Evidence for the protonation of two internal carboxylic groups during the photocycle of bacteriorhodopsin: Investigation of kinetic infrared spectroscopy

Origin of the Difference in Proton Transport Direction between Inward and Outward Proton-Pumping Rhodopsins

ConspectusActive transport is a vital and ubiquitous process in biological phenomena. Ion-pumping rhodopsins are light-driven active ion transporters that share a heptahelical transmembrane structural scaffold in which the all-trans retinal chromophore is covalently bonded through a Schiff base to a conserved lysine residue in the seventh transmembrane helix. Bacteriorhodopsin from Halobacterium salinarum was the first ion-pumping rhodopsin to be discovered and was identified as an outward proton-pumping rhodopsin. Since the discovery of bacteriorhodopsin in 1971, many more ion-pumping rhodopsins have been isolated from diverse microorganisms spanning three domains (bacteria, archaea, and eukaryotes) and giant viruses. In addition to proton-pumping rhodopsins, chloride ion- and sodium ion-pumping rhodopsins have also been discovered. Furthermore, diversity of ion-pumping rhodopsins was found in the direction of ion transport; i.e., rhodopsins that pump protons inward have recently been discovered. Very intriguingly, the inward proton-pumping rhodopsins share structural features and many conserved key residues with the outward proton-pumping rhodopsins. However, a central question remains unchanged despite the increasing variety: how and why do the ion-pumping rhodopsins undergo interlocking conformational changes that allow unidirectional ion transfer within proteins? In this regard, it is an effective strategy to compare the structures and their evolutions in the proton-pumping processes of both inward and outward proton-pumping rhodopsins because the comparison sheds light on key elements for the unidirectional proton transport. We elucidated the proton-pumping mechanism of the inward and outward proton-pumping rhodopsins by time-resolved resonance Raman spectroscopy, a powerful technique for tracking the structural evolutions of proteins at work that are otherwise inaccessible.In this Account, we primarily review our endeavors in the elucidation of the proton-pumping mechanisms and determination factors for the transport directions of inward and outward proton-pumping rhodopsins. We begin with a brief summary of previous findings on outward proton-pumping rhodopsins revealed by vibrational spectroscopy. Next, we provide insights into the mechanism of inward proton-pumping rhodopsins, schizorhodopsins, obtained in our studies. Time-resolved resonance Raman spectroscopy provided valuable information about the structures of the retinal chromophore in the unphotolyzed state and intermediates of schizorhodopsins. As we ventured further into our investigations, we succeeded in uncovering the factors determining the directions of proton release and uptake in the retinal Schiff base. While it is intriguing that the proton-pumping rhodopsins actively transport protons against a concentration gradient, it is even more curious that proteins with structural similarities transport protons in opposite directions. Solving the second mystery led to solving the first. When we considered our findings, we realized that we would probably not have been able to elucidate the mechanism if we had studied only the outward pump. Our Account concludes by outlining future opportunities and challenges in the growing research field of ion-pumping rhodopsins, with a particular emphasis on elucidating their sequence-structure-function relationships. We aim to inspire further advances toward the understanding and creation of light-driven active ion transporters.

Versatile Femtosecond Laser Synchronization for Multiple-Timescale Transient Infrared Spectroscopy

Several ways to electronically synchronize different types of amplified femtosecond laser systems are presented based on a single freely programmable electronics hardware: arbitrary-detuning asynchronous optical sampling (ADASOPS), as well as actively locking two femtosecond laser oscillators, albeit not necessarily to the same round-trip frequency. They allow us to rapidly probe a very wide range of timescales, from picoseconds to potentially seconds, in a single transient absorption experiment without the need to move any delay stage. Experiments become possible that address a largely unexplored aspect of many photochemical reactions, in particular in the context of photo-catalysis as well as photoactive proteins, where an initial femtosecond trigger very often initiates a long-lasting cascade of follow-up processes. The approach is very versatile and allows us to synchronize very different lasers, such as a Ti:Sa amplifier and a 100 kHz Yb-laser system. The jitter of the synchronization, and therewith the time-resolution in the transient experiment, lies in the range from 1 to 3 ps, depending on the method. For illustration, transient IR measurements of the excited state solvation and decay of a metal carbonyl complex as well as the full reaction cycle of bacteriorhodopsin are shown. The pros and cons of the various methods are discussed, with regard to the scientific question one might want to address, and also with regard to the laser systems that might be already existent in a laser lab.

Vibrational Spectroscopy of Phytochromes

Phytochromes are biological photoswitches that translate light into physiological functions. Spectroscopic techniques are essential tools for molecular research into these photoreceptors. This review is directed at summarizing how resonance Raman and IR spectroscopy contributed to an understanding of the structure, dynamics, and reaction mechanism of phytochromes, outlining the substantial experimental and theoretical challenges and describing the strategies to master them. It is shown that the potential of the various vibrational spectroscopic techniques can be most efficiently exploited using integral approaches via a combination of theoretical methods as well as other experimental techniques.

Interdisciplinary biophysical studies of membrane proteins bacteriorhodopsin and rhodopsin

The centenary of the birth of H. Gobind Khorana provides an auspicious opportunity to review the origins and evolution of parallel advances in biophysical methodology and molecular genetics technology used to study membrane proteins. Interdisciplinary work in the Khorana laboratory in the late 1970s and for the next three decades led to productive collaborations and fostered three subsequent scientific generations whose biophysical work on membrane proteins has led to detailed elucidation of the molecular mechanisms of energy transduction by the light-driven proton pump bacteriorhodopsin (bR) and signal transduction by the G protein-coupled receptor (GPCR) rhodopsin. This review will highlight the origins and advances of biophysical studies of membrane proteins made possible by the application of molecular genetics approaches to engineer site-specific alterations of membrane protein structures.

Electrocatalytic evidence of the diversity of the oxygen reaction in the bacterial bd oxidase from different organisms

Cytochrome bd oxidase is a bacterial terminal oxygen reductase that was suggested to enable adaptation to different environments and to confer resistance to stress conditions. An electrocatalytic study of the cyt bd oxidases from Escherichia coli, Corynebacterium glutamicum and Geobacillus thermodenitrificans gives evidence for a different reactivity towards oxygen. An inversion of the redox potential values of the three hemes is found when comparing the enzymes from different bacteria. This inversion can be correlated with different protonated glutamic acids as evidenced by reaction induced FTIR spectroscopy. The influence of the microenvironment of the hemes on the reactivity towards oxygen is discussed.

