X-ray structures of catalytic intermediates of cytochrome c oxidase provide insights into its O2 activation and unidirectional proton-pump mechanisms

Resonance Raman spectral analysis of the heme site structure of cytochrome c oxidase with its positive regulator CHCHD2

Cytochrome c oxidase (CcO) reduces O2, pumps protons in the mitochondrial respiratory chain, and is essential for oxygen consumption in the cell. The coiled-coil-helix-coiled-coil-helix domain-containing 2 (CHCHD2; also known as mitochondrial nuclear retrograde regulator 1 [MNRR1], Parkinson's disease 22 [PARK22] and aging-associated gene 10 protein [AAG10]) is a protein that binds to CcO from the intermembrane space and positively regulates the activity of CcO. Despite the importance of CHCHD2 in mitochondrial function, the mechanism of action of CHCHD2 and structural information regarding its binding to CcO remain unknown. Here, we utilized visible resonance Raman spectroscopy to investigate the structural changes around the hemes in CcO in the reduced and CO-bound states upon CHCHD2 binding. We found that CHCHD2 has a significant impact on the structure of CcO in the reduced state. Mapping of the heme peripheries that result in Raman spectral changes in the structure of CcO highlighted helices IX and X near the hemes as sites where CHCHD2 takes action. Part of helix IX is exposed in the intermembrane space, whereas helix X, located between both hemes, may play a key role in proton uptake to a proton-loading site in the reduced state for proton pumping. Taken together, our results suggested that CHCHD2 binds near helix IX and induces a structural change in helix X, accelerating proton uptake.

Structural and functional mechanisms of cytochrome c oxidase

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

Membrane-Bound Redox Enzyme Cytochrome bd-I Promotes Carbon Monoxide-Resistant Escherichia coli Growth and Respiration

The terminal oxidases of bacterial aerobic respiratory chains are redox-active electrogenic enzymes that catalyze the four-electron reduction of O2 to 2H2O taking out electrons from quinol or cytochrome c. Living bacteria often deal with carbon monoxide (CO) which can act as both a signaling molecule and a poison. Bacterial terminal oxidases contain hemes; therefore, they are potential targets for CO. However, our knowledge of this issue is limited and contradictory. Here, we investigated the effect of CO on the cell growth and aerobic respiration of three different Escherichia coli mutants, each expressing only one terminal quinol oxidase: cytochrome bd-I, cytochrome bd-II, or cytochrome bo3. We found that following the addition of CO to bd-I-only cells, a minimal effect on growth was observed, whereas the growth of both bd-II-only and bo3-only strains was severely impaired. Consistently, the degree of resistance of aerobic respiration of bd-I-only cells to CO is high, as opposed to high CO sensitivity displayed by bd-II-only and bo3-only cells consuming O2. Such a difference between the oxidases in sensitivity to CO was also observed with isolated membranes of the mutants. Accordingly, O2 consumption of wild-type cells showed relatively low CO sensitivity under conditions favoring the expression of a bd-type oxidase.

Time-resolved serial crystallography to track the dynamics of carbon monoxide in the active site of cytochrome c oxidase

Cytochrome c oxidase (CcO) is part of the respiratory chain and contributes to the electrochemical membrane gradient in mitochondria as well as in many bacteria, as it uses the energy released in the reduction of oxygen to pump protons across an energy-transducing biological membrane. Here, we use time-resolved serial femtosecond crystallography to study the structural response of the active site upon flash photolysis of carbon monoxide (CO) from the reduced heme a3 of ba3-type CcO. In contrast with the aa3-type enzyme, our data show how CO is stabilized on CuB through interactions with a transiently ordered water molecule. These results offer a structural explanation for the extended lifetime of the CuB-CO complex in ba3-type CcO and, by extension, the extremely high oxygen affinity of the enzyme.

