Oxygen Activation Mechanism at the Binuclear Site of Heme-Copper Oxidase Superfamily as Revealed by Time-Resolved Resonance Raman Spectroscopy. In: Karlin KD, editor. Progress in Inorganic Chemistry. Hoboken: John Wiley & Sons, Inc.; 1996. pp. 431-79. [DOI: 10.1002/9780470166468.ch6]

Factors that Impact Photochemical Cage Escape Yields

The utilization of visible light to mediate chemical reactions in fluid solutions has applications that range from solar fuel production to medicine and organic synthesis. These reactions are typically initiated by electron transfer between a photoexcited dye molecule (a photosensitizer) and a redox-active quencher to yield radical pairs that are intimately associated within a solvent cage. Many of these radicals undergo rapid thermodynamically favored "geminate" recombination and do not diffuse out of the solvent cage that surrounds them. Those that do escape the cage are useful reagents that may undergo subsequent reactions important to the above-mentioned applications. The cage escape process and the factors that determine the yields remain poorly understood despite decades of research motivated by their practical and fundamental importance. Herein, state-of-the-art research on light-induced electron transfer and cage escape that has appeared since the seminal 1972 review by J. P. Lorand entitled "The Cage Effect" is reviewed. This review also provides some background for those new to the field and discusses the cage escape process of both homolytic bond photodissociation and bimolecular light induced electron transfer reactions. The review concludes with some key goals and directions for future research that promise to elevate this very vibrant field to even greater heights.

Investigation of the Mechanism of Membrane Potential Generation by Heme-Copper Respiratory Oxidases in a Real Time Mode

Heme-copper respiratory oxidases are highly efficient molecular machines. These membrane enzymes catalyze the final step of cellular respiration in eukaryotes and many prokaryotes: the transfer of electrons from cytochromes or quinols to molecular oxygen and oxygen reduction to water. The free energy released in this redox reaction is converted by heme-copper respiratory oxidases into the transmembrane gradient of the electrochemical potential of hydrogen ions H+). Heme-copper respiratory oxidases have a unique mechanism for generating H+, namely, a redox-coupled proton pump. A combination of direct electrometric method for measuring the kinetics of membrane potential generation with the methods of prestationary kinetics and site-directed mutagenesis in the studies of heme-copper oxidases allows to obtain a unique information on the translocation of protons inside the proteins in real time. The review summarizes the data of studies employing time-resolved electrometry to decipher the mechanisms of functioning of these important bioenergetic enzymes.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

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.

Infrared and Raman spectroscopic investigation of the reaction mechanism of cytochrome c oxidase

Recent progress in studies on the proton-pumping and O₂reduction mechanisms of cytochrome c oxidase (CcO) elucidated by infrared (IR) and resonance Raman (rR) spectroscopy, is reviewed. CcO is the terminal enzyme of the respiratory chain and its O₂reduction reaction is coupled with H⁺ pumping activity across the inner mitochondrial membrane. The former is catalyzed by heme a3 and its mechanism has been determined using a rR technique, while the latter used the protein moiety and has been investigated with an IR technique. The number of H⁺ relative to e⁻ transferred in the reaction is 1:1, and their coupling is presumably performed by heme a and nearby residues. To perform this function, different parts of the protein need to cooperate with each other spontaneously and sequentially. It is the purpose of this article to describe the structural details on the coupling on the basis of the vibrational spectra of certain specified residues and chromophores involved in the reaction. Recent developments in time-resolved IR and Raman technology concomitant with protein manipulation methods have yielded profound insights into such structural changes. In particular, the new IR techniques that yielded the breakthrough are reviewed and assessed in detail. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

Reaction mechanism of mammalian mitochondrial cytochrome c oxidase

Cytochrome c oxidase (COX) is the terminal oxidase of the mitochondrial respiratory system. This enzyme reduces molecular oxygen (O(2)) to water in a reaction coupled with the pumping of protons across the mitochondrial inner membrane. Progress in investigating the reaction mechanism of this enzyme has been limited by the resolution of its X-ray structure. Bovine heart COX has provided the highest resolution (1.8 Å) X-ray structure presently available among the terminal oxidases. The reaction mechanism of the bovine heart enzyme has been the most extensively studied, particularly with respect to (1) the reduction of O(2) to water without release of reactive oxygen species, (2) the mechanism of coupling between the O(2) reduction process and proton pumping, (3) the structural basis for unidirectional proton transfer (proton pumping), and (4) the effective prevention of proton leakage from the proton-pumping pathway to the proton pathway used for generation of water molecules. In this chapter, we will review recent structural studies of bovine heart COX and discuss the mechanisms described earlier in context of the structural data.