Infrared Difference Spectroscopy of Proteins: From Bands to Bonds

Infrared difference spectroscopy probes vibrational changes of proteins upon their perturbation. Compared with other spectroscopic methods, it stands out by its sensitivity to the protonation state, H-bonding, and the conformation of different groups in proteins, including the peptide backbone, amino acid side chains, internal water molecules, or cofactors. In particular, the detection of protonation and H-bonding changes in a time-resolved manner, not easily obtained by other techniques, is one of the most successful applications of IR difference spectroscopy. The present review deals with the use of perturbations designed to specifically change the protein between two (or more) functionally relevant states, a strategy often referred to as reaction-induced IR difference spectroscopy. In the first half of this contribution, I review the technique of reaction-induced IR difference spectroscopy of proteins, with special emphasis given to the preparation of suitable samples and their characterization, strategies for the perturbation of proteins, and methodologies for time-resolved measurements (from nanoseconds to minutes). The second half of this contribution focuses on the spectral interpretation. It starts by reviewing how changes in H-bonding, medium polarity, and vibrational coupling affect vibrational frequencies, intensities, and bandwidths. It is followed by band assignments, a crucial aspect mostly performed with the help of isotopic labeling and site-directed mutagenesis, and complemented by integration and interpretation of the results in the context of the studied protein, an aspect increasingly supported by spectral calculations. Selected examples from the literature, predominately but not exclusively from retinal proteins, are used to illustrate the topics covered in this review.

The unusual redox properties of C-type oxidases

Cytochrome cbb₃ (also known as C-type) oxidases belong to the family of heme-copper terminal oxidases which couple at the end of the respiratory chain the reduction of molecular oxygen into water and the pumping of protons across the membrane. They are expressed most often at low pressure of O₂ and they exhibit a low homology of sequence with the cytochrome aa₃ (A-type) oxidases found in mitochondria. Their binuclear active site comprises a high-spin heme b₃ associated with a Cu_B center. The protein also contains one low-spin heme b and 3 hemes c. We address here the redox properties of cbb₃ oxidases from three organisms, Rhodobacter sphaeroides, Vibrio cholerae and Pseudomonas stutzeri by means of electrochemical and spectroscopic techniques. We show that the redox potential of the heme b₃ exhibits a relatively low midpoint potential, as in related cytochrome c-dependent nitric oxide reductases. Potential implications for the coupled electron transfer and proton uptake mechanism of C-type oxidases are discussed.

Lipid-Protein Interactions in the Regulated Betaine Symporter BetP Probed by Infrared Spectroscopy

The Na(+)-coupled betaine symporter BetP senses changes in the membrane state and increasing levels of cytoplasmic K(+) during hyperosmotic stress latter via its C-terminal domain and regulates transport activity according to both stimuli. This intriguing sensing and regulation behavior of BetP was intensively studied in the past. It was shown by several biochemical studies that activation and regulation depends crucially on the lipid composition of the surrounding membrane. In fact, BetP is active and regulated only when negatively charged lipids are present. Recent structural studies have revealed binding of phosphatidylglycerol lipids to functional important parts of BetP, suggesting a functional role of lipid interactions. However, a regulatory role of lipid interactions could only be speculated from the snapshot provided by the crystal structure. Here, we investigate the nature of lipid-protein interactions of BetP reconstituted in closely packed two-dimensional crystals of negatively charged lipids and probed at the molecular level with Fourier transform infrared (FTIR) spectroscopy. The FTIR data indicate that K(+) binding weakens the interaction of BetP especially with the anionic lipid head groups. We suggest a regulation mechanism in which lipid-protein interactions, especially with the C-terminal domain and the functional important gating helices transmembrane helice 3 (TMH3) and TMH12, confine BetP to its down-regulated transport state. As BetP is also activated by changes in the physical state of the membrane, our results point toward a more general mechanism of how active transport can be modified by dynamic lipid-protein interactions.

FTIR studies of metal ligands, networks of hydrogen bonds, and water molecules near the active site Mn₄CaO₅ cluster in Photosystem II

The photosynthetic conversion of water to molecular oxygen is catalyzed by the Mn₄CaO₅ cluster in Photosystem II and provides nearly our entire supply of atmospheric oxygen. The Mn₄CaO₅ cluster accumulates oxidizing equivalents in response to light-driven photochemical events within Photosystem II and then oxidizes two molecules of water to oxygen. The Mn₄CaO₅ cluster converts water to oxygen much more efficiently than any synthetic catalyst because its protein environment carefully controls the cluster's reactivity at each step in its catalytic cycle. This control is achieved by precise choreography of the proton and electron transfer reactions associated with water oxidation and by careful management of substrate (water) access and proton egress. This review describes the FTIR studies undertaken over the past two decades to identify the amino acid residues that are responsible for this control and to determine the role of each. In particular, this review describes the FTIR studies undertaken to characterize the influence of the cluster's metal ligands on its activity, to delineate the proton egress pathways that link the Mn₄CaO₅ cluster with the thylakoid lumen, and to characterize the influence of specific residues on the water molecules that serve as substrate or as participants in the networks of hydrogen bonds that make up the water access and proton egress pathways. This information will improve our understanding of water oxidation by the Mn₄CaO₅ catalyst in Photosystem II and will provide insight into the design of new generations of synthetic catalysts that convert sunlight into useful forms of storable energy. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

Evidence from FTIR difference spectroscopy of an extensive network of hydrogen bonds near the oxygen-evolving Mn(4)Ca cluster of photosystem II involving D1-Glu65, D2-Glu312, and D1-Glu329

