The location of cytochrome c on the surface of ultrathin lipid multilayer films using x-ray diffraction.

X-ray diffraction and spectroscopic techniques were used to characterize ultrathin fatty acid multilayers having a bound surface layer of cytochrome c. Three to six monolayers of arachidic acid were deposited onto an alkylated glass surface, using the Langmuir-Blodgett method. These fatty acid multilayer films were stored either in a 1 mM NaHCO3 pH 7.5 solution or a buffered 10 microM cytochrome c solution, pH 7.5. After washing extensively with buffer, these multilayer films were assayed for bound cytochrome c by optical spectroscopy. It was found that the cytochrome c bound only to the odd-numbered monolayer films (which have hydrophilic surfaces). The theoretical number of cytochrome c molecules bound to the ultrathin multilayer films having three or five monolayers was calculated as N = 1.2 x 10(13)/cm2 (assuming a hexagonally close-packed monolayer of protein), which would produce an optical density of 0.002 at a wavelength of 550 nm; for a three or five monolayer ultrathin film that was incubated with cytochrome c, OD550 approximately equal to 0.002. The protein was released from the film when treated with greater than 100 mM KCl solution, as would be expected for an electrostatic interaction. Meridional x-ray diffraction data were collected from the arachidic acid films with and without a bound cytochrome c layer. A box refinement technique, previously shown to be effective in deriving the profile structures of nonperiodic ultrathin films, was used to determine the multilayer electron density profiles. The electron density profiles and their autocorrelation functions showed that bound cytochrome c resulted in an additional electron dense feature on the multilayer surface, consistent with a bound cytochrome c monolayer. The position of the bound protein relative to the multilayer surface was independent of the number of fatty acid monolayers in the multilayer. Future studies will use these methods to investigate the structures of membrane protein complexes bound directly to the surface of multilayer films.     

Structural investigation of the covalent and electrostatic binding of yeast cytochrome c to the surface of various ultrathin lipid multilayers using x-ray diffraction.

X-Ray diffraction was used to characterize the profile structures of ultrathin lipid multilayers having a bound surface layer of cytochrome c. The lipid multilayers were formed on an alkylated glass surface, using the Langmuir-Blodgett method. The ultrathin lipid multilayers of this study were: five monolayers of arachidic acid, four monolayers of arachidic acid with a surface monolayer of dimyristoyl phosphatidylserine, and four monolayers of arachidic acid acid with a surface monolayer of thioethyl stearate. Both the phosphatidylserine and the thioethyl stearate surfaces were found previously to covalently bind yeast cytochrome c, while the arachidic acid surface electrostatically binds yeast cytochrome c. Meridional x-ray diffraction data were collected from these lipid multilayer films with and without a bound yeast cytochrome c surface layer. A box refinement technique, previously shown to be effective in deriving the profile structures of ultrathin multilayer lipid films with and without electrostatically bound cytochrome c, was used to determine the multilayer electron density profiles. The surface monolayer of bound cytochrome c was readily apparent upon comparison of the multilayer electron density profiles for the various pairs of ultrathin multilayer films plus/minus cytochrome c for all cases. In addition, cytochrome c binding to the multilayer surface significantly perturbs the underlying lipid monolayers.     

Location of the heme-Fe atoms within the profile structure of a monolayer of cytochrome c bound to the surface of an ultrathin lipid multilayer film.