Symmetry states of metalloporphyrin π-cation radicals, models for peroxidase compound I

Oxoferryl porphyrin π-cation radical active sites of compound I intermediates which are found in enzymes such as peroxidases and catalases have been extensively modeled by oxidized synthetic metalloporphyrins. The electronic symmetry states of these compounds were initially assigned on the basis of electronic absorption data. In recent years new experimental and theoretical results have become available which have led to a re-evaluation and modification of the original assignments. A historical perspective of these developments is provided in the context of recent NMR, resonance Raman, and other spectroscopic data and theoretical calculations for the synthetic models and enzymatic systems.

Structural studies on bovine heart cytochrome c oxidase

Among the X-ray structures of bovine heart cytochrome c oxidase (CcO), reported thus far, the highest resolution is 1.8Å. CcO includes 13 different protein subunits, 7 species of phospholipids, 7 species of triglycerides, 4 redox-active metal sites (Cu(A), heme a (Fe(a)), Cu(B), heme a(3) (Fe(a3))) and 3 redox-inactive metal sites (Mg(2+), Zn(2+) and Na(+)). The effects of various O(2) analogs on the X-ray structure suggest that O(2) molecules are transiently trapped at the Cu(B) site before binding to Fe(a3)(2+) to provide O(2)(-). This provides three possible electron transfer pathways from Cu(B), Fe(a3) and Tyr244 via a water molecule. These pathways facilitate non-sequential 3 electron reduction of the bound O(2)(-) to break the OO bond without releasing active oxygen species. Bovine heart CcO has a proton conducting pathway that includes a hydrogen-bond network and a water-channel which, in tandem, connect the positive side phase with the negative side phase. The hydrogen-bond network forms two additional hydrogen-bonds with the formyl and propionate groups of heme a. Thus, upon oxidation of heme a, the positive charge created on Fe(a) is readily delocalized to the heme peripheral groups to drive proton-transport through the hydrogen-bond network. A peptide bond in the hydrogen-bond network and a redox-coupled conformational change in the water channel are expected to effectively block reverse proton transfer through the H-pathway. These functions of the pathway have been confirmed by site-directed mutagenesis of bovine CcO expressed in HeLa cells.

Resonance Raman applications in investigations of cytochrome c oxidase

Recent applications of resonance Raman (RR) spectroscopy in investigations of cytochrome c oxidase (CcO) are reviewed. Red-excited RR spectra for the fully oxidized "as-isolated" CcO tuned to the ligand-to-metal charge transfer absorption band at 655nm exhibit a Raman band at 755cm(-1) assignable to the ν(OO) stretching mode of a peroxide. Binding of CN(-) diminishes the RR band concomitant with the loss of the charge transfer absorption band. This suggests that a peroxide forms a bridge between heme a(3) and Cu(B). Time-resolved RR spectroscopy of whole mitochondria identified a band at 571cm(-1) arising from the oxygenated intermediate at Δt=0.4, 0.6 and 1.4ms. Bands at 804 and 780cm(-1) of the P and F intermediates were observed at Δt=0.6 and 1.4ms, respectively. The coordination geometries of the three intermediates are essentially the same as the respective species observed for solubilized CcO. However, the lifetime of the oxygenated intermediate in mitochondria was significantly longer than the lifetime of this intermediate determined for solubilized CcO. This phenomenon is due either to the pH effect of mitochondrial matrix, the effect of ΔpH and/or ΔΨ across the membrane, or the effect of interactions with other membrane components and/or phospholipids.

Active site intermediates in the reduction of O(2) by cytochrome oxidase, and their derivatives

The mechanism of dioxygen activation and reduction in cell respiration, as catalysed by cytochrome c oxidase, has a long history. The work by Otto Warburg, David Keilin and Britton Chance defined the dioxygen-binding heme iron centre, viz. das Atmungsferment, or cytochrome a(3). Chance brought the field further in the mid-1970's by ingenious low-temperature studies that for the first time identified the primary enzyme-substrate (ES) Michaelis complex of cell respiration, the dioxygen adduct of heme a(3), which he termed Compound A. Further work using optical, resonance Raman, EPR, and other sophisticated spectroscopic techniques, some of which with microsecond time resolution, has brought us to the situation today, where major principles of how O(2) reduction occurs in respiration are well understood. Nonetheless, some questions have remained open, for example concerning the precise structures, catalytic roles, and spectroscopic properties of the breakdown products of Compound A that have been called P, F (for peroxy and ferryl), and O (oxidised). This nomenclature has been known to be inadequate for some time already, and an alternative will be suggested here. In addition, the multiple forms of P, F and O states have been confusing, a situation that we endeavour to help clarifying. The P and F states formed artificially by reacting cytochrome oxidase with hydrogen peroxide are especially scrutinised, and some novel interpretations will be given that may account for previously unexplained observations.