Analyses of the refined X-ray crystallographic structures of photosystem II (PSII) at 2.9-3.5 A have revealed the presence of possible channels for the removal of protons from the catalytic Mn(4)Ca cluster during the water-splitting reaction. As an initial attempt to verify these channels experimentally, the presence of a network of hydrogen bonds near the Mn(4)Ca cluster was probed with FTIR difference spectroscopy in a spectral region sensitive to the protonation states of carboxylate residues and, in particular, with a negative band at 1747 cm(-1) that is often observed in the S(2)-minus-S(1) FTIR difference spectrum of PSII from the cyanobacterium Synechocystis sp. PCC 6803. On the basis of its 4 cm(-1) downshift in D(2)O, this band was assigned to the carbonyl stretching vibration (C horizontal lineO) of a protonated carboxylate group whose pK(a) decreases during the S(1) to S(2) transition. The positive charge that forms on the Mn(4)Ca cluster during the S(1) to S(2) transition presumably causes structural perturbations that are transmitted to this carboxylate group via electrostatic interactions and/or an extended network of hydrogen bonds. In an attempt to identify the carboxylate group that gives rise to this band, the FTIR difference spectra of PSII core complexes from the mutants D1-Asp61Ala, D1-Glu65Ala, D1-Glu329Gln, and D2-Glu312Ala were examined. In the X-ray crystallographic models, these are the closest carboxylate residues to the Mn(4)Ca cluster that do not ligate Mn or Ca and all are highly conserved. The 1747 cm(-1) band is present in the S(2)-minus-S(1) FTIR difference spectrum of D1-Asp61Ala but absent from the corresponding spectra of D1-Glu65Ala, D2-Glu312Ala, and D1-Glu329Gln. The band is also sharply diminished in magnitude in the wild type when samples are maintained at a relative humidity of </=85%. It is proposed that D1-Glu65, D2-Glu312, and D1-Glu329 participate in a common network of hydrogen bonds that includes water molecules and the carboxylate group that gives rise to the 1747 cm(-1) band. It is further proposed that the mutation of any of these three residues, or partial dehydration caused by maintaining samples at a relative humidity of <or=85%, disrupts the network sufficiently that the structural perturbations associated with the S(1) to S(2) transition are no longer transmitted to the carboxylate group that gives rise to the 1747 cm(-1) band. Because D1-Glu329 is located approximately 20 A from D1-Glu65 and D2-Glu312, the postulated network of hydrogen bonds must extend for at least 20 A across the lumenal face of the Mn(4)Ca cluster. The D1-Asp61Ala, D1-Glu65Ala, and D2-Glu312Ala mutations also appear to substantially decrease the fraction of PSII reaction centers that undergo the S(3) to S(0) transition in response to a saturating flash. This behavior is consistent with D1-Asp61, D1-Glu65, and D2-Glu312 participating in a dominant proton egress channel that links the Mn(4)Ca cluster with the thylakoid lumen.

Monitoring the redox and protonation dependent contributions of cardiolipin in electrochemically induced FTIR difference spectra of the cytochrome bc(1) complex from yeast

Biochemical studies have shown that cardiolipin is essential for the integrity and activity of the cytochrome bc(1) complex and many other membrane proteins. Recently the direct involvement of a bound cardiolipin molecule (CL) for proton uptake at center N, the site of quinone reduction, was suggested on the basis of a crystallographic study. In the study presented here, we probe the low frequency infrared spectroscopy region as a technique suitable to detect the involvement of the lipids in redox induced reactions of the protein. First the individual infrared spectroscopic features of lipids, typically present in the yeast membrane, have been monitored for different pH values in micelles and vesicles. The pK(a) values for cardiolipin molecule have been observed at 4.7+/-0.3 and 7.9+/-1.3, respectively. Lipid contributions in the electrochemically induced FTIR spectra of the bc(1) complex from yeast have been identified by comparing the spectra of the as isolated form, with samples where the lipids were digested by lipase-A(2). Overall, a noteworthy perturbation in the spectral region typical for the protein backbone can be reported. Interestingly, signals at 1159, 1113, 1039 and 980 cm(-1) have shifted, indicating the perturbation of the protonation state of cardiolipin coupled to the reduction of the hemes. Additional shifts are found and are proposed to reflect lipids reorganizing due to a change in their direct environment upon the redox reaction of the hemes. In addition a small shift in the alpha band from 559 to 556 nm can be seen after lipid depletion, reflecting the interaction with heme b(H) and heme c. Thus, our work highlights the role of lipids in enzyme reactivity and structure.

Microbial rhodopsins: scaffolds for ion pumps, channels, and sensors

Microbial rhodopsins have been intensively researched for the last three decades. Since the discovery of bacteriorhodopsin, the scope of microbial rhodopsins has been considerably extended, not only in view of the large number of family members, but also their functional properties as pumps, sensors, and channels. In this review, we give a short overview of old and newly discovered microbial rhodopsins, the mechanism of signal transfer and ion transfer, and we discuss structural and mechanistic aspects of phototaxis.

Infrared spectroscopy of proteins

This review discusses the application of infrared spectroscopy to the study of proteins. The focus is on the mid-infrared spectral region and the study of protein reactions by reaction-induced infrared difference spectroscopy.

Infrared spectra and molar absorption coefficients of the 20 alpha amino acids in aqueous solutions in the spectral range from 1800 to 500 cm(-1)

In this work, we present the absorption spectra and molar coefficients of all 20 amino acids in aqueous solutions down to 500 cm(-1). The spectral region between 1200 and 500 cm(-1) was yet disregarded for protein infrared spectroscopy, mainly due to the strong H(2)O absorption. Absorption spectra were obtained mainly for physiological relevant pH region. Intense bands for aromatic amino acids, histidine and such with OH group could clearly be identified throughout the given spectral region. For sulfur-containing amino acids cysteine and methionine some strong bands besides the weak carbon-sulfur stretching vibration was shown. Effects of aqueous solution environment, pH, protonation states were discussed, together with previously reported data from theoretical approaches. With this complete set of spectral information application to proteins in the whole mid infrared region could be described precise and the potential of the lower spectral region to study typical cofactor ligands like histidine, shown.

Monitoring redox-dependent contribution of lipids in Fourier transform infrared difference spectra of complex I fromEscherichia coli

Biochemical and crystallographic studies have shown that phospholipids are essential for the integrity and activity of membrane proteins. In the study presented here, we use electrochemically induced Fourier transform infrared (FTIR) spectroscopy to demonstrate variations occurring upon the presence and absence of lipids in NADH:ubiquinone oxidoreductase (complex I) from Escherchia coli by following the C=O vibration of the lipid molecule. Complex I is activated in the presence of lipids. Interestingly, in electrochemically induced FTIR difference spectra of complex I from E. coli, a new signal at 1744/1730 cm(-1) appears after addition of E. coli polar lipids, concomitant with the oxidized or reduced form, respectively. Absorbance spectra of liposomes from mixed lipids at different pH values demonstrate shifts for the carbonyl vibration depending on the environment. On this basis we suggest that lipids, though not redox active themselves, contribute in reaction-induced FTIR difference spectra, if a change occurs in the direct environment of the lipid during the observed reaction or coupled processes.