We have recently developed x-ray diffraction methods to derive the profile structure of ultrathin lipid multilayer films having one to five bilayers (e.g., Skita, V., W. Richardson, M. Filipkowski, A.F. Garito, and J.K. Blasie. 1987. J. Physique. 47:1849-1855). Furthermore, we have employed these techniques to determine the location of a monolayer of cytochrome c bound to the carboxyl group surface of various ultrathin lipid multilayer substrates via nonresonance x-ray diffraction (Pachence, J.M., and J.K. Blasie. 1987. Biophys. J. 52:735-747). Here an intense tunable source of x-rays (beam line X9-A at the National Synchrotron Light Source at the Brookhaven National Laboratory) was utilized to measure the resonance x-ray diffraction effect from the heme-Fe atoms within the cytochrome c molecular monolayer located on the carboxyl surface of a five monolayer arachidic acid film. Lamellar x-ray diffraction was recorded for energies above, below, and at the Fe K-absorption edge (E = 7,112 eV). An analysis of the resonance x-ray diffraction effect is presented, whereby the location of the heme-Fe atoms within the electron density profile of the cytochrome c/arachidic acid ultrathin multilayer film is indicated to +/- 3 A accuracy.     

Vectorially oriented monolayers of the cytochrome c/cytochrome oxidase bimolecular complex.

Vectorially oriented monolayers of yeast cytochrome c and its bimolecular complex with bovine heart cytochrome c oxidase have been formed by self-assembly from solution. Both quartz and Ge/Si multilayer substrates were chemical vapor deposited with an amine-terminated alkylsiloxane monolayer that was then reacted with a hetero-bifunctional cross-linking reagent, and the resulting maleimide endgroup surface then provided for covalent interactions with the naturally occurring single surface cysteine 102 of the yeast cytochrome c. The bimolecular complex was formed by further incubating these cytochrome c monolayers in detergent-solubilized cytochrome oxidase. The sequential formation of such monolayers and the vectorially oriented nature of the cytochrome oxidase was studied via meridional x-ray diffraction, which directly provided electron density profiles of the protein(s) along the axis normal to the substrate plane. The nature of these profiles is consistent with previous work performed on vectorially oriented monolayers of either cytochrome c or cytochrome oxidase alone. Furthermore, optical spectroscopy has indicated that the rate of binding of cytochrome oxidase to the cytochrome c monolayer is an order of magnitude faster than the binding of cytochrome oxidase to an amine-terminated surface that was meant to mimic the ring of lysine residues around the heme edge of cytochrome c, which are known to be involved in the binding of this protein to cytochrome oxidase.     

Adsorption of cytochrome c to phospholipid monolayers studied by reflection spectroscopy

The uptake of cytochrome c by charged and neutral lipid monolayers was studied by using reflection spectroscopy. The method was shown to be a very sensitive and useful technique in studies of lipid-protein interactions. It was found that cytochrome c is preferentially bound to monolayers of negatively charged monolayers in the solid phase. Polarized light under oblique incidence was used to determine the average orientation of chromophores in cytochrome c bound to lipid monolayer. The transition moments of chromophore are oriented parallel to the monolayer plane.     

Heme structure and orientation in single monolayers of cytochrome c on polar and nonpolar soft surfaces

Polarized x-ray absorption fine structure (XAFS) spectroscopy has been performed in fluorescence mode under total external reflection conditions on frozen hydrated single monolayers of yeast cytochrome c (YCC). The protein molecules were vectorially oriented within the monolayer by tethering their naturally occurring and unique surface cysteine residues to the sulfhydryl-endgroups at the surface of a mixed organic self-assembled monolayer, itself covalently attached to an ultrapure silicon wafer. The sulfhydryl-endgroups were isolated by dilution with either methyl- or hydroxyl-endgroups, producing macroscopically nonpolar or uncharged-polar soft surfaces, respectively. Independent information on the heme-plane orientation relative to the monolayer plane was obtained experimentally via optical linear dichroism. The polarized XAFS data have been analyzed both qualitatively and by a global mapping approach limited to systematically altering the various iron-ligand distances within a model for the local atomic environment of the heme prosthetic group, and comparing the theoretically generated XAFS spectra with those obtained experimentally. A similar analysis of unpolarized XAFS data from a frozen solution of YCC was performed using either the heme environment from the NMR solution or the x-ray crystallographic data for YCC as the model structure. All resulting iron-ligand distances were then used in molecular dynamics (MD) computer simulations of YCC in these three systems to investigate the possible effects of anisotropic ligand motions on the fits of the calculated to the experimental XAFS spectra.     

