Role of subunit III and its lipids in the molecular mechanism of cytochrome c oxidase

Identification of proteins involved in intracellular ubiquinone trafficking in Saccharomyces cerevisiae using artificial ubiquinone probe

Ubiquinone (UQ) is an essential player in the respiratory electron transfer system. In Saccharomyces cerevisiae strains lacking the ability to synthesize UQ6, exogenously supplied UQs can be taken up and delivered to mitochondria through an unknown mechanism, restoring the growth of UQ6-deficient yeast in non-fermentable medium. Since elucidating the mechanism responsible may markedly contribute to therapeutic strategies for patients with UQ deficiency, many attempts have been made to identify the machinery involved in UQ trafficking in the yeast model. However, definite experimental evidence of the direct interaction of UQ with a specific protein(s) has not yet been demonstrated. To gain insight into intracellular UQ trafficking via a chemistry-based strategy, we synthesized a hydrophobic UQ probe (pUQ5), which has a photoreactive diazirine group attached to a five-unit isoprenyl chain and a terminal alkyne to visualize and/or capture the labeled proteins via click chemistry. pUQ5 successfully restored the growth of UQ6-deficient S. cerevisiae (Δcoq2) on a non-fermentable carbon source, indicating that this UQ was taken up and delivered to mitochondria, and served as a UQ substrate of respiratory enzymes. Through photoaffinity labeling of the mitochondria isolated from Δcoq2 yeast cells cultured in the presence of pUQ5, we identified many labeled proteins, including voltage-dependent anion channel 1 (VDAC1) and cytochrome c oxidase subunit 3 (Cox3). The physiological relevance of UQ binding to these proteins is discussed.

CG7630 is the Drosophila melanogaster homolog of the cytochrome c oxidase subunit COX7B

The mitochondrial respiratory chain (MRC) is composed of four multiheteromeric enzyme complexes. According to the endosymbiotic origin of mitochondria, eukaryotic MRC derives from ancestral proteobacterial respiratory structures consisting of a minimal set of complexes formed by a few subunits associated with redox prosthetic groups. These enzymes, which are the “core” redox centers of respiration, acquired additional subunits, and increased their complexity throughout evolution. Cytochrome c oxidase (COX), the terminal component of MRC, has a highly interspecific heterogeneous composition. Mammalian COX consists of 14 different polypeptides, of which COX7B is considered the evolutionarily youngest subunit. We applied proteomic, biochemical, and genetic approaches to investigate the COX composition in the invertebrate model Drosophila melanogaster. We identified and characterized a novel subunit which is widely different in amino acid sequence, but similar in secondary and tertiary structures to COX7B, and provided evidence that this object is in fact replacing the latter subunit in virtually all protostome invertebrates. These results demonstrate that although individual structures may differ the composition of COX is functionally conserved between vertebrate and invertebrate species.

Mitochondrial Cytochrome c Oxidase Defects Alter Cellular Homeostasis of Transition Metals

The redox activity of cytochrome c oxidase (COX), the terminal oxidase of the mitochondrial respiratory chain (MRC), depends on the incorporation of iron and copper into its catalytic centers. Many mitochondrial proteins have specific roles for the synthesis and delivery of metal-containing cofactors during COX biogenesis. In addition, a large set of different factors possess other molecular functions as chaperones or translocators that are also necessary for the correct maturation of these complexes. Pathological variants in genes encoding structural MRC subunits and these different assembly factors produce respiratory chain deficiency and lead to mitochondrial disease. COX deficiency in Drosophila melanogaster, induced by downregulated expression of three different assembly factors and one structural subunit, resulted in decreased copper content in the mitochondria accompanied by different degrees of increase in the cytosol. The disturbances in metal homeostasis were not limited only to copper, as some changes in the levels of cytosolic and/or mitochondrial iron, manganase and, especially, zinc were observed in several of the COX-deficient groups. The altered copper and zinc handling in the COX defective models resulted in a transcriptional response decreasing the expression of copper transporters and increasing the expression of metallothioneins. We conclude that COX deficiency is generally responsible for an altered mitochondrial and cellular homeostasis of transition metals, with variations depending on the origin of COX assembly defect.