Fourier transform infrared spectrum of the secondary quinone electron acceptor Q(B) in photosystem II

Fourier transform infrared difference spectra upon single reduction of the secondary quinone electron acceptor Q(B) in photosystem II (PSII), without a contribution from the electron donor-side signals, were obtained for the first time using Mn-depleted PSII core complexes of the thermophilic cyanobacterium Thermosynechococcus elongatus. The Q(B)(-)/Q(B) difference spectrum exhibited a strong C...O stretching band of the semiquinone anion at 1480 cm(-)(1), the frequency higher by 2 cm(-)(1) than that of the corresponding band of Q(A)(-), in agreement with the previous S(2)Q(B)(-)/S(1)Q(B) spectrum of the PSII membranes of spinach [Zhang, H., Fischer, G., and Wydrzynski, T. (1998) Biochemistry 37, 5511-5517]. Also, several peaks originating from the Fermi resonance of coupled His modes with its strongly H-bonded NH vibration were observed in the 2900-2600 cm(-)(1) region, where the peak frequencies were higher by 7-24 cm(-)(1) compared with those of the Q(A)(-)/Q(A) spectrum. These frequency differences suggest that H-bond interactions of the CO groups, especially with a His side chain, are different between Q(B)(-) and Q(A)(-). Furthermore, a prominent positive peak was observed at 1745 cm(-)(1) in the C=O stretching region of COOH or ester groups in the Q(B)(-)/Q(B) spectrum. The peak frequency was unaffected by D(2)O substitution, indicating that this peak does not arise from a COOH group but probably from the 10a-ester C=O group of the pheophytin molecule adjacent to Q(B). The absence of protonation of carboxylic amino acids upon Q(B)(-) formation in contrast to the previous observation in the purple bacterium Rhodobacter sphaeroides suggests that the protonation mechanism of Q(B) in PSII is different from that of bacterial reaction centers.

Sulfur Dehydrogenase of Paracoccus pantotrophus: The Heme-2 Domain of the Molybdoprotein Cytochrome c Complex Is Dispensable for Catalytic Activity

Sulfur dehydrogenase, Sox(CD)(2), is an essential part of the sulfur-oxidizing enzyme system of the chemotrophic bacterium Paracoccus pantotrophus. Sox(CD)(2) is a alpha(2)beta(2) complex composed of the molybdoprotein SoxC (43 442 Da) and the hybrid diheme c-type cytochrome SoxD (37 637 Da). Sox(CD)(2) catalyzes the oxidation of protein-bound sulfur to sulfate with a unique six-electron transfer. Amino acid sequence analysis identified the heme-1 domain of SoxD proteins to be specific for sulfur dehydrogenases and to contain a novel ProCysMetXaaAspCys motif, while the heme-2 domain is related to various cytochromes c(2). Purification of sulfur dehydrogenase without protease inhibitor yielded a dimeric SoxCD(1) complex consisting of SoxC and SoxD(1) of 30 kDa, which contained only the heme-1 domain. The heme-2 domain was isolated as a new cytochrome SoxD(2) of about 13 kDa. Both hemes of SoxD in Sox(CD)(2) are redox-active with midpoint potentials at E(m)1 = 218 +/- 10 mV and E(m)2 = 268 +/- 10 mV, while SoxCD(1) and SoxD(2) both exhibit a midpoint potential of E(m) = 278 +/- 10 mV. Electrochemically induced FTIR difference spectra of Sox(CD)(2), SoxCD(1), and SoxD(2) were distinct. A carboxy group is protonated upon reduction of the SoxD(1) heme but not for SoxD(2). The specific activity of SoxCD(1) and Sox(CD)(2) was identical as was the yield of electrons with thiosulfate in the reconstituted Sox enzyme system. To examine the physiological significance of the heme-2 domain, a mutant was constructed that was deleted for the heme-2 domain, which produced SoxCD(1) and transferred electrons from thiosulfate to oxygen. These data demonstrated the crucial role of the heme-1 domain of SoxD for catalytic activity, electron yield, and transfer of the electrons to the cytoplasmic membrane, while the heme-2 domain mediated the alpha(2)beta(2) tetrameric structure of sulfur dehydrogenase.

A1 Reduction in Intact Cyanobacterial Photosystem I Particles Studied by Time-Resolved Step-Scan Fourier Transform Infrared Difference Spectroscopy and Isotope Labeling

Time-resolved step-scan Fourier transform infrared (FTIR) difference spectroscopy, with 5 mus time resolution, has been used to produce P700(+)A(1)(-)/P700A(1) FTIR difference spectra in intact photosystem I particles from Synechococcus sp. 7002 and Synechocystis sp. 6803 at 77 K. Corresponding spectra were also obtained for fully deuterated photosystem I particles from Synechococcus sp. 7002 as well as fully (15)N- and (13)C-labeled photosystem I particles from Synechocystis sp. 6803. Static P700(+)/P700 FTIR difference spectra at 77 K were also obtained for all of the unlabeled and labeled photosystem I particles. From the time-resolved and static FTIR difference spectra, A(1)(-)/A(1) FTIR difference spectra were constructed. The A(1)(-)/A(1) FTIR difference spectra obtained for unlabeled trimeric photosystem I particles from both cyanobacterial strains are very similar. There are some mode frequency differences in spectra obtained for monomeric and trimeric PS I particles. However, the spectra can be interpreted in an identical manner, with the proposed band assignments being compatible with all of the data obtained for labeled and unlabeled photosystem I particles. In A(1)(-)/A(1) FTIR difference spectra obtained for unlabeled photosystem I particles, negative bands are observed at 1559 and 1549-1546 cm(-)(1). These bands are assigned to amide II protein vibrations, as they downshift approximately 86 cm(-)(1) upon deuteration and approximately 13 cm(-)(1) upon (15)N labeling. Difference band features at 1674-1677(+) and 1666(-) cm(-)(1) display isotope-induced shifts that are consistent with these bands being due to amide I protein vibrations. The observed amide modes suggest alteration of the protein backbone (possibly in the vicinity of A(1)) upon A(1) reduction. A difference band at 1754(+)/1748(-) cm(-)(1) is observed in unlabeled spectra from both strains. The frequency of this difference band, as well as the observed isotope-induced shifts, indicate that this difference band is due to a 13(3) ester carbonyl group of chlorophyll a species, most likely the A(0) chlorophyll a molecule that is in close proximity to A(1). Thus A(1) reduction perturbs A(0), probably via a long-range electrostatic interaction. A negative band is observed at 1693 cm(-)(1). The isotope shifts associated with this band are consistent with this band being due to the 13(1) keto carbonyl group of chlorophyll a, again, most likely the 13(1) keto carbonyl group of the A(0) chlorophyll a that is close to A(1). Semiquinone anion bands are resolved at approximately 1495(+) and approximately 1414(+) cm(-)(1) in the A(1)(-)/A(1) FTIR difference spectra for photosystem I particles from both cyanobacterial strains. The isotope-induced shifts of these bands could suggest that the 1495(+) and 1414(+) cm(-)(1) bands are due to C-O and C-C modes of A(1)(-), respectively.