Interactions of cytochrome c and [14C]-carboxymethylated cytochrome c with monolayers of phosphatidylcholine, phosphatidic acid and cardiolipin

1. The interactions between cytochrome c (native and [(14)C]carboxymethylated) and monolayers of phosphatidylcholine, phosphatidic acid and cardiolipin at the air/water interface was investigated by measurements of surface radioactivity, pressure and potential. 2. On a subphase of 10mm-or m-sodium chloride, penetration of cytochrome c into egg phosphatidylcholine monolayers, as measured by an increase of surface pressure, and the number of molecules penetrating, as judged by surface radioactivity, were inversely proportional to the initial pressure of the monolayer and became zero at 20dynes/cm. The constant of proportionality was increased when the cytochrome c was carboxymethylated or decreased when the phospholipid was hydrogenated, but the cut-off point remained at 20dynes/cm. 3. Penetrated cytochrome c could be removed almost entirely by compression of the phosphatidylcholine monolayer above 20dynes/cm. 4. With phosphatidic acid and cardiolipin monolayers on 10mm-sodium chloride the binding of cytochrome c was much stronger and cytochrome c penetrated into films nearing the collapse pressure (>40dynes/cm.). The penetration was partly electrostatically facilitated, since it was decreased by carrying out the reaction on a subphase of m-sodium chloride, and the relationship between the surface pressure increment and the initial film pressure moved nearer to that observed with phosphatidylcholine. 5. Surface radioactivity determinations showed that [(14)C]carboxymethylated cytochrome c was still adsorbed on phosphatidic acid and cardiolipin monolayers after the cessation of penetration. This adsorption was primarily electrostatic in nature because it could be prevented and substantially reversed by adding m-sodium chloride to the subphase and there was no similar adsorption on phosphatidylcholine films. 6. The penetration into and adsorption on the three phospholipid monolayers was examined as a function of the pH of the subphase and compared with the state of ionization of both the phospholipid and the protein, and the area occupied by the latter at an air/water interface. 7. It is concluded that the binding of cytochrome c to phospholipids can only be partially understood by a consideration of the ionic interaction between the components and that subtle conformational changes in the protein must affect the magnitude and stability of the complex. 8. If cytochrome c is associated with a phospholipid in mitochondria then cardiolipin would fulfil the characteristics of the binding most adequately.     

Vectorially oriented membrane protein monolayers: profile structures via x-ray interferometry/holography.

X-ray interferometry/holography was applied to meridional x-ray diffraction data to determine uniquely the profile structures of a single monolayer of an integral membrane protein and a peripheral membrane protein, each tethered to the surface of a solid inorganic substrate. Bifunctional, organic self-assembled monolayers (SAMs) were utilized to tether the proteins to the surface of Ge/Si multilayer substrates, fabricated by molecular beam epitaxy, to facilitate the interferometric/holographic x-ray structure determination. The peripheral membrane protein yeast cytochrome c was covalently tethered to the surface of a sulfhydryl-terminated 11-siloxyundecanethiol SAM via a disulfide linkage with residue 102. The detergent-solubilized, photosynthetic reaction center integral membrane protein was electrostatically tethered to the surface of an analogous amine-terminated SAM. Optical absorption measurements performed on these two tethered protein monolayer systems were consistent with the x-ray diffraction results indicating the reversible formation of densely packed single monolayers of each fully functional membrane protein on the surface of the respective SAM. The importance of utilizing the organic self-assembled monolayers (as opposed to Langmuir-Blodgett) lies in their ability to tether specifically both soluble peripheral membrane proteins and detergent-solubilized integral membrane proteins. The vectorial orientations of the cytochrome c and the reaction center molecules were readily distinguishable in the profile structure of each monolayer at a spatial resolution of 7 A.     

The interaction of cytochrome c with monolayers of phosphatidylethanolamine

1. The interaction between [(14)C]carboxymethylated cytochrome c and monolayers of egg phosphatidylethanolamine at the air/water interface has been investigated by measurements of surface radioactivity, pressure and potential. 2. On adding (14)C-labelled cytochrome c to the subphase under monolayers with a surface pressure below 24dynes/cm. there was an initial surface pressure increment as the protein penetrated, followed by an adsorption that could be detected only by a continued increase in the surface radioactivity. 3. Above film pressures of 24dynes/cm. only adsorption was observed, i.e. an increment in surface radioactivity with none in surface pressure. 4. The changes in surface parameters with penetration of cytochrome c added to the subphase were indirectly proportional to the initial pressure of the monolayer. With hydrogenated phosphatidylethanolamine the constant of proportionality was increased but penetration again ceased at 24dynes/cm. 5. On compressing a phosphatidylethanolamine film containing penetrated cytochrome c to 40dynes/cm. only a proportion of the protein was ejected on a subphase of 10mm-sodium chloride, whereas on a subphase of m-sodium chloride nearly all the protein was lost. 6. With both penetration and adsorption only a small proportion of the added cytochrome c interacted with the phospholipid films, and initially the amount bound was proportional to the added protein concentration. There was no evidence of a stoicheiometric relationship between the protein and phospholipid or the build-up of multilayers. The bonded protein was not released by removing cytochrome c from the subphase. 7. The addition of m-sodium chloride to the subphase delays the rate of protein penetration into low-pressure films, but the final surface-pressure increment is not appreciably decreased. In contrast, m-sodium chloride almost completely stops adsorption on to films at all pressures. 8. When sodium chloride is added to the subphase below cytochrome c adsorbed to monolayers at high pressures, so that the final concentration is 1m, only a proportion of the protein is desorbed and this decreases as the time of the interaction increases. This indicates that adsorption is initially electrostatic, followed by the formation of non-ionic bonds. 9. Alteration of the subphase pH under a high-pressure film leads to a steady increase in adsorption from pH3 to 8.5 followed by a rapid fall to zero adsorption at pH11. 10. The penetration into phospholipid monolayers at 10dynes/cm. shows a rate that is consistent with the relative electrostatic status of the two components of the interaction as the subphase pH is varied between 3 and 10.5. The final equilibrium penetration shows a pronounced peak in the increments of surface pressure at pH9.0 although a similar peak is not observed in the surface radioactivity. This indicates that more residues of the protein are penetrating into the film at about this pH. 11. Determinations were made of the electrophoretic mobilities of phosphatidylethanolamine particles both alone and after interaction with cytochrome c. 12. The electrophoretic mobilities of cytochrome c adsorbed on lipid particles showed an isoelectric point below that of cytochrome c. This and the observations on the monolayers suggest that, with cytochrome c, protein-protein interactions are weak compared with other proteins.     

Studies on the orientations of the mitochondrial redox carriers. II. Orientation of the mitochondrial chromophores with respect to the plane of the membrane in hydrated, oriented mitochondrial multilayers.

Orientations of the active site chromophores of the mitochondrial redox carriers have been investigated in hydrated, oriented multilayers of mitochondrial membranes using optical and EPR spectroscopy. The hemes of cytochrome c oxidase, cytochrome c1, and cytochromes b were found to be oriented in a similar manner, with the normal to their heme planes lying approximately in the plane of the mitochondrial membrane. The heme of cytochrome c was either less oriented in general or was oriented at an angle closer to the plane of the mitochondrial membrane than were the hemes of the "tightly bound" mitochondrial cytochromes. EPR spectra of the azide, sulfide and formate complexes of cytochrome c oxidase in mitochondria in situ obtained as a function of the orientation of the applied magnetic field relative to the planes of the membrane multilayers showed that both hemes of the oxidase were oriented in such a way that the angle between the heme normal and the membrane normal was approx. 90 degrees.     