Crystallographic cyanide-probing for cytochrome c oxidase reveals structural bases suggesting that a putative proton transfer H-pathway pumps protons

Cytochrome c oxidase (CcO) reduces O2 in the O2-reduction site by sequential four-electron donations through the low-potential metal sites (CuA and Fea). Redox-coupled X-ray crystal structural changes have been identified at five distinct sites including Asp51, Arg438, Glu198, the hydroxyfarnesyl ethyl group of heme a, and Ser382, respectively. These sites interact with the putative proton-pumping H-pathway. However, the metal sites responsible for each structural change have not been identified, since these changes were detected as structural differences between the fully reduced and fully oxidized CcOs. Thus, the roles of these structural changes in the CcO function are yet to be revealed. X-ray crystal structures of cyanide-bound CcOs under various oxidation states showed that the O2-reduction site controlled only the Ser382-including site, while the low-potential metal sites induced the other changes. This finding indicates that these low-potential site-inducible structural changes are triggered by sequential electron-extraction from the low-potential sites by the O2-reduction site and that each structural change is insensitive to the oxidation and ligand-binding states of the O2-reduction site. Because the proton/electron coupling efficiency is constant (1:1), regardless of the reaction progress in the O2-reduction site, the structural changes induced by the low-potential sites are assignable to those critically involved in the proton pumping, suggesting that the H-pathway, facilitating these low-potential site-inducible structural changes, pumps protons. Furthermore, a cyanide-bound CcO structure suggests that a hypoxia-inducible activator, Higd1a, activates the O2-reduction site without influencing the electron transfer mechanism through the low-potential sites, kinetically confirming that the low-potential sites facilitate proton pump.

New insights into the proton pumping mechanism of ba3 cytochrome c oxidase: the functions of key residues and water

As the terminal oxidase of cell respiration in mitochondria and aerobic bacteria, the proton pumping mechanism of ba3-type cytochrome c oxidase (CcO) of Thermus thermophiles is still not fully understood. Especially, the functions of key residues which were considered as the possible proton loading sites (PLSs) above the catalytic center, as well as water located above and within the catalytic center, remain unclear. In this work, molecular dynamic simulations were performed on a set of designed mutants of key residues (Asp287, Asp372, His376, and Glu126II). The results showed that Asp287 may not be a PLS, but it could modulate the ability of the proton transfer pathway to transfer protons through its salt bridge with Arg225. Maintaining the closed state of the water pool above the catalytic center is necessary for the participation of inside water molecules in proton transfer. Water molecules inside the water pool can form hydrogen bond chains with PLS to facilitate proton transfer. Additional quantum cluster models of the Fe-Cu metal catalytic center are established, indicating that when the proton is transferred from Tyr237, it is more likely to reach the OCu atom directly through only one water molecule. This work provides a more profound understanding of the functions of important residues and specific water molecules in the proton pumping mechanism of CcO.

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

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

Recent progress in experimental studies on the catalytic mechanism of cytochrome c oxidase

Cytochrome c oxidase (CcO) reduces molecular oxygen (O₂) to water, coupled with a proton pump from the N-side to the P-side, by receiving four electrons sequentially from the P-side to the O₂-reduction site-including Fe_a3 and Cu_B-via the two low potential metal sites; Cu_A and Fe_a. The catalytic cycle includes six intermediates as follows, R (Fe_a3 ²⁺, Cu_B ¹⁺, Tyr244OH), A (Fe_a3 ²⁺-O₂, Cu_B ¹⁺, Tyr244OH), P_m (Fe_a3 ⁴⁺ = O^2-, Cu_B ²⁺-OH^-, Tyr244O•), F (Fe_a3 ⁴⁺ = O^2-, Cu_B ²⁺-OH^-, Tyr244OH), O (Fe_a3 ³⁺-OH^-, Cu_B ²⁺-OH^-, Tyr244OH), and E (Fe_a3 ³⁺-OH^-, Cu_B ¹⁺-H₂O, Tyr244OH). CcO has three proton conducting pathways, D, K, and H. The D and K pathways connect the N-side surface with the O₂-reduction site, while the H-pathway is located across the protein from the N-side to the P-side. The proton pump is driven by electrostatic interactions between the protons to be pumped and the net positive charges created during the O₂ reduction. Two different proton pump proposals, each including either the D-pathway or H-pathway as the proton pumping site, were proposed approximately 30 years ago and continue to be under serious debate. In our view, the progress in understanding the reaction mechanism of CcO has been critically rate-limited by the resolution of its X-ray crystallographic structure. The improvement of the resolutions of the oxidized/reduced bovine CcO up to 1.5/1.6 Å resolution in 2016 provided a breakthrough in the understanding of the reaction mechanism of CcO. In this review, experimental studies on the reaction mechanism of CcO before the appearance of the 1.5/1.6 Å resolution X-ray structures are summarized as a background description. Following the summary, we will review the recent (since 2016) experimental findings which have significantly improved our understanding of the reaction mechanism of CcO including: 1) redox coupled structural changes of bovine CcO; 2) X-ray structures of all six intermediates; 3) spectroscopic findings on the intermediate species including the Tyr244 radical in the P_m form, a peroxide-bound form between the A and Pm forms, and F_r, a one-electron reduced F-form; 4) time resolved X-ray structural changes during the photolysis of CO-bound fully reduced CcO using XFEL; 5) a simulation analysis for the Pm→Pr→F transition.