FTIR difference spectroscopy in combination with isotope labeling for identification of the carbonyl modes of P700 and P700+ in photosystem I

Room temperature, light induced (P700(+)-P700) Fourier transform infrared (FTIR) difference spectra have been obtained using photosystem I (PS I) particles from Synechocystis sp. PCC 6803 that are unlabeled, uniformly (2)H labeled, and uniformly (15)N labeled. Spectra were also obtained for PS I particles that had been extensively washed and incubated in D(2)O. Previously, we have found that extensive washing and incubation of PS I samples in D(2)O does not alter the (P700(+)-P700) FTIR difference spectrum, even with approximately 50% proton exchange. This indicates that the P700 binding site is inaccessible to solvent water. Upon uniform (2)H labeling of PS I, however, the (P700(+)-P700) FTIR difference spectra are considerably altered. From spectra obtained using PS I particles grown in D(2)O and H(2)O, a ((1)H-(2)H) isotope edited double difference spectrum was constructed, and it is shown that all difference bands associated with ester/keto carbonyl modes of the chlorophylls of P700 and P700(+) downshift 4-5/1-3 cm(-1) upon (2)H labeling, respectively. It is also shown that the ester and keto carbonyl modes of the chlorophylls of P700 need not be heterogeneously distributed in frequency. Finally, we find no evidence for the presence of a cysteine mode in our difference spectra. The spectrum obtained using (2)H labeled PS I particles indicates that a negative difference band at 1698 cm(-1) is associated with at least two species. The observed (15)N and (2)H induced band shifts strongly support the idea that the two species are the 13(1) keto carbonyl modes of both chlorophylls of P700. We also show that a negative difference band at approximately 1639 cm(-1) is somewhat modified in intensity, but unaltered in frequency, upon (2)H labeling. This indicates that this band is not associated with a strongly hydrogen bonded keto carbonyl mode of one of the chlorophylls of P700.

Role of Arginine-82 in Fast Proton Release during the Bacteriorhodopsin Photocycle: A Time-Resolved FT-IR Study of Purple Membranes Containing 15N-Labeled Arginine

Arginine-82 has long been recognized as an important residue in bacteriorhodopsin (bR), because its mutation usually results in loss of fast H(+) release, an important step in the normal light-induced H(+) transport mechanism. To help to clarify the structural changes in Arg-82 associated with the H(+)-release step, we have measured time-resolved FT-IR difference spectra of wild-type bR containing either natural-abundance isotopes ((14)N-Arg-bR) or all seven arginines selectively and uniformly labeled with (15)N at the two eta-nitrogens ((15)N-Arg-bR). Comparison of the spectra from the two isotopic variants shows that a 1556 cm(-1) vibrational difference band due to the M photocycle intermediate of (14)N-Arg-bR loses substantial intensity in (15)N-Arg-bR. However, this isotope-sensitive arginine vibrational difference band is only observed at pH 7 and not at pH 4 where fast H(+) release is blocked. These observations support the earlier conclusion, based on site-directed mutagenesis and chemical labeling, that a strong C-N stretch vibration of Arg-82 can be assigned to a highly perturbed frequency near 1555 cm(-1) in the M state of wild-type bR [Hutson et al. (2000) Biochemistry 39, 13189-13200]. Furthermore, alkylguanidine model compound spectra indicate that the unusually low arginine C-N stretch frequency in the M state is consistent with a nearly stoichiometric light-induced deprotonation of an arginine side chain within bR, presumably arginine-82.

FTIR spectroscopic characterization of the cytochrome aa3 from Acidianus ambivalens: evidence for the involvement of acidic residues in redox coupled proton translocation

The aa(3)-type quinol oxidase from Acidianus ambivalens is a divergent member of the heme-copper oxidases superfamily, namely, concerning the putative channels for intraprotein proton conduction. In this study, we used electrochemically induced FTIR difference spectroscopy to identify residues involved in redox-coupled protonation changes. In the spectral region characteristic for the nu(C=O) mode from protonated aspartic or glutamic acid side chains, a number of prominent features can be observed between 1790 and 1710 cm(-)(1), clearly indicating the reorganization or protonation of more than four protonatable residues upon electron transfer. A direct comparison of the Fourier-transform infrared difference spectra at different pH values reveals the noteworthy high pK of >8 for some of these residues, and the protonation of two of them. These acidic residues may play a role in the proton transport to the oxygen reducing site, in proton pumping pathways, or in protonation reactions concomitant with quinone reduction. Whereas the residues contributing between 1790 and 1750 cm(-)(1) have the typical position of an aspartic/glutamic acid side chain buried in the protein, a position closer to the surface is suggested for the residues contributing below approximately 1730 cm(-)(1). The possible involvement of residues contributing between 1750 and 1720 cm(-)(1) in the quinone binding site is demonstrated on the basis of experiments in the presence and absence of ubiquinone-2 and of the native electron carrier of the A. ambivalens respiratory chain, caldariella quinone. Most signals seen here are not observable in comparable spectra of typical members of the heme copper oxidase superfamily and thus reflect unique features of the enzyme from the hyperthermoacidophilic archaeon A. ambivalens.