Electrostatic orientation of the electron-transfer complex between plastocyanin and cytochrome c.

To understand the specificity and efficiency of protein-protein interactions promoting electron transfer, we evaluated the role of electrostatic forces in precollision orientation by the development of two new methods, computer graphics alignment of protein electrostatic fields and a systematic orientational search of intermolecular electrostatic energies for two proteins at present separation distances. We applied these methods to the plastocyanin/cytochrome c interaction, which is faster than random collision, but too slow for study by molecular dynamics techniques. Significant electrostatic potentials were concentrated on one-fourth (969 A2) of the plastocyanin surface, with the greatest negative potential centered on the Tyr-83 hydroxyl within the acidic patch, and on one-eighth (632 A2) of the cytochrome c surface, with the greatest positive potential centered near the exposed heme edge. Coherent electrostatic fields occurred only over these regions, suggesting that local, rather than global, charge complementarity controls productive recognition. The three energetically favored families of pre-collision orientations all directed the positive region surrounding the heme edge of cytochrome c toward the acidic patch of plastocyanin but differed in heme plane orientation. Analysis of electrostatic fields, electrostatic energies of precollision orientations with 12 and 6 A separation distances, and surface topographies suggested that the favored orientations should converge to productive complexes promoting a single electron-transfer pathway from the cytochrome c heme edge to Tyr-83 of plastocyanin. Direct interactions of the exposed Cu ligand in plastocyanin with the cytochrome c heme edge are not unfavorable sterically or electrostatically but should occur no faster than randomly, indicating that this is not the primary pathway for electron transfer.     

Interaction of a nonspecific wheat lipid transfer protein with phospholipid monolayers imaged by fluorescence microscopy and studied by infrared spectroscopy.

The interaction of a nonspecific wheat lipid transfer protein (LTP) with phospholipids has been studied using the monolayer technique as a simplified model of biological membranes. The molecular organization of the LTP-phospholipid monolayer has been determined by using polarized attenuated total internal reflectance infrared spectroscopy, and detailed information on the microstructure of the mixed films has been investigated by using epifluorescence microscopy. The results show that the incorporation of wheat LTP within the lipid monolayers is surface-pressure dependent. When LTP is injected into the subphase under a dipalmytoylphosphatidylglycerol monolayer at low surface pressure (< 20 mN/m), insertion of the protein within the lipid monolayer leads to an expansion of dipalmytoylphosphatidylglycerol surface area. This incorporation leads to a decrease in the conformational order of the lipid acyl chains and results in an increase in the size of the solid lipid domains, suggesting that LTP penetrates both expanded and solid domains. By contrast, when the protein is injected under the lipid at high surface pressure (> or = 20 mN/m) the presence of LTP leads neither to an increase of molecular area nor to a change of the lipid order, even though some protein molecules are bound to the surface of the monolayer, which leads to an increase of the exposure of the lipid ester groups to the aqueous environment. On the other hand, the conformation of LTP, as well as the orientation of alpha-helices, is surface-pressure dependent. At low surface pressure, the alpha-helices inserted into the monolayers are rather parallel to the monolayer plane. In contrast, at high surface pressure, the alpha-helices bound to the surface of the monolayers are neither parallel nor perpendicular to the interface but in an oblique orientation.     

Compartmentation of ATP within renal proximal tubular cells

Temperature-dependent spin changes of the heme iron atom on cytochrome P-450scc were studied by optical absorption and circular dichroism measurements. The optical absorption and circular dichroism spectra of cholesterol-free cytochrome P-450scc did not change between 10 and 26 degrees C. In contrast, the absorbance at 390 nm and the ellipticity at 330 nm of cholesterol-bound cytochrome P-450scc decreased upon temperature elevation, and the absorbance at 424 nm correspondingly increased. These spectral changes were reversible in respect of temperature. The far-ultraviolet circular dichroism spectra of both cholesterol-bound and -free cytochrome P-450scc were not affected by temperature. In addition, bound cholesterol molecule is not released from the cytochrome molecule by increasing temperature. From these results, we propose that temperature modulates specific interactions between the heme protein and bound cholesterol rather than the gross secondary structural changes of the protein.     