Structural basis for functional properties of cytochrome c oxidase

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

The accelerated evolution of human cytochrome c oxidase - Selection for reduced rate and proton pumping efficiency?

The cytochrome c oxidase complex, complex VI (CIV), catalyzes the terminal step of the mitochondrial electron transport chain where the reduction of oxygen to water by cytochrome c is coupled to the generation of a protonmotive force that drive the synthesis of ATP. CIV evolution was greatly accelerated in humans and other anthropoid primates and appears to be driven by adaptive selection. However, it is not known if there are significant functional differences between the anthropoid primates CIV, and other mammals. Comparison of the high-resolution structures of bovine CIV, mouse CIV and human CIV shows structural differences that are associated with anthropoid-specific substitutions. Here I examine the possible effects of these substitutions in four CIV peptides that are known to affect proton pumping: the mtDNA-coded subunits I, II and III, and the nuclear-encoded subunit VIa2. I conclude that many of the anthropoid-specific substitutions could be expected to modulate the rate and/or the efficiency of proton pumping. These results are compatible with the previously proposed hypothesis that the accelerated evolution of CIV in anthropoid primates is driven by selection pressure to lower the mitochondrial protonmotive force and thus decrease the rate of superoxide generation by mitochondria.

Radical in the Peroxide-Produced F-Type Ferryl Form of Bovine Cytochrome c Oxidase

The reduction of O₂ in respiratory cytochrome c oxidases (CcO) is associated with the generation of the transmembrane proton gradient by two mechanisms. In one of them, the proton pumping, two different types of the ferryl intermediates of the catalytic heme a₃-Cu_B center P and F forms, participate. Equivalent ferryl states can be also formed by the reaction of the oxidized CcO (O) with H₂O₂. Interestingly, in acidic solutions a single molecule of H₂O₂ can generate from the O an additional F-type ferryl form (F^•) that should contain, in contrast to the catalytic F intermediate, a free radical at the heme a₃-Cu_B center. In this work, the formation and the endogenous decay of both the ferryl iron of heme a₃ and the radical in F^• intermediate were examined by the combination of four experimental approaches, isothermal titration calorimetry, electron paramagnetic resonance, and electronic absorption spectroscopy together with the reduction of this form by the defined number of electrons. The results are consistent with the generation of radicals in F^• form. However, the radical at the catalytic center is more rapidly quenched than the accompanying ferryl state of heme a₃, very likely by the intrinsic oxidation of the enzyme itself.

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

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

Impact of Hydrogen Sulfide on Mitochondrial and Bacterial Bioenergetics

This review focuses on the effects of hydrogen sulfide (H2S) on the unique bioenergetic molecular machines in mitochondria and bacteria-the protein complexes of electron transport chains and associated enzymes. H2S, along with nitric oxide and carbon monoxide, belongs to the class of endogenous gaseous signaling molecules. This compound plays critical roles in physiology and pathophysiology. Enzymes implicated in H2S metabolism and physiological actions are promising targets for novel pharmaceutical agents. The biological effects of H2S are biphasic, changing from cytoprotection to cytotoxicity through increasing the compound concentration. In mammals, H2S enhances the activity of FoF1-ATP (adenosine triphosphate) synthase and lactate dehydrogenase via their S-sulfhydration, thereby stimulating mitochondrial electron transport. H2S serves as an electron donor for the mitochondrial respiratory chain via sulfide quinone oxidoreductase and cytochrome c oxidase at low H2S levels. The latter enzyme is inhibited by high H2S concentrations, resulting in the reversible inhibition of electron transport and ATP production in mitochondria. In the branched respiratory chain of Escherichia coli, H2S inhibits the bo3 terminal oxidase but does not affect the alternative bd-type oxidases. Thus, in E. coli and presumably other bacteria, cytochrome bd permits respiration and cell growth in H2S-rich environments. A complete picture of the impact of H2S on bioenergetics is lacking, but this field is fast-moving, and active ongoing research on this topic will likely shed light on additional, yet unknown biological effects.

Proton Pumping and Non-Pumping Terminal Respiratory Oxidases: Active Sites Intermediates of These Molecular Machines and Their Derivatives

Terminal respiratory oxidases are highly efficient molecular machines. These most important bioenergetic membrane enzymes transform the energy of chemical bonds released during the transfer of electrons along the respiratory chains of eukaryotes and prokaryotes from cytochromes or quinols to molecular oxygen into a transmembrane proton gradient. They participate in regulatory cascades and physiological anti-stress reactions in multicellular organisms. They also allow microorganisms to adapt to low-oxygen conditions, survive in chemically aggressive environments and acquire antibiotic resistance. To date, three-dimensional structures with atomic resolution of members of all major groups of terminal respiratory oxidases, heme-copper oxidases, and bd-type cytochromes, have been obtained. These groups of enzymes have different origins and a wide range of functional significance in cells. At the same time, all of them are united by a catalytic reaction of four-electron reduction in oxygen into water which proceeds without the formation and release of potentially dangerous ROS from active sites. The review analyzes recent structural and functional studies of oxygen reduction intermediates in the active sites of terminal respiratory oxidases, the features of catalytic cycles, and the properties of the active sites of these enzymes.

The therapeutic potential of mitochondrial toxins

When screening active compounds by phenotypic assays, we often encounter mitochondrial toxins, which are compounds that can affect mitochondrial functions. In normal cells, these toxins may have relatively low toxicity but can nonetheless show measurable effects even at low concentrations. On the other hand, in animals, mitochondrial toxins can exert severe toxicity. Mitochondrial toxins that act as inhibitors of respiratory chain complexes in oxidative phosphorylation (OXPHOS) are typically avoided during drug discovery efforts, as such compounds can directly promote lethal inhibition of pulmonary respiration. However, mitochondrial toxins could in fact have beneficial therapeutic effects. Anti-cancer strategies that target mitochondrial functions, particularly OXPHOS, have received increasing attention in recent years. In this review article we examine the significance of OXPHOS inhibitors as anti-cancer drug candidates and discuss compounds having microbial origins.

The three-spin intermediate at the O-O cleavage and proton-pumping junction in heme-Cu oxidases

[Figure: see text].

Biochemical and crystallographic studies of monomeric and dimeric bovine cytochrome c oxidase

Cytochrome c oxidase (CcO), a terminal oxidase in the respiratory chain, catalyzes the reduction of O2 to water coupled with the proton pump across the membrane. Mitochondrial CcO exists in monomeric and dimeric forms, and as a monomer as part of the respiratory supercomplex, although the enzymatic reaction proceeds in the CcO monomer. Recent biochemical and crystallographic studies of monomeric and dimeric CcOs have revealed functional and structural differences among them. In solubilized mitochondrial membrane, the monomeric form is dominant, and a small amount of dimer is observed. The activity of the monomeric CcO is higher than that of the dimer, suggesting that the monomer is the active form. In the structure of monomeric CcO, a hydrogen bond network of water molecules is formed at the entrance of the proton transfer K-pathway, and in dimeric CcO, this network is altered by a cholate molecule binding between monomers. The specific binding of the cholate molecule at the dimer interface suggests that the binding of physiological ligands similar in size or shape to cholate could also trigger dimer formation as a physiological standby form. Because the dimer interface also contains weak interactions of nonspecifically bound lipid molecules, hydrophobic interactions between the transmembrane helices, and a Met-Met interaction between the extramembrane regions, these interactions could support the stabilization of the standby form. Structural analyses also suggest that hydrophobic interactions of cardiolipins bound to the trans-membrane surface of CcO are involved in forming the supercomplex.