A Fourier transform infrared study of Neurospora rhodopsin: similarities with archaeal rhodopsins

The NOP-1 gene from the eukaryote Neurospora crassa, a filamentous fungus, has recently been shown to encode an archaeal rhodopsin-like protein NOP-1. To explore the functional mechanism of NOP-1 and its possible similarities to archaeal and visual rhodopsins, static and time-resolved Fourier transform infrared difference spectra were measured from wild-type NOP-1 and from a mutant containing an Asp-->Glu substitution in the Schiff base (SB) counterion, Asp131 (D131E). Several conclusions could be drawn about the molecular mechanism of NOP-1: (1) the NOP-1 retinylidene chromophore undergoes an all-trans to 13-cis isomerization, which is typical of archaeal rhodopsins, and closely resembles structural changes of the chromophore in sensory rhodopsin II; (2) the NOP-1 SB counterion, Asp131, has a very similar environment and behavior compared with the SB counterions in bacteriorhodopsin (BR) and sensory rhodopsin II; (3) the O-H stretching of a structurally active water molecule(s) in NOP-1 is similar to water detected in BR and is most likely located near the SB and SB counterion in these proteins; and (4) one or more cysteine residues undergo structural changes during the NOP-1 photocycle. Overall, these results indicate that many features of the active sites of the archaeal rhodopsins are conserved in NOP-1, despite its eukaryotic origin.

Time-resolved long-lived infrared emission from bacteriorhodopsin during its photocycle

The infrared emission observed below 2000 cm(-1) upon exciting retinal in bacteriorhodopsin (bR) is found to have a rise time in the submicrosecond time regime and to relax with two exponential components on the submillisecond to millisecond time scale. These time scales, together with the assignment of this emission to hot vibrations from the all-trans retinal (in bR) and the 13-cis retinal (in the K intermediate), support the recent assignment of the J-intermediate as an electronically excited species (Atkinson et al., J. Phys. Chem. A. 104:4130-4139, 2000) rather than a vibrationally hot K intermediate. A discussion of these time scales of the observed infrared emission is given in terms of the competition between radiative and nonradiative relaxation processes of the vibrational states involved.

Proton transfers in the photochemical reaction cycle of proteorhodopsin

The spectral and photochemical properties of proteorhodopsin (PR) were determined to compare its proton transport steps to those of bacteriorhodopsin (BR). Static and time-resolved measurements on wild-type PR and several mutants were done in the visible and infrared (FTIR and FT-Raman). Assignment of the observed C=O stretch bands indicated that Asp-97 and Glu-108 serve as the proton acceptor and donor, respectively, to the retinal Schiff base, as do the residues at corresponding positions in BR, but there are numerous spectral and kinetic differences between the two proteins. There is no detectable dark-adaptation in PR, and the chromophore contains nearly entirely all-trans retinal. Because the pK(a) of Asp-97 is relatively high (7.1), the proton-transporting photocycle is produced only at alkaline pH. It contains at least seven transient states with decay times in the range from 10 micros to 200 ms, but the analysis reveals only three distinct spectral forms. The first is a red-shifted K-like state. Proton release does not occur during the very slow (several milliseconds) rise of the second, M-like, intermediate, consistent with lack of the residues facilitating extracellular proton release in BR. Proton uptake from the bulk, presumably on the cytoplasmic side, takes place prior to release (tau approximately 2 ms), and coincident with reprotonation of the retinal Schiff base. The intermediate produced by this process contains 13-cis retinal as does the N state of BR, but its absorption maximum is red-shifted relative to PR (like the O state of BR). The decay of this N-like state is coupled to reisomerization of the retinal to all-trans, and produces a state that is O-like in its C-C stretch bands, but has an absorption maximum apparently close to that of unphotolyzed PR.

FTIR spectroscopic evidence for the involvement of an acidic residue in quinone binding in cytochrome bd from Escherichia coli

In this work, FTIR difference spectroscopy is used to search for possible binding partners and protonable groups involved in the binding of the quinol to cytochrome bd from Escherichia coli. In addition, the electrochemically induced FTIR difference spectra are compared for preparations of the enzyme isolated from cells grown at different oxygen levels in which the quinone content of the membrane is altered. On this basis, difference signals can be tentatively attributed to the vibrational modes of the different quinones types that are associated with the enzyme depending on growth conditions. Furthermore, vibrational modes due to the redox-dependent reorganization of the protein vary depending on the quinone associated with the isolated enzyme. Of particular interest are the observations that a mode at 1738 cm(-1) is decreased and a mode at 1595 cm(-1) is increased as observed in direct comparison to the data obtained from samples grown anaerobically. These signals indicate a change in the protonation state of an aspartic or glutamic acid. Since these changes are observed when the ubiquinone ratio in the preparation increases, the data provide evidence for the modulation of the binding site by the interacting quinone and the involvement of an acidic group in the binding site. The tentative assignments of the vibrational modes are supported by electrochemically induced FTIR difference spectra of cytochrome bd in the presence of the specific quinone binding site inhibitors heptylhydroxyquinoline-N-oxide (HQNO) or 2-methyl-3-undecylquinolone-4. Whereas HQNO leads to strong shifts in the FTIR redox difference spectrum, 2-methyl-3-undecylquinolone-4 induces a specific shift of a mode at 1635 cm(-1), which likely originates from the displacement of the C=O group of the bound quinone.

Exploring the function of Tyr83 in bacteriorhodopsin: features of the Y83F and Y83N mutants

Tyrosine-83, a residue which is conserved in all halobacterial retinal proteins, is located at the extracellular side in helix C of bacteriorhodopsin. Structural studies indicate that its hydroxyl group is hydrogen bonded to Trp189 and possibly to Glu194, a residue which is part of the proton release complex (PRC) in bacteriorhodopsin. To elucidate the role of Tyr83 in proton transport, we studied the Y83F and Y83N mutants. The Y83F mutation causes an 11 nm blue shift of the absorption spectrum and decreases the size of the absorption changes seen upon dark adaptation. The light-induced fast proton release, which accompanies formation of the M intermediate, is observed only at pH above 7 in Y83F. The pK(a) of the PRC in M is elevated in Y83F to about 7.3 (compared to 5.8 in WT). The rate of the recovery of the initial state (the rate of the O --> BR transition) and light-induced proton release at pH below 7 is very slow in Y83F (ca. 30 ms at pH 6). The amount of the O intermediate is decreased in Y83F despite the longer lifetime of O. The Y83N mutant shows a similar phenotype in respect to proton release. As in Y83F, the recovery of the initial state is slowed several fold in Y83N. The O intermediate is not seen in this mutant. The data indicate that the PRC is functional in Y83F and Y83N but its pK(a) in M is increased by about 1.5 pK units compared to the WT. This suggests that Tyr83 is not the main source for the proton released upon M formation in the WT; however, Tyr83 is involved in the proton release affecting the pK(a) of the PRC in M and the rate of proton transport from Asp85 to PRC during the O --> bR transition. Both the Y83F and the Y83N mutations lead to a greatly decreased functionality of the pigment at high pH because most of the pigment is converted into the inactive P480 species, with a pK(a) 8-9.