Tackling Dysfunction of Mitochondrial Bioenergetics in the Brain

Oxidative phosphorylation (OxPhos) is the basic function of mitochondria, although the landscape of mitochondrial functions is continuously growing to include more aspects of cellular homeostasis. Thanks to the application of -omics technologies to the study of the OxPhos system, novel features emerge from the cataloging of novel proteins as mitochondrial thus adding details to the mitochondrial proteome and defining novel metabolic cellular interrelations, especially in the human brain. We focussed on the diversity of bioenergetics demand and different aspects of mitochondrial structure, functions, and dysfunction in the brain. Definition such as 'mitoexome', 'mitoproteome' and 'mitointeractome' have entered the field of 'mitochondrial medicine'. In this context, we reviewed several genetic defects that hamper the last step of aerobic metabolism, mostly involving the nervous tissue as one of the most prominent energy-dependent tissues and, as consequence, as a primary target of mitochondrial dysfunction. The dual genetic origin of the OxPhos complexes is one of the reasons for the complexity of the genotype-phenotype correlation when facing human diseases associated with mitochondrial defects. Such complexity clinically manifests with extremely heterogeneous symptoms, ranging from organ-specific to multisystemic dysfunction with different clinical courses. Finally, we briefly discuss the future directions of the multi-omics study of human brain disorders.

Modular assembly of yeast mitochondrial ATP synthase and cytochrome oxidase

The respiratory pathway of mitochondria is composed of four electron transfer complexes and the ATP synthase. In this article, we review evidence from studies of Saccharomyces cerevisiae that both ATP synthase and cytochrome oxidase (COX) are assembled from independent modules that correspond to structurally and functionally identifiable components of each complex. Biogenesis of the respiratory chain requires a coordinate and balanced expression of gene products that become partner subunits of the same complex, but are encoded in the two physically separated genomes. Current evidence indicates that synthesis of two key mitochondrial encoded subunits of ATP synthase is regulated by the F1 module. Expression of COX1 that codes for a subunit of the COX catalytic core is also regulated by a mechanism that restricts synthesis of this subunit to the availability of a nuclear-encoded translational activator. The respiratory chain must maintain a fixed stoichiometry of the component enzyme complexes during cell growth. We propose that high-molecular-weight complexes composed of Cox6, a subunit of COX, and of the Atp9 subunit of ATP synthase play a key role in establishing the ratio of the two complexes during their assembly.

Mitochondrial disorders of the OXPHOS system

Mitochondrial disorders are among the most frequent inborn errors of metabolism, their primary cause being the dysfunction of the oxidative phosphorylation system (OXPHOS). OXPHOS is composed of the electron transport chain (ETC), formed by four multimeric enzymes and two mobile electron carriers, plus an ATP synthase [also called complex V (cV)]. The ETC performs the redox reactions involved in cellular respiration while generating the proton motive force used by cV to synthesize ATP. OXPHOS biogenesis involves multiple steps, starting from the expression of genes encoded in physically separated genomes, namely the mitochondrial and nuclear DNA, to the coordinated assembly of components and cofactors building each individual complex and eventually the supercomplexes. The genetic cause underlying around half of the diagnosed mitochondrial disease cases is currently known. Many of these cases result from pathogenic variants in genes encoding structural subunits or additional factors directly involved in the assembly of the ETC complexes. Here, we review the historical and most recent findings concerning the clinical phenotypes and the molecular pathological mechanisms underlying this particular group of disorders.

Role of cytochrome c oxidase nuclear-encoded subunits in health and disease

Cytochrome c oxidase (COX), the terminal enzyme of mitochondrial electron transport chain, couples electron transport to oxygen with generation of proton gradient indispensable for the production of vast majority of ATP molecules in mammalian cells. The review summarizes current knowledge of COX structure and function of nuclear-encoded COX subunits, which may modulate enzyme activity according to various conditions. Moreover, some nuclear-encoded subunits posess tissue-specific and development-specific isoforms, possibly enabling fine-tuning of COX function in individual tissues. The importance of nuclear-encoded subunits is emphasized by recently discovered pathogenic mutations in patients with severe mitopathies. In addition, proteins substoichiometrically associated with COX were found to contribute to COX activity regulation and stabilization of the respiratory supercomplexes. Based on the summarized data, a model of three levels of quaternary COX structure is postulated. Individual structural levels correspond to subunits of the i) catalytic center, ii) nuclear-encoded stoichiometric subunits and iii) associated proteins, which may constitute several forms of COX with varying composition and differentially regulated function.

