitochondria from potato tubers were investigated using the swelling technique and membrane potential measurements. Proton-dependent swelling of fatty acid-depleted mitochondria in potassium acetate with valinomycin was possible only in the presence of fatty acids (linoleic acid and 12-(4-azido-2-nitrophenylamino)dodecanoic acid) and was inhibited by various purine nucleotides including ATP, GDP, and GTP. Swelling representing uptake of hexanesulfonate was also inhibited by purine nucleotides. Also, the membrane potential of fatty acid-depleted potato mitochondria energized by succinate declined upon the addition of linoleic acid or 12-(4-azido-2-nitrophenylamino)dodecanoic acid, and this decrease was prevented by ATP and other purine nucleotides. These transport activities are identical to those reported for brown adipose tissue mitochondria and related to the uncoupling protein; therefore, we ascribed them to the plant mitochondrial uncoupling protein (PUMP). A major difference between plant and mammalian uncoupling protein is that PUMP transports small hydrophilic anions such as Cl- very slowly, if at all. We suggest that PUMP may play an important role in plant physiology, where a regulated uncoupling and thermogenesis can proceed during fruit and seed development.

Photomodification of mitochondrial proteins by azido fatty acids and its effect on mitochondrial energetics. Further evidence for the role of the ADP/ATP carrier in fatty-acid-mediated uncoupling

Azido derivatives of long-chain fatty acids, 12-(4-azido-2-nitrophenylamino)dodecanoic acid (N3-NpNH-Lau) and 16-(4-azido-2-nitrophenylamino)hexadecanoic acid (N3-NpNH-Pam), were used to study the mechanism of the protonophoric function of long-chain fatty acids in mitochondrial membranes. N3-NpNH-Lau was found to increase resting-state respiration and decrease the membrane potential in a dose-dependent way in a manner similar to that of the natural fatty acid, myristate. Both effects of N3-NpNH-Lau as well as of the myristate were reversed or prevented by the inhibitor of the mitochondrial ADP/ATP carrier (AAC), carboxyatractyloside. This protective effect of carboxyatractyloside was well expressed in rat heart mitochondria and less so in mitochondria within digitonin-permeabilized Ehrlich ascites tumour cells. Photomodification of Ehrlich ascites tumour mitochondria by ultraviolet irradiation in the presence of N3-NpNH-Lau made them more resistant to the uncoupling effect of myristate, and photomodification of rat heart mitochondria resulted in a strong inhibition of AAC which could not be reversed by serum albumin. Photolabelling of rat heart mitochondria with tritiated N3-NpNH-Pam revealed around 10 labelled bands on SDS/polyacrylamide gel electrophoresis. Based on immunodetection with a specific antibody, one of them, corresponding to 30 kDa, was identified as AAC. Specific interaction of AAC with azido fatty acids was confirmed by a high radiolabelling of this band. The role of fatty acids in fine control of the efficiency of oxidative phosphorylation is discussed.

Photoaffinity labelling of the mitochondrial uncoupling protein by [3H]azido fatty acid affects the anion channel

Brown adipose tissue (BAT) mitochondria were incubated with the azido derivative of fatty acid (hexadecanoic) containing four tritium atoms, [3H]AzHA, and among all mitochondrial proteins only a few proteins were photolabelled after irradiation with UV. It suggests the existence of specific fatty acid binding sites on mitochondrial proteins. It was also possible to label with [3H]AzHA the isolated uncoupling protein (UcP) of BAT mitochondria with a low stoichiometry--lower than one AzHA per dimeric UcP. These results together with the observed competition (i.e. prevention of photolabelling) of various UcP anionic substrates with [3H]AzHA and its dodecanoic acid analogue, suggest the existence of the specific fatty acid binding site on UcP identical with the anion channel or anion translocating site.

On the mechanism of fatty acid-induced proton transport by mitochondrial uncoupling protein

Uncoupling protein mediates electrophoretic transport of protons and anions across the inner membrane of brown adipose tissue mitochondria. The mechanism and site of proton transport, the mechanism by which fatty acids activate proton transport, and the relationship between fatty acids and anion transport are unknown. We used fluorescent probes to measure H+ and anion transport in vesicles reconstituted with purified uncoupling protein and carried out a comparative study of the effects of laurate and its close analogue, undecanesulfonate. Undecanesulfonate was transported by uncoupling protein with a Km value similar to that observed for laurate as it activated H+ transport. Both laurate and undecanesulfonate inhibited Cl- with competitive kinetics. Undecanesulfonate inhibited laurate-induced H+ transport with competitive kinetics. Undecanesulfonate and laurate differed in two important respects. (i) Laurate caused uncoupling protein-mediated H+ transport, whereas undecanesulfonate did not. (ii) Lauric acid was rapidly transported across the bilayer by nonionic diffusion, whereas undecanesulfonic was not. We infer that the role of uncoupling protein in H+ transport is to transport fatty acid anions and that fatty acids induce H+ transport because they can diffuse electroneutrally across the membrane. According to this hypothesis, uncoupling protein is a pure anion porter and does not transport protons; rather it is designed to enable fatty acids to behave as cycling protonophores.

EPR spectroscopy of 5-DOXYL-stearic acid bound to the mitochondrial uncoupling protein reveals its competitive displacement by alkylsulfonates in the channel and allosteric displacement by ATP

Competition of fatty acids (FA) and alkylsulfonates with 5-DOXYL-stearic acid (5-SASL) binding to isolated mitochondrial uncoupling protein (UcP) is demonstrated using EPR spectroscopy. A distinct peak of the bound 5-SASL (h+1I) decreased with increasing concentration of competitors. Since alkylsulfonates are UcP substrates, it suggests that the FA binding site is located in the anion channel. Moreover, with increasing ATP the h+1I peak decreased and was smoothed with the 'micellar' peak into a single wider peak. A pH of 8.5 reversed this effect. It could reflect an allosteric release of 5-SASL from the ATP binding site which mimics the ATP gating mechanism.