The structure of a cytochrome oxidase-lipid model membrane.

The structure of "membranous cytochrome oxidase" has been investigated by X-ray diffraction, optical polarization spectroscopy and EPR spectroscopy. These studies indicate that the cytochrome oxidase molecules are oriented symmetrically in the membrane profile with a significant portion of their mass occurring within the extravesicular surface of the membrane; the oxidase molecultes span the membrane profile; the distribution of the oxidase molecules over the plane of these membranes is non-crystalline; the oxidase molecules contain bundles of alpha-helical polypeptide chain segments where the average orientation of the helices is normal to the membrane plane; and the average heme orientation within the oxidase molecules is such that the normal to the heme plane lies in the plane of the membrane.     

The structure of a cytochrome oxidase-lipid model membrane

The structure of “membranous cytochrome oxidase” has been investigated by X-ray diffraction, optical polarization spectroscopy and EPR spectroscopy. These studies indicate that the cytochrome oxidase molecules are oriented asymmetrically in the membrane profile with a significant portion of their mass occurring within the extravesicular surface of the membrane; the oxidase molecultes span the membrane profile; the distribution of the oxidase molecules over the plane of these membranes is non-crystalline; the oxidase molecules contain bundles of α-helical polypeptide chain segments where the average orientation of the helices is normal to the membrane plane; and the average heme orientation within the oxidase molecules is such that the normal to the heme plane lies in the plane of the membrane.     

Lateral mobility of phospholipid molecules in thin liquid films

The Fluorescence Recovery After Photobleaching (FRAP) method was applied to measure the lateral mobility of the fluorescent lipid analog, dioctadecylindocarbocyanine perchlorate (Dil-C18), in microscopic thin liquid films (Foam Films (FFs)). The foam film structures were comprised of two phosphatidylcholine monolayers adsorbed at air/water interfaces which sandwiched a thin liquid core. Lateral diffusion of the DiI molecules in the plane of the monolayers was determined as a function of the thickness of the thin liquid core of the film between the FF monolayers. The results obtained indicated that the diffusion coefficient was strongly dependent both on the distance between the FF monolayers in the range 4 nm to 85 nm (corresponding to the FF thickness) and on the film type. The applicability of the FRAP method for studying the molecular mobility in phospholipid FFs was demonstrated. Considerable differences in the surface diffusion coefficient of Dil were observed, ranging between 2 × 10^–8 cm²/s and 22 × 10^–8 cm²/s in so called yellow, gray, common black and Newton black FFs. The effect of the presence of polyethylene glycol (PEG-400) in the liquid core of lecithin FFs on surface diffusion was also studied. The surface diffusion results from the FF studies were compared with data from black lipid membranes (BLMs). These structures are related in thickness terms but the molecular orientation in FFs is the reverse of that in BLMs. Correspondence to: Z. Lalchev     

Monolayers of a model anesthetic-binding membrane protein: formation, characterization, and halothane-binding affinity

hbAP0 is a model membrane protein designed to possess an anesthetic-binding cavity in its hydrophilic domain and a cation channel in its hydrophobic domain. Grazing incidence x-ray diffraction shows that hbAP0 forms four-helix bundles that are vectorially oriented within Langmuir monolayers at the air-water interface. Single monolayers of hbAP0 on alkylated solid substrates would provide an optimal system for detailed structural and dynamical studies of anesthetic-peptide interaction via x-ray and neutron scattering and polarized spectroscopic techