Water exit pathways and proton pumping mechanism in B-type cytochrome c oxidase from molecular dynamics simulations

Reaction pathways, proton transfer, and proton pumping in ba3 class cytochrome c oxidase: perspectives from DFT quantum chemistry and molecular dynamics

After drawing comparisons between the reaction pathways of cytochrome c oxidase (CcO, Complex 4) and the preceding complex cytochrome bc1 (Complex 3), both being proton pumping complexes along the electron transport chain, we provide an analysis of the reaction pathways in bacterial ba3 class CcO, comparing spectroscopic results and kinetics observations with results from DFT calculations. For an important arc of the catalytic cycle in CcO, we can trace the energy pathways for the chemical protons and show how these pathways drive proton pumping of the vectorial protons. We then explore the proton loading network above the Fe heme a3-CuB catalytic center, showing how protons are loaded in and then released by combining DFT-based reaction energies with molecular dynamics simulations over states of that cycle. We also propose some additional reaction pathways for the chemical and vector protons based on our recent work with spectroscopic support.

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.

Interplay of Hydration and Protonation Dynamics in the K-Channel of Cytochrome c Oxidase

Cytochrome c oxidase is a membrane protein of the respiratory chain that consumes protons and molecular oxygen to produce water and uses the resulting energy to pump protons across the membrane. Our molecular dynamics simulations with an excess proton located at different positions in one of the proton-conducting channels, the K-channel, show a clear dependence of the number of water molecules inside the channel on the proton position. A higher hydration level facilitates the formation of hydrogen-bonded chains along which proton transfer can occur. However, a sufficiently high hydration level for such proton transport is observed only when the excess proton is located above S365, i.e., the lower third of the channel. From the channel entrance up to this point, proton transport is via water molecules as proton carriers. These hydronium ions move with their surrounding water molecules, up to K362, filling and widening the channel. The conformation of K362 depends on its own protonation state and on the hydration level, suggesting its role to be proton transport from a hydronium ion at the height of K362 to the upper part of the channel via a conformational change. The protonation-dependent conformational dynamics of E101 at the bottom of the channel renders proton transfer via E101 unlikely. Instead, its role is rather that of an amplifier of H96’s proton affinity, suggesting H96 as the initial proton acceptor.

Electric fields control water-gated proton transfer in cytochrome c oxidase

Aerobic life is powered by membrane-bound enzymes that catalyze the transfer of electrons to oxygen and protons across a biological membrane. Cytochrome c oxidase (CcO) functions as a terminal electron acceptor in mitochondrial and bacterial respiratory chains, driving cellular respiration and transducing the free energy from O₂ reduction into proton pumping. Here we show that CcO creates orientated electric fields around a nonpolar cavity next to the active site, establishing a molecular switch that directs the protons along distinct pathways. By combining large-scale quantum chemical density functional theory (DFT) calculations with hybrid quantum mechanics/molecular mechanics (QM/MM) simulations and atomistic molecular dynamics (MD) explorations, we find that reduction of the electron donor, heme a, leads to dissociation of an arginine (Arg438)-heme a₃ D-propionate ion-pair. This ion-pair dissociation creates a strong electric field of up to 1 V Å^-1 along a water-mediated proton array leading to a transient proton loading site (PLS) near the active site. Protonation of the PLS triggers the reduction of the active site, which in turn aligns the electric field vectors along a second, "chemical," proton pathway. We find a linear energy relationship of the proton transfer barrier with the electric field strength that explains the effectivity of the gating process. Our mechanism shows distinct similarities to principles also found in other energy-converting enzymes, suggesting that orientated electric fields generally control enzyme catalysis.

Coupled transport of electrons and protons in a bacterial cytochrome c oxidase-DFT calculated properties compared to structures and spectroscopies

After a general introduction to the features and mechanisms of cytochrome c oxidases (CcOs) in mitochondria and aerobic bacteria, we present DFT calculated physical and spectroscopic properties for the catalytic reaction cycle compared with experimental observations in bacterial ba3 type CcO, also with comparisons/contrasts to aa3 type CcOs. The Dinuclear Complex (DNC) is the active catalytic reaction center, containing a heme a3 Fe center and a near lying Cu center (called CuB) where by successive reduction and protonation, molecular O2 is transformed to two H2O molecules, and protons are pumped from an inner region across the membrane to an outer region by transit through the CcO integral membrane protein. Structures, energies and vibrational frequencies for Fe-O and O-O modes are calculated by DFT over the catalytic cycle. The calculated DFT frequencies in the DNC of CcO are compared with measured frequencies from Resonance Raman spectroscopy to clarify the composition, geometry, and electronic structures of different intermediates through the reaction cycle, and to trace reaction pathways. X-ray structures of the resting oxidized state are analyzed with reference to the known experimental reaction chemistry and using DFT calculated structures in fitting observed electron density maps. Our calculations lead to a new proposed reaction pathway for coupling the PR → F → OH (ferryl-oxo → ferric-hydroxo) pathway to proton pumping by a water shift mechanism. Through this arc of the catalytic cycle, major shifts in pKa's of the special tyrosine and a histidine near the upper water pool activate proton transfer. Additional mechanisms for proton pumping are explored, and the role of the CuB+ (cuprous state) in controlling access to the dinuclear reaction site is proposed.