Emerging Diversity in Lipid-Protein Interactions

Membrane lipids interact with proteins in a variety of ways, ranging from providing a stable membrane environment for proteins to being embedded in to detailed roles in complicated and well-regulated protein functions. Experimental and computational advances are converging in a rapidly expanding research area of lipid-protein interactions. Experimentally, the database of high-resolution membrane protein structures is growing, as are capabilities to identify the complex lipid composition of different membranes, to probe the challenging time and length scales of lipid-protein interactions, and to link lipid-protein interactions to protein function in a variety of proteins. Computationally, more accurate membrane models and more powerful computers now enable a detailed look at lipid-protein interactions and increasing overlap with experimental observations for validation and joint interpretation of simulation and experiment. Here we review papers that use computational approaches to study detailed lipid-protein interactions, together with brief experimental and physiological contexts, aiming at comprehensive coverage of simulation papers in the last five years. Overall, a complex picture of lipid-protein interactions emerges, through a range of mechanisms including modulation of the physical properties of the lipid environment, detailed chemical interactions between lipids and proteins, and key functional roles of very specific lipids binding to well-defined binding sites on proteins. Computationally, despite important limitations, molecular dynamics simulations with current computer power and theoretical models are now in an excellent position to answer detailed questions about lipid-protein interactions.

Computational Modeling of Realistic Cell Membranes

Cell membranes contain a large variety of lipid types and are crowded with proteins, endowing them with the plasticity needed to fulfill their key roles in cell functioning. The compositional complexity of cellular membranes gives rise to a heterogeneous lateral organization, which is still poorly understood. Computational models, in particular molecular dynamics simulations and related techniques, have provided important insight into the organizational principles of cell membranes over the past decades. Now, we are witnessing a transition from simulations of simpler membrane models to multicomponent systems, culminating in realistic models of an increasing variety of cell types and organelles. Here, we review the state of the art in the field of realistic membrane simulations and discuss the current limitations and challenges ahead.

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.

Atomistic insights into cardiolipin binding sites of cytochrome c oxidase

Mitochondrial cytochrome c oxidase couples the reduction of oxygen to proton pumping. Despite an overall good understanding of its molecular mechanism, the role of cardiolipin in protein function is not understood. Here, we have studied the cardiolipin-protein interactions in a dynamic context by means of atomistic molecular dynamics simulations performed on the entire structure of monomeric and dimeric forms of the enzyme. Several microseconds of simulation data reveal that the crystallographic cardiolipin molecules that glue two monomers together bind weakly in hybrid and single-component lipid bilayers and dissociate rapidly. Atomistic simulations performed in the absence of tightly bound cardiolipin molecules strongly perturb the structural integrity of subunits III and VIIa, thereby highlighting an indispensable nature of lipid-protein interactions in enzyme function such as proton uptake and oxygen channeling. Our results demonstrate the strength of molecular simulations in providing direct atomic description of lipid-protein processes that are difficult to achieve experimentally.

Oxygen Activation and Energy Conservation by Cytochrome c Oxidase

This review focuses on the type A cytochrome c oxidases (CcO), which are found in all mitochondria and also in several aerobic bacteria. CcO catalyzes the respiratory reduction of dioxygen (O₂) to water by an intriguing mechanism, the details of which are fairly well understood today as a result of research for over four decades. Perhaps even more intriguingly, the membrane-bound CcO couples the O₂ reduction chemistry to translocation of protons across the membrane, thus contributing to generation of the electrochemical proton gradient that is used to drive the synthesis of ATP as catalyzed by the rotary ATP synthase in the same membrane. After reviewing the structure of the core subunits of CcO, the active site, and the transfer paths of electrons, protons, oxygen, and water, we describe the states of the catalytic cycle and point out the few remaining uncertainties. Finally, we discuss the mechanism of proton translocation and the controversies in that area that still prevail.