Critical roles of the CuB site in efficient proton pumping as revealed by crystal structures of mammalian cytochrome c oxidase catalytic intermediates

Mammalian cytochrome c oxidase (CcO) reduces O₂ to water in a bimetallic site including Fe_a3 and Cu_B giving intermediate molecules, termed A-, P-, F-, O-, E-, and R-forms. From the P-form on, each reaction step is driven by single-electron donations from cytochrome c coupled with the pumping of a single proton through the H-pathway, a proton-conducting pathway composed of a hydrogen-bond network and a water channel. The proton-gradient formed is utilized for ATP production by F-ATPase. For elucidation of the proton pumping mechanism, crystal structural determination of these intermediate forms is necessary. Here we report X-ray crystallographic analysis at ∼1.8 Å resolution of fully reduced CcO crystals treated with O₂ for three different time periods. Our disentanglement of intermediate forms from crystals that were composed of multiple forms determined that these three crystallographic data sets contained ∼45% of the O-form structure, ∼45% of the E-form structure, and ∼20% of an oxymyoglobin-type structure consistent with the A-form, respectively. The O- and E-forms exhibit an unusually long Cu_B²⁺-OH^- distance and Cu_B¹⁺-H₂O structure keeping Fe_a3³⁺-OH^- state, respectively, suggesting that the O- and E-forms have high electron affinities that cause the O→E and E→R transitions to be essentially irreversible and thus enable tightly coupled proton pumping. The water channel of the H-pathway is closed in the O- and E-forms and partially open in the R-form. These structures, together with those of the recently reported P- and F-forms, indicate that closure of the H-pathway water channel avoids back-leaking of protons for facilitating the effective proton pumping.

Peroxide stimulated transition between the ferryl intermediates of bovine cytochrome c oxidase

During catalysis of cytochrome c oxidases (CcO) several ferryl intermediates of the catalytic heme a₃-Cu_B center are observed. In the P_M ferryl state, produced by the reaction of two-electron reduced CcO with O₂, the ferryl iron of heme a₃ and a free radical are present at the catalytic center. The radical reduction stimulates the transition of the P_M into another ferryl F state. Similar ferryl states can be also generated from the oxidized CcO (O) in the reaction with H₂O₂. The P_M, the product of the reaction of the O with one molecule of peroxide, is transformed into the F state by the second molecule of H₂O₂. However, the chemical nature of this transition has not been unambiguously elucidated yet. Here, we examined the redox state of the peroxide-produced P_M and F states by the one-electron reduction. The F form and interestingly also the major fraction of the P_M sample, likely another P-type ferryl form (P_R), were found to be the one oxidizing equivalent above the O state. However, the both P-type forms are transformed into the F state by additional molecule of H₂O₂. It is suggested that the P_R-to-F transition is due to the binding of H₂O₂ to Cu_B triggering a structural change together with the uptake of H⁺ at the catalytic center. In the P_M-to-F conversion, these two events are complemented with the annihilation of radical by the intrinsic oxidation of the enzyme.

The 1.3-Å resolution structure of bovine cytochrome c oxidase suggests a dimerization mechanism

Cytochrome c oxidase (CcO) in the respiratory chain catalyzes oxygen reduction by coupling electron and proton transfer through the enzyme and proton pumping across the membrane. Although the functional unit of CcO is monomeric, mitochondrial CcO forms a monomer and a dimer, as well as a supercomplex with respiratory complexes I and III. A recent study showed that dimeric CcO has lower activity than monomeric CcO and proposed that dimeric CcO is a standby form for enzymatic activation in the mitochondrial membrane. Other studies have suggested that the dimerization is dependent on specifically arranged lipid molecules, peptide segments, and post-translationally modified amino acid residues. To re-examine the structural basis of dimerization, we improved the resolution of the crystallographic structure to 1.3 Å by optimizing the method for cryoprotectant soaking. The observed electron density map revealed many weakly bound detergent and lipid molecules at the interface of the dimer. The dimer interface also contained hydrogen bonds with tightly bound cholate molecules, hydrophobic interactions between the transmembrane helices, and a Met-Met interaction between the extramembrane regions. These results imply that binding of physiological ligands structurally similar to cholate could trigger dimerization in the mitochondrial membrane and that non-specifically bound lipid molecules at the transmembrane surface between monomers support the stabilization of the dimer. The weak interactions involving the transmembrane helices and extramembrane regions may play a role in positioning each monomer at the correct orientation in the dimer.