Identifying the proton loading site cluster in the ba3 cytochrome c oxidase that loads and traps protons

Cytochrome c Oxidase (CcO) is the terminal electron acceptor in aerobic respiratory chain, reducing O₂ to water. The released free energy is stored by pumping protons through the protein, maintaining the transmembrane electrochemical gradient. Protons are held transiently in a proton loading site (PLS) that binds and releases protons driven by the electron transfer reaction cycle. Multi-Conformation Continuum Electrostatics (MCCE) was applied to crystal structures and Molecular Dynamics snapshots of the B-type Thermus thermophilus CcO. Six residues are identified as the PLS, binding and releasing protons as the charges on heme b and the binuclear center are changed: the heme a₃ propionic acids, Asp287, Asp372, His376 and Glu126B. The unloaded state has one proton and the loaded state two protons on these six residues. Different input structures, modifying the PLS conformation, show different proton distributions and result in different proton pumping behaviors. One loaded and one unloaded protonation states have the loaded/unloaded states close in energy so the PLS binds and releases a proton through the reaction cycle. The alternative proton distributions have state energies too far apart to be shifted by the electron transfers so are locked in loaded or unloaded states. Here the protein can use active states to load and unload protons, but has nearby trapped states, which stabilize PLS protonation state, providing new ideas about the CcO proton pumping mechanism. The distance between the PLS residues Asp287 and His376 correlates with the energy difference between loaded and unloaded states.

On the Elementary Chemical Mechanisms of Unidirectional Proton Transfers: A Nonadiabatic Electron-Wavepacket Dynamics Study

We propose a set of chemical reaction mechanisms of unidirectional proton transfers, which may possibly work as an elementary process in chemical and biological systems. Being theoretically derived based on our series of studies on charge separation dynamics in water splitting by Mn oxides, the present mechanisms have been constructed after careful exploration over the accumulated biological studies on cytochrome c oxidase (CcO) and bacteriorhodopsin. In particular, we have focused on the biochemical findings in the literature that unidirectional transfers of approximately two protons are driven by one electron passage through the reaction center (binuclear center) in CcO, whereas no such dissipative electron transfer is believed to be demanded in the proton transport in bacteriorhodopsin. The proposed basic mechanisms of unidirectional proton transfers are further reduced to two elementary dynamical processes, namely, what we call the coupled proton and electron-wavepacket transfer (CPEWT) and the inverse CPEWT. To show that the proposed mechanisms can indeed be materialized in a molecular level, we construct model systems with possible molecules that are rather familiar in biological chemistry, for which we perform the ab initio calculations of full-dimensional nonadiabatic electron-wavepacket dynamics coupled with all nuclear motions including proton transfers.

Multiscale Simulations of Biological Membranes: The Challenge To Understand Biological Phenomena in a Living Substance

Biological membranes are tricky to investigate. They are complex in terms of molecular composition and structure, functional over a wide range of time scales, and characterized by nonequilibrium conditions. Because of all of these features, simulations are a great technique to study biomembrane behavior. A significant part of the functional processes in biological membranes takes place at the molecular level; thus computer simulations are the method of choice to explore how their properties emerge from specific molecular features and how the interplay among the numerous molecules gives rise to function over spatial and time scales larger than the molecular ones. In this review, we focus on this broad theme. We discuss the current state-of-the-art of biomembrane simulations that, until now, have largely focused on a rather narrow picture of the complexity of the membranes. Given this, we also discuss the challenges that we should unravel in the foreseeable future. Numerous features such as the actin-cytoskeleton network, the glycocalyx network, and nonequilibrium transport under ATP-driven conditions have so far received very little attention; however, the potential of simulations to solve them would be exceptionally high. A major milestone for this research would be that one day we could say that computer simulations genuinely research biological membranes, not just lipid bilayers.