Time-resolved Fourier transform infrared spectroscopy of the polarizable proton continua and the proton pump mechanism of bacteriorhodopsin

Nanosecond-to-microsecond time-resolved Fourier transform infrared (FTIR) spectroscopy in the 3000-1000-cm(-1) region has been used to examine the polarizable proton continua observed in bacteriorhodopsin (bR) during its photocycle. The difference in the transient FTIR spectra in the time domain between 20 ns and 1 ms shows a broad absorption continuum band in the 2100-1800-cm(-1) region, a bleach continuum band in the 2500-2150-cm(-1) region, and a bleach continuum band above 2700 cm(-1). According to Zundel (G., J. Mol. Struct. 322:33-42), these continua appear in systems capable of forming polarizable hydrogen bonds. The formation of a bleach continuum suggests the presence of a polarizable proton in the ground state that changes during the photocycle. The appearance of a transient absorption continuum suggests a change in the polarizable proton or the appearance of new ones. It is found that each continuum has a rise time of less than 80 ns and a decay time component of approximately 300 micros. In addition, it is found that the absorption continuum in the 2100-1800-cm(-1) region has a slow rise component of 190 ns and a fast decay component of approximately 60 micros. Using these results and those of the recent x-ray structural studies of bR(570) and M(412) (H. Luecke, B. Schobert, H.T. Richter, J.-P. Cartailler, and J. K., Science 286:255-260), together with the already known spectroscopic properties of the different intermediates in the photocycle, the possible origins of the polarizable protons giving rise to these continua during the bR photocycle are proposed. Models of the proton pump are discussed in terms of the changes in these polarizable protons and the hydrogen-bonded chains and in terms of previously known results such as the simultaneous deprotonation of the protonated Schiff base (PSB) and Tyr185 and the disappearance of water molecules in the proton release channel during the proton pump process.

Trapping and Spectroscopic Identification of the Photointermediates of Bacteriorhodopsin at Low Temperatures¶

Light-driven transmembrane proton pumping by bacteriorhodopsin occurs in the photochemical cycle, which includes a number of spectroscopically identifiable intermediates. The development of methods to crystallize bacteriorhodopsin have allowed it to be studied with high-resolution X-ray diffraction, opening the possibility to advance substantially our knowledge of the structure and mechanism of this light-driven proton pump. A key step is to obtain the structures of the intermediate states formed during the photocycle of bacteriorhodopsin. One difficulty in these studies is how to trap selectively the intermediates at low temperatures and determine quantitatively their amounts in a photosteady state. In this paper we review the procedures for trapping the K, L, M and N intermediates of the bacteriorhodopsin photocycle and describe the difference absorption spectra accompanying the transformation of the all-trans-bacteriorhodopsin into each intermediate. This provides the means for quantitative analysis of the light-induced mixtures of different intermediates produced by illumination of the pigment at low temperatures.

Role of internal water molecules in bacteriorhodopsin

Internal water molecules are considered to play a crucial role in the functional processes of proton pump proteins. They may participate in hydrogen-bonding networks inside proteins that constitute proton pathways. In addition, they could participate in the switch reaction by mediating an essential proton transfer at the active site. Nevertheless, little has been known about the structure and function of internal water molecules in such proteins. Recent progress in infrared spectroscopy and X-ray crystallography provided new information on water molecules inside bacteriorhodopsin, the light-driven proton pump. The accumulated knowledge on bacteriorhodopsin in the last decade of the 20th century will lead to a realistic picture of internal water molecules at work in the 21st century. In this review, I describe how the role of water molecules has been studied in bacteriorhodopsin, and what should be known about the role of water molecules in the future.

Two groups control light-induced Schiff base deprotonation and the proton affinity of Asp85 in the Arg82 his mutant of bacteriorhodopsin

Arg(82) is one of the four buried charged residues in the retinal binding pocket of bacteriorhodopsin (bR). Previous studies show that Arg(82) controls the pK(a)s of Asp(85) and the proton release group and is essential for fast light-induced proton release. To further investigate the role of Arg(82) in light-induced proton pumping, we replaced Arg(82) with histidine and studied the resulting pigment and its photochemical properties. The main pK(a) of the purple-to-blue transition (pK(a) of Asp(85)) is unusually low in R82H: 1.0 versus 2.6 in wild type (WT). At pH 3, the pigment is purple and shows light and dark adaptation, but almost no light-induced Schiff base deprotonation (formation of the M intermediate) is observed. As the pH is increased from 3 to 7 the M yield increases with pK(a) 4.5 to a value approximately 40% of that in the WT. A transition with a similar pK(a) is observed in the pH dependence of the rate constant of dark adaptation, k(da). These data can be explained, assuming that some group deprotonates with pK(a) 4.5, causing an increase in the pK(a) of Asp(85) and thus affecting k(da) and the yield of M. As the pH is increased from 7 to 10.5 there is a further 2.5-fold increase in the yield of M and a decrease in its rise time from 200 micros to 75 micros with pK(a) 9. 4. The chromophore absorption band undergoes a 4-nm red shift with a similar pK(a). We assume that at high pH, the proton release group deprotonates in the unphotolyzed pigment, causing a transformation of the pigment into a red-shifted "alkaline" form which has a faster rate of light-induced Schiff base deprotonation. The pH dependence of proton release shows that coupling between Asp(85) and the proton release group is weakened in R82H. The pK(a) of the proton release group in M is 7.2 (versus 5.8 in the WT). At pH < 7, most of the proton release occurs during O --> bR transition with tau approximately 45 ms. This transition is slowed in R82H, indicating that Arg(82) is important for the proton transfer from Asp(85) to the proton release group. A model describing the interaction of Asp(85) with two ionizable residues is proposed to describe the pH dependence of light-induced Schiff base deprotonation and proton release.

Bioenergetics of the Archaea

In the late 1970s, on the basis of rRNA phylogeny, Archaea (archaebacteria) was identified as a distinct domain of life besides Bacteria (eubacteria) and Eucarya. Though forming a separate domain, Archaea display an enormous diversity of lifestyles and metabolic capabilities. Many archaeal species are adapted to extreme environments with respect to salinity, temperatures around the boiling point of water, and/or extremely alkaline or acidic pH. This has posed the challenge of studying the molecular and mechanistic bases on which these organisms can cope with such adverse conditions. This review considers our cumulative knowledge on archaeal mechanisms of primary energy conservation, in relationship to those of bacteria and eucarya. Although the universal principle of chemiosmotic energy conservation also holds for Archaea, distinct features have been discovered with respect to novel ion-transducing, membrane-residing protein complexes and the use of novel cofactors in bioenergetics of methanogenesis. From aerobically respiring Archaea, unusual electron-transporting supercomplexes could be isolated and functionally resolved, and a proposal on the organization of archaeal electron transport chains has been presented. The unique functions of archaeal rhodopsins as sensory systems and as proton or chloride pumps have been elucidated on the basis of recent structural information on the atomic scale. Whereas components of methanogenesis and of phototrophic energy transduction in halobacteria appear to be unique to Archaea, respiratory complexes and the ATP synthase exhibit some chimeric features with respect to their evolutionary origin. Nevertheless, archaeal ATP synthases are to be considered distinct members of this family of secondary energy transducers. A major challenge to future investigations is the development of archaeal genetic transformation systems, in order to gain access to the regulation of bioenergetic systems and to overproducers of archaeal membrane proteins as a prerequisite for their crystallization.

Opening the Schiff base moiety of bacteriorhodopsin by mutation of the four extracellular Glu side chains

The quadruple bacteriorhodopsin (BR) mutant E9Q+E74Q+E194Q+E204Q shows a lambda(max) of about 500 nm in water at neutral pH and a great influence of pH and salts on the visible absorption spectrum. Accessibility to the Schiff base is strongly increased, as detected by the rapid bleaching effect of hydroxylamine in the dark as well as in light. Both the proton release kinetics and the photocycle are altered, as indicated by a delayed proton release after proton uptake and changed M kinetics. Moreover, affinity of the color-controlling cation(s) is found to be decreased. We suggest that the four Glu side chains are essential elements of the extracellular structure of BR.

Time-resolved step-scan Fourier transform infrared spectroscopy reveals differences between early and late M intermediates of bacteriorhodopsin

In this report, from time-resolved step-scan Fourier transform infrared investigations from 15 ns to 160 ms, we provide evidence for the subsequent rise of three different M states that differ in their structures. The first state rises with approximately 3 microseconds to only a small percentage. Its structure as judged from amide I/II bands differs in small but well-defined aspects from the L state. The next M state, which appears in approximately 40 microseconds, has almost all of the characteristics of the "late" M state, i.e., it differs considerably from the first one. Here, the L left arrow over right arrow M equilibrium is shifted toward M, although some percentage of L still persists. In the last M state (rise time approximately 130 microseconds), the equilibrium is shifted toward full deprotonation of the Schiff base, and only small additional structural changes take place. In addition to these results obtained for unbuffered conditions or at pH 7, experiments performed at lower and higher pH are presented. These results are discussed in terms of the molecular changes postulated to occur in the M intermediate to allow the shift of the L/M equilibrium toward M and possibly to regulate the change of the accessibility of the Schiff base necessary for effective proton pumping.

Characterization of the photoreduction of the secondary quinone QB in the photosynthetic reaction center from rhodobacter capsulatus with FTIR spectroscopy

The photoreduction of the secondary quinone acceptor QB in reaction centers (RCs) of the photosynthetic bacteria Rhodobacter (Rb.) capsulatus has been investigated by light-induced FTIR difference spectroscopy in 1H2O and 2H2O. The Q-B/QB FTIR spectra reflect reorganization of the protein upon electron transfer, changes of protonation state of carboxylic acid groups, and (semi)quinone-protein interactions. As expected from the conservation of most of the amino acids near QB in the RCs from Rb. capsulatus and Rb. sphaeroides, several protein and quinone IR bands are common to both spectra, e.g., the 1728 cm-1 band is assigned to proton uptake by a carboxylic acid residue, most probably Glu L212 as previously proposed for Rb. sphaeroides RCs. However, noticeable changes are observed at 1709 cm-1 (deprotonation of a Glu or Asp residue), 1674 and 1659 cm-1 (side chain and/or backbone), around 1540 cm-1 (amide II), and in the semiquinone absorption range. This FTIR study demonstrates that the environment of the secondary quinone in Rb. capsulatus is close but not identical to that in Rb. sphaeroides suggesting slight differences in the structural organization of side chains and/or ordered water molecules near QB.

The proton release group of bacteriorhodopsin controls the rate of the final step of its photocycle at low pH

The factors determining the pH dependence of the formation and decay of the O photointermediate of the bacteriorhodopsin (bR) photocycle were investigated in the wild-type (WT) pigment and in the mutants of Glu-194 and Glu-204, key residues of the proton release group (PRG) in bR. We have found that in the WT the rate constant of O --> bR transition decreases 30-fold upon decreasing the pH from 6 to 3 with a pKa of about 4.3. D2O slows the rise and decay of the O intermediate in the WT at pH 3.5 by a factor of 5.5. We suggest that the rate of the O --> bR transition (which reflects the rate of deprotonation of the primary proton acceptor Asp-85) at low pH is controlled by the deprotonation of the PRG. To test this hypothesis, we studied the E194D mutant. We show that the pKa of the PRG in the ground state of the E194D mutant, when Asp-85 is protonated, is increased by 1.2 pK units compared to that of the WT. We found a similar increase in the pKa of the rate constant of the O --> bR transition in E194D. This provides further evidence that the rate of the O --> bR transition is controlled by the PRG. In a further test, the E194Q mutation, which disables the PRG and slows proton release, almost completely eliminates the pH dependence of O decay at pHs below 6. A second phenomenon we investigated was that in the WT at neutral and alkaline pH the fraction of the O intermediate decreases with pKa 7.5. A similar pH dependence is observed in the mutants in which the PRG is disabled, E194Q and E204Q, suggesting that the decrease in the fraction of the O intermediate with pKa ca. 7.5 is not controlled by the PRG. We propose that the group with pKa 7.5 is Asp-96. The slowing of the reprotonation of Asp-96 at high pH is the cause of the decrease in the rate of the N --> O transition, leading to the decrease in the fraction of O.