Adaptive Patterns of Mitogenome Evolution Are Associated with the Loss of Shell Scutes in Turtles

The mitochondrial genome encodes several protein components of the oxidative phosphorylation (OXPHOS) pathway and is critical for aerobic respiration. These proteins have evolved adaptively in many taxa, but linking molecular-level patterns with higher-level attributes (e.g., morphology, physiology) remains a challenge. Turtles are a promising system for exploring mitochondrial genome evolution as different species face distinct respiratory challenges and employ multiple strategies for ensuring efficient respiration. One prominent adaptation to a highly aquatic lifestyle in turtles is the secondary loss of keratenized shell scutes (i.e., soft-shells), which is associated with enhanced swimming ability and, in some species, cutaneous respiration. We used codon models to examine patterns of selection on mitochondrial protein-coding genes along the three turtle lineages that independently evolved soft-shells. We found strong evidence for positive selection along the branches leading to the pig-nosed turtle (Carettochelys insculpta) and the softshells clade (Trionychidae), but only weak evidence for the leatherback (Dermochelys coriacea) branch. Positively selected sites were found to be particularly prevalent in OXPHOS Complex I proteins, especially subunit ND2, along both positively selected lineages, consistent with convergent adaptive evolution. Structural analysis showed that many of the identified sites are within key regions or near residues involved in proton transport, indicating that positive selection may have precipitated substantial changes in mitochondrial function. Overall, our study provides evidence that physiological challenges associated with adaptation to a highly aquatic lifestyle have shaped the evolution of the turtle mitochondrial genome in a lineage-specific manner.

Comparative biochemistry of cytochrome c oxidase in animals

Cytochrome c oxidase (COX), the terminal enzyme of the electron transport system, is central to aerobic metabolism of animals. Many aspects of its structure and function are highly conserved, yet, paradoxically, it is also an important model for studying the evolution of the metabolic phenotype. In this review, part of a special issue honouring Peter Hochachka, we consider the biology of COX from the perspective of comparative and evolutionary biochemistry. The approach is to consider what is known about the enzyme in the context of conventional biochemistry, but focus on how evolutionary researchers have used this background to explore the role of the enzyme in biochemical adaptation of animals. In synthesizing the conventional and evolutionary biochemistry, we hope to identify synergies and future research opportunities. COX represents a rare opportunity for researchers to design studies that span the breadth of biology: molecular genetics, protein biochemistry, enzymology, metabolic physiology, organismal performance, evolutionary biology, and phylogeography.

Evolution of Cytochrome c Oxidase in Hypoxia Tolerant Sculpins (Cottidae, Actinopterygii)

Vertebrate hypoxia tolerance can emerge from modifications to the oxygen (O2) transport cascade, but whether there is adaptive variation to O2 binding at the terminus of this cascade, mitochondrial cytochrome c oxidase (COX), is not known. In order to address the hypothesis that hypoxia tolerance is associated with enhanced O2 binding by mitochondria we undertook a comparative analysis of COX O2 kinetics across species of intertidal sculpins (Cottidae, Actinopterygii) that vary in hypoxia tolerance. Our analysis revealed a significant relationship between hypoxia tolerance (critical O2 tension of O2 consumption rate; Pcrit), mitochondrial O2 binding affinity (O2 tension at which mitochondrial respiration was half maximal; P50), and COX O2-binding affinity (apparent Michaelis-Menten constant for O2 binding to COX; Km,app O2). The more hypoxia tolerant species had both a lower mitochondrial P50 and lower COX Km,app O2, facilitating the maintenance of mitochondrial function to a lower O2 tension than in hypoxia intolerant species. Additionally, hypoxia tolerant species had a lower overall COX Vmax but higher mitochondrial COX respiration rate when expressed relative to maximal electron transport system respiration rate. In silico analyses of the COX3 subunit postulated as the entry point for O2 into the COX protein catalytic core, points to variation in COX3 protein stability (estimated as free energy of unfolding) contributing to the variation in COX Km,app O2. We propose that interactions between COX3 and cardiolipin at four amino acid positions along the same alpha-helix forming the COX3 v-cleft represent likely determinants of interspecific differences in COX Km,app O2.