DEPC modification of the CuA protein from Thermus thermophilus

The Cu_A center is the initial electron acceptor in cytochrome c oxidase, and it consists of two copper ions bridged by two cysteines and ligated by two histidines, a methionine, and a carbonyl in the peptide backbone of a nearby glutamine. The two ligating histidines are of particular interest as they may influence the electronic and redox properties of the metal center. To test for the presence of reactive ligating histidines, a portion of cytochrome c oxidase from the bacteria Thermus thermophilus that contains the Cu_A site (the TtCu_A protein) was treated with the chemical modifier diethyl pyrocarbonate (DEPC) and the reaction followed through UV-visible, circular dichroism, and electron paramagnetic resonance spectroscopies at pH 5.0-9.0. A mutant protein (H40A/H117A) with the non-ligating histidines removed was similarly tested. Introduction of an electron-withdrawing DEPC-modification onto the ligating histidine 157 of TtCu_A increased the reduction potential by over 70 mV, as assessed by cyclic voltammetry. Results from both proteins indicate that DEPC reacts with one of the two ligating histidines, modification of a ligating histidine raises the reduction potential of the Cu_A site, and formation of the DEPC adduct is reversible at room temperature. The existence of the reactive ligating histidine suggests that this residue may play a role in modulating the electronic and redox properties of TtCu_A through kinetically-controlled proton exchange with the solvent. Lack of reactivity by the metalloproteins Sco and azurin, both of which contain a mononuclear copper center, indicate that reactivity toward DEPC is not a characteristic of all ligating histidines.

Hydrogen-Bonded Network and Water Dynamics in the D-channel of Cytochrome c Oxidase

Proton transfer in cytochrome c oxidase (CcO) from the cellular inside to the binuclear redox centre as well as proton pumping through the membrane takes place through proton entrance via two distinct pathways, the D- and K-channel. Both channels show a dependence of their hydration level on the protonation states of their key residues, K362 for the K-channel, and E286 or D132 for the D-channel. In the oxidative half of CcO's catalytic cycle the D-channel is the proton-conducting path. For this channel, an interplay of protonation state of the D-channel residues with the water and hydrogen-bond dynamics has been observed in molecular dynamics simulations of the CcO protein, embedded in a lipid bi-layer, modelled in different protonation states. Protonation of residue E286 at the end of the D-channel results in a hydrogen-bonded network pointing from E286 to N139, that is against proton transport, and favouring N139 conformations which correspond to a closed asparagine gate (formed by residues N121 and N139). Consequently, the hydration level is lower than with unprotonated E286. In those models, the Asn gate is predominantly open, allowing water molecules to pass and thus increase the hydration level. The hydrogen-bonded network in these states exhibits longer life times of the Asn residues with water than other models and shows the D-channel to be traversable from the entrance, D132, to exit, E286. The D-channel can thus be regarded as auto-regulated with respect to proton transport, allowing proton passage only when required, that is the proton is located at the lower part of the D-channel (D132 to Asn gate) and not at the exit (E286).

A Water Dimer Shift Activates a Proton Pumping Pathway in the PR → F Transition of ba3 Cytochrome c Oxidase

Broken-symmetry density functional calculations have been performed on the [Fe_a3⁴⁺,Cu_B²⁺] state of the dinuclear center (DNC) for the P_R → F part of the catalytic cycle of ba₃ cytochrome c oxidase (CcO) from Thermus thermophilus (Tt), using the OLYP-D3-BJ functional. The calculations show that the movement of the H₂O molecules in the DNC affects the pK_a values of the residue side chains of Tyr237 and His376⁺, which are crucial for proton transfer/pumping in ba₃ CcO from Tt. The calculated lowest energy structure of the DNC in the [Fe_a3⁴⁺,Cu_B²⁺] state (state F) is of the form Fe_a3⁴⁺═O^2-···Cu_B²⁺, in which the H₂O ligand that resulted from protonation of the OH^- ligand in the P_R state is dissociated from the Cu_B²⁺ site. The calculated Fe_a3⁴⁺═O^2- distance in F (1.68 Å) is 0.03 Å longer than that in P_R (1.65 Å), which can explain the different Fe_a3⁴⁺═O^2- stretching modes in P (804 cm^-1) and F (785 cm^-1) identified by resonance Raman experiments. In this F state, the Cu_B²⁺···O^2- (ferryl-oxygen) distance is only around 2.4 Å. Hence, the subsequent O_H state [Fe_a3³⁺-OH^--Cu_B²⁺] with a μ-hydroxo bridge can be easily formed, as shown by our calculations.