Proton-pumping mechanism of cytochrome C oxidase

Cytochrome c oxidase (CcO), as the terminal oxidase of cellular respiration, coupled with a proton-pumping process, reduces molecular oxygen (O(2)) to water. This intriguing and highly organized chemical process represents one of the most critical aspects of cellular respiration. It employs transition metals (Fe and Cu) at the O(2) reduction site and has been considered one of the most challenging research subjects in life science. Extensive X-ray structural and mutational analyses have provided two different proposals with regard to the mechanism of proton pumping. One mechanism is based on bovine CcO and includes an independent pathway for the pumped protons. The second mechanistic proposal includes a common pathway for the pumped and chemical protons and is based upon bacterial CcO. Here, recent progress in experimental evaluations of these proposals is reviewed and strategies for improving our understanding of the mechanism of this physiologically important process are discussed.

Reaction mechanism of bovine heart cytochrome c oxidase

The 1.9 A resolution X-ray structure of the O2 reduction site of bovine heart cytochrome c oxidase in the fully reduced state indicates trigonal planar coordination of CuB by three histidine residues. One of the three histidine residues has a covalent link to a tyrosine residue to ensure retention of the tyrosine at the O2 reduction site. These moieties facilitate a four electron reduction of O2, and prevent formation of active oxygen species. The combination of a redox-coupled conformational change of an aspartate residue (Asp51) located near the intermembrane surface of the enzyme molecule and the existence of a hydrogen bond network connecting Asp51 to the matrix surface suggest that the proton-pumping process is mediated at Asp51. Mutation analyses using a gene expression system of the Asp51-containing enzyme subunit yield results in support of the proposal that Asp51 plays a critical role in the proton pumping process.

Structures of reaction intermediates of bovine cytochrome c oxidase probed by time-resolved vibrational spectroscopy

Structures of reaction intermediates of bovine cytochrome c oxidase (CcO) in the reactions of its fully reduced form with O2 and fully oxidized form with H2O2 were investigated with time-resolved resonance Raman (RR) and infrared spectroscopy. Six oxygen-associated RR bands were observed for the reaction of CcO with O2. The isotope shifts for an asymmetrically labeled dioxygen, (16)O(18)O, has established that the primary intermediate of cytochrome a3 is an end-on type dioxygen adduct and the subsequent intermediate (P) is an oxoiron species with Fe=O stretch (nu(Fe=O)) at 804/764 cm(-1) for (16)O2/(18)O2 derivatives, although it had been long postulated to be a peroxy species. The P intermediate is converted to the F intermediate with nu(Fe=O) at 785/751 cm(-1) and then to a ferric hydroxy species with nu(Fe-OH) at 450/425 cm(-1) (443/417 cm(-1) in D2O). The rate of reaction from P to F intermediates is significantly slower in D2O than in H2O. The reaction of oxidized CcO with H2O2 yields the same oxygen isotope-sensitive bands as those of P and F, indicating the identity of intermediates. Time-resolved infrared spectroscopy revealed that deprotonation of carboxylic acid side chain takes place upon deligation of a ligand from heme a3. UV RR spectrum gave a prominent band due to cis C=C stretch of phospholipids tightly bound to purified CcO.

X-ray structure and the reaction mechanism of bovine heart cytochrome c oxidase

X-ray structure of bovine heart cytochrome c oxidase in the fully oxidized state shows a peroxide bridging between Fe2+ and Cu2+ in the O2 reduction site. The bond distances for Fe-O and Cu-O are 2.52 and 2.16 A, respectively. The structure is consistent with antiferromagnetic coupling between the two metals, which has long been known and to recent redox titration results [J. Biol. Chem. 274 (1999) 33403]. The trigonal planer coordination of Cu1+ in the O2 reduction site is consistent with the very weak interaction between Cu1+ and O2 bound at Fe2+ revealed by time-resolved resonance Raman investigations. One of the three histidine imidazoles coordinated to the Cu ion in the O2 reduction site fixes a tyrosine phenol group near the O2 reduction site with the direct covalent link between the two groups. The structure suggests that the phenol group is the site for donating protons to the bound O2. Redox-coupled conformational change in an extramembrane loop indicates that an aspartate (Asp51) in the loop apart from the O2 reduction site is the site for proton pumping.

Probing molecular structure of dioxygen reduction site of bacterial quinol oxidases through ligand binding to the redox metal centers

Cytochromes bo and bd are structurally unrelated terminal ubiquinol oxidases in the aerobic respiratory chain of Escherichia coli. The high-spin heme o-CuB binuclear center serves as the dioxygen reduction site for cytochrome bo, and the heme b595-heme d binuclear center for cytochrome bd. CuB coordinates three histidine ligands and serves as a transient ligand binding site en route to high-spin heme o one-electron donor to the oxy intermediate, and a binding site for bridging ligands like cyanide. In addition, it can protect the dioxygen reduction site through binding of a peroxide ion in the resting state, and connects directly or indirectly Tyr288 and Glu286 to carry out redox-driven proton pumping in the catalytic cycle. Contrary, heme b595 of cytochrome bd participate a similar role to CuB in ligand binding and dioxygen reduction but cannot perform such versatile roles because of its rigid structure.