Role of cardiolipin in stability of integral membrane proteins

Cardiolipin (CL) is a unique phospholipid with a dimeric structure having four acyl chains and two phosphate groups found almost exclusively in certain membranes of bacteria and of mitochondria of eukaryotes. CL interacts with numerous proteins and has been implicated in function and stabilization of several integral membrane proteins (IMPs). While both functional and stabilization roles of CL in IMPs has been generally acknowledged, there are, in fact, only limited number of quantitative analysis that support this function of CL. This is likely caused by relatively complex determination of parameters characterizing stability of IMPs and particularly intricate assessment of role of specific phospholipids such as CL in IMPs stability. This review aims to summarize quantitative findings regarding stabilization role of CL in IMPs reported up to now.

Understanding the essential proton-pumping kinetic gates and decoupling mutations in cytochrome c oxidase

Cytochrome c oxidase (CcO) catalyzes the reduction of oxygen to water and uses the released free energy to pump protons against the transmembrane proton gradient. To better understand the proton-pumping mechanism of the wild-type (WT) CcO, much attention has been given to the mutation of amino acid residues along the proton translocating D-channel that impair, and sometimes decouple, proton pumping from the chemical catalysis. Although their influence has been clearly demonstrated experimentally, the underlying molecular mechanisms of these mutants remain unknown. In this work, we report multiscale reactive molecular dynamics simulations that characterize the free-energy profiles of explicit proton transport through several important D-channel mutants. Our results elucidate the mechanisms by which proton pumping is impaired, thus revealing key kinetic gating features in CcO. In the N139T and N139C mutants, proton back leakage through the D-channel is kinetically favored over proton pumping due to the loss of a kinetic gate in the N139 region. In the N139L mutant, the bulky L139 side chain inhibits timely reprotonation of E286 through the D-channel, which impairs both proton pumping and the chemical reaction. In the S200V/S201V double mutant, the proton affinity of E286 is increased, which slows down both proton pumping and the chemical catalysis. This work thus not only provides insight into the decoupling mechanisms of CcO mutants, but also explains how kinetic gating in the D-channel is imperative to achieving high proton-pumping efficiency in the WT CcO.

Atomistic determinants of co-enzyme Q reduction at the Qi-site of the cytochrome bc1 complex

The cytochrome (cyt) bc₁ complex is an integral component of the respiratory electron transfer chain sustaining the energy needs of organisms ranging from humans to bacteria. Due to its ubiquitous role in the energy metabolism, both the oxidation and reduction of the enzyme’s substrate co-enzyme Q has been studied vigorously. Here, this vast amount of data is reassessed after probing the substrate reduction steps at the Q_i-site of the cyt bc₁ complex of Rhodobacter capsulatus using atomistic molecular dynamics simulations. The simulations suggest that the Lys251 side chain could rotate into the Q_i-site to facilitate binding of half-protonated semiquinone – a reaction intermediate that is potentially formed during substrate reduction. At this bent pose, the Lys251 forms a salt bridge with the Asp252, thus making direct proton transfer possible. In the neutral state, the lysine side chain stays close to the conserved binding location of cardiolipin (CL). This back-and-forth motion between the CL and Asp252 indicates that Lys251 functions as a proton shuttle controlled by pH-dependent negative feedback. The CL/K/D switching, which represents a refinement to the previously described CL/K pathway, fine-tunes the proton transfer process. Lastly, the simulation data was used to formulate a mechanism for reducing the substrate at the Q_i-site.

Tralopyril bioconcentration and effects on the gill proteome of the Mediterranean mussel Mytilus galloprovincialis

Antifouling (AF) systems are used worldwide as one of the most cost-effective ways of protecting submerged structures against heavy biofouling. The emergence of environmentally friendly AF biocides requires knowledge on their environmental fate and toxicity. In this study we measured the bioconcentration of the emerging AF biocide tralopyril (TP) in the Mediterranean mussel Mytilus galloprovincialis and investigated the effects of TP on the mussel gill proteome following acute (2days) and chronic (30days) exposure, as well as after a 10-day depuration period. The experiments were carried out with 1μg/L TP; blank and solvent (5×10(-5)% DMSO) controls were also included. Proteomics analysis was performed by mass spectrometry-based multidimensional protein identification technology (MudPIT). Differentially expressed proteins were identified using a label-free approach based on spectral counts and G-test. Our results show that TP is rapidly accumulated by mussels at concentrations up to 362ng/g dw (whole tissues), reaching steady-state condition within 13days. Ten days of depuration resulted in 80% elimination of accumulated TP from the organism, suggesting that a complete elimination could be reached with longer depuration times. In total, 46 proteins were found to be regulated in the different exposure scenarios. Interestingly, not only TP but also DMSO alone significantly modulated the protein expression in mussel gills following acute and chronic exposure. Both compounds regulated proteins involved in bioenergetics, immune system, active efflux and oxidative stress, often in the opposite way. Alterations of several proteins, notably several cytoskeletal ones, were still observed after the depuration period. These may reflect either the continuing chemical effect due to incomplete elimination or an onset of recovery processes in the mussel gills. Our study shows that exposure of adult mussels to sublethal TP concentration results in the bioconcentration of this biocide in the tissues and modulates the expression of several proteins that may intervene in important metabolic pathways.

Multiscale simulations reveal key features of the proton-pumping mechanism in cytochrome c oxidase

Cytochrome c oxidase (CcO) reduces oxygen to water and uses the released free energy to pump protons across the membrane. We have used multiscale reactive molecular dynamics simulations to explicitly characterize (with free-energy profiles and calculated rates) the internal proton transport events that enable proton pumping during first steps of oxidation of the fully reduced enzyme. Our results show that proton transport from amino acid residue E286 to both the pump loading site (PLS) and to the binuclear center (BNC) are thermodynamically driven by electron transfer from heme a to the BNC, but that the former (i.e., pumping) is kinetically favored whereas the latter (i.e., transfer of the chemical proton) is rate-limiting. The calculated rates agree with experimental measurements. The backflow of the pumped proton from the PLS to E286 and from E286 to the inside of the membrane is prevented by large free-energy barriers for the backflow reactions. Proton transport from E286 to the PLS through the hydrophobic cavity and from D132 to E286 through the D-channel are found to be strongly coupled to dynamical hydration changes in the corresponding pathways and, importantly, vice versa.

Role of charged lipids in membrane structures - Insight given by simulations

Lipids and proteins are the main components of cell membranes. It is becoming increasingly clear that lipids, in addition to providing an environment for proteins to work in, are in many cases also able to modulate the structure and function of those proteins. Particularly charged lipids such as phosphatidylinositols and phosphatidylserines are involved in several examples of such effects. Molecular dynamics simulations have proved an invaluable tool in exploring these aspects. This so-called computational microscope can provide both complementing explanations for the experimental results and guide experiments to fruitful directions. In this paper, we review studies that have utilized molecular dynamics simulations to unravel the roles of charged lipids in membrane structures. We focus on lipids as active constituents of the membranes, affecting both general membrane properties as well as non-lipid membrane components, mainly proteins. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Delipidation of cytochrome c oxidase from Rhodobacter sphaeroides destabilizes its quaternary structure

Delipidation of detergent-solubilized cytochrome c oxidase isolated from Rhodobacter sphaeroides (Rbs-CcO) has no apparent structural and/or functional effect on the protein, however affects its resistance against thermal or chemical denaturation. Phospholipase A2 (PLA2) hydrolysis of phospholipids that are co-purified with the enzyme removes all but two tightly bound phosphatidylethanolamines. Replacement of the removed phospholipids with nonionic detergent decreases both thermal stability of the enzyme and its resilience against the effect of chemical denaturants such as urea. In contrast to nondelipidated Rbs-CcO, the enzymatic activity of PLA2-treated Rbs-CcO is substantially diminished after exposure to high (>4 M) urea concentration at room temperature without an alteration of its secondary structure. Absorbance spectroscopy and sedimentation velocity experiments revealed a strong correlation between intact tertiary structure of heme regions and quaternary structure, respectively, and the enzymatic activity of the protein. We concluded that phospholipid environment of Rbs-CcO has the protective role for stability of its tertiary and quaternary structures.