Reactive oxygen species leave a damage trail that reveals water channels in Photosystem II

Photosystem II (PSII), a unique membrane-bound oxidoreductase, catalyzes light-driven oxidation of water to molecular oxygen. Although high-resolution structures of PSII are known, the exact path of the substrate water molecules to the catalytic Mn₄CaO₅ center within the PSII complex remains poorly understood. PSII produces reactive oxygen species (ROS), responsible for the frequent damage and turnover of this megacomplex that occur under physiological conditions. Such ROS are known to specifically modify PSII proteins. Using high-resolution tandem mass spectrometry, we identified oxidative modifications on 36 amino acid residues on the lumenal side of PSII, in the core PSII proteins D1, D2, and CP43 of the cyanobacterium Synechocystis sp. PCC 6803. Remarkably, these oxidized residues clustered into three nearly continuous formations, tracking the pathways of ROS diffusion from the manganese center all the way out to the surface of PSII. We suggest that these profiles of oxidized residues reveal the locations of water channels within PSII. Our results provide the most comprehensive experimental evidence to date of physiologically relevant oxidized residues in PSII and illuminate three possible channels for water between the catalytic Mn cluster in the PSII complex and the bulk medium around it.

Time-resolved generation of membrane potential by ba3 cytochrome c oxidase from Thermus thermophilus coupled to single electron injection into the O and OH states

Two electrogenic phases with characteristic times of ~14μs and ~290μs are resolved in the kinetics of membrane potential generation coupled to single-electron reduction of the oxidized "relaxed" O state of ba₃ oxidase from T. thermophilus (O→E transition). The rapid phase reflects electron redistribution between Cu_A and heme b. The slow phase includes electron redistribution from both Cu_A and heme b to heme a₃, and electrogenic proton transfer coupled to reduction of heme a₃. The distance of proton translocation corresponds to uptake of a proton from the inner water phase into the binuclear center where heme a₃ is reduced, but there is no proton pumping and no reduction of Cu_B. Single-electron reduction of the oxidized "unrelaxed" state (O_H→E_H transition) is accompanied by electrogenic reduction of the heme b/heme a₃ pair by Cu_A in a "fast" phase (~22μs) and transfer of protons in "middle" and "slow" electrogenic phases (~0.185ms and ~0.78ms) coupled to electron redistribution from the heme b/heme a₃ pair to the Cu_B site. The "middle" and "slow" electrogenic phases seem to be associated with transfer of protons to the proton-loading site (PLS) of the proton pump, but when all injected electrons reach Cu_B the electronic charge appears to be compensated by back-leakage of the protons from the PLS into the binuclear site. Thus proton pumping occurs only to the extent of ~0.1 H⁺/e^-, probably due to the formed membrane potential in the experiment.

Insights into proton translocation in cbb3 oxidase from MD simulations

Heme-copper oxidases are membrane protein complexes that catalyse the final step of the aerobic respiration, namely the reduction of oxygen to water. The energy released during catalysis is coupled to the active translocation of protons across the membrane, which contributes to the establishment of an electrochemical gradient that is used for ATP synthesis. The distinctive C-type (or cbb₃) cytochrome c oxidases, which are mostly present in proteobacteria, exhibit a number of unique structural and functional features, including high catalytic activity at low oxygen concentrations. At the moment, the functioning mechanism of C-type oxidases, in particular the proton transfer/pumping mechanism presumably via a single proton channel, is still poorly understood. In this work we used all-atom molecular dynamics simulations and continuum electrostatics calculations to obtain atomic-level insights into the hydration and dynamics of a cbb₃ oxidase. We provide the details of the water dynamics and proton transfer pathways for both the "chemical" and "pumped" protons, and show that formation of protonic connections is strongly affected by the protonation state of key residues, namely H243, E323 and H337.

Data for molecular dynamics simulations of B-type cytochrome c oxidase with the Amber force field

Cytochrome c oxidase (CcO) is a vital enzyme that catalyzes the reduction of molecular oxygen to water and pumps protons across mitochondrial and bacterial membranes. This article presents parameters for the cofactors of ba₃-type CcO that are compatible with the all-atom Amber ff12SB and ff14SB force fields. Specifically, parameters were developed for the Cu_A pair, heme b, and the dinuclear center that consists of heme a₃ and Cu_B bridged by a hydroperoxo group. The data includes geometries in XYZ coordinate format for cluster models that were employed to compute proton transfer energies and derive bond parameters and point charges for the force field using density functional theory. Also included are the final parameter files that can be employed with the Amber leap program to generate input files for molecular dynamics simulations with the Amber software package. Based on the high resolution (1.8 Å) X-ray crystal structure of the ba₃-type CcO from Thermus thermophilus (Protein Data Bank ID number PDB: 3S8F), we built a model that is embedded in a POPC lipid bilayer membrane and solvated with TIP3P water molecules and counterions. We provide PDB data files of the initial model and the equilibrated model that can be used for further studies.