Heme-mediated oxygen activation in biology: cytochrome c oxidase and nitric oxide synthase

Major advances have been made in our understanding of cytochrome c oxidase owing to continued crystallographic work on important intermediates. This, together with a wealth of data derived from selective mutations and sophisticated spectroscopic probes, has provided significant new insights into oxidase dioxygen chemistry and proton pumping activities. Recent advances have also been made for nitric oxide synthase, owing to the crystal structure determination of the heme domain for two of three nitric oxide synthase isoforms.

Proton-coupled electron transfer

Proton-coupled electron transfer (PCET) is an important mechanism for charge transfer in a wide variety of systems including biology- and materials-oriented venues. We review several areas where the transfer of an electron and proton is tightly coupled and discuss model systems that can provide an experimental basis for a test of PCET theory. In a PCET reaction, the electron and proton may transfer consecutively (ET/PT) or concertedly (ETPT). The distinction between these processes is formulated, and rate-constant expressions for the two reaction channels are presented. Methods for the evaluation of these rate constants are discussed that are based on dielectric continuum theory. Electron donor hydrogen-bonded-interface electron acceptor systems displaying PCET reactivity are presented, and the rate-constant expressions corresponding to the ETPT and ET/PT channels for several model reaction complexes are evaluated.

Cytochrome c oxidase: structure and spectroscopy

Cytochrome c oxidase, the terminal enzyme of the respiratory chains of mitochondria and aerobic bacteria, catalyzes electron transfer from cytochrome c to molecular oxygen, reducing the latter to water. Electron transfer is coupled to proton translocation across the membrane, resulting in a proton and charge gradient that is then employed by the F0F1-ATPase to synthesize ATP. Over the last years, substantial progress has been made in our understanding of the structure and function of this enzyme. Spectroscopic techniques such as EPR, absorbance and resonance Raman spectroscopy, in combination with site-directed mutagenesis work, have been successfully applied to elucidate the nature of the cofactors and their ligands, to identify key residues involved in proton transfer, and to gain insight into the catalytic cycle and the structures of its intermediates. Recently, the crystal structures of a bacterial and a mitochondrial cytochrome c oxidase have been determined. In this review, we provide an overview of the crystal structures, summarize recent spectroscopic work, and combine structural and spectroscopic data in discussing mechanistic aspects of the enzyme. For the latter, we focus on the structure of the oxygen intermediates, proton-transfer pathways, and the much-debated issue of how electron transfer in the enzyme might be coupled to proton translocation.

Ferrocyanide-peroxidase activity of cytochrome c oxidase

Redox interaction of mitochondrial cytochrome c oxidase (COX) with ferrocyanide/ferricyanide couple is greatly accelerated by polycations, such as poly-l-lysine [Musatov et al. (1991) Biological Membranes 8, 229-234]. This has allowed us to study ferrocyanide oxidation by COX at very high redox potentials of the ferrocyanide/ferricyanide couple either following spectrophotometrically ferricyanide accumulation or measuring proton uptake associated with water formation in the reaction. At low [ferrocyanide]/[ferricyanide] ratios (Eh values around 500 mV) and ambient oxygen concentration, the ferrocyanide-oxidase activity of COX becomes negligibly small as compared to the reaction rate observed with pure ferrocyanide. Oxidation of ferrocyanide under these conditions, is greatly stimulated by H2O2 or ethylhydroperoxide indicating peroxidatic reaction involved. The ferrocyanide-peroxidase activity of COX is strictly polylysine-dependent and is inhibited by heme a3 ligands such as KCN and NaN3. Apparently the reaction involves normal electron pathway, i.e. electron donation through CuA and oxidation via heme a3. The peroxidase reaction shows a pH-dependence similar to that of the cytochrome c oxidase activity of COX. When COX is preequilibrated with excess H2O2, addition of ferrocyanide shifts the initial steady-state concentrations of the Ferryl-Oxo and Peroxy compounds towards approximately 2:1 ratio of the two intermediates. It is suggested that in the peroxidase cycleferrocyanide donates electrons to both P and F intermediates with a comparable efficiency. Isolation of a partial redox activity of COX opens a possibility to study separately proton translocation coupled to the peroxidase half-reaction of the COX reaction cycle. Copyright 1998

Beef heart cytochrome c oxidase

During the past two years, the crystal structures of beef heart cytochrome c oxidase with 13 subunits and the bacterial enzyme with four subunits have been reported at atomic resolution, ushering in a new era for cytochrome c oxidase research. Different proton pumping mechanisms have been proposed for the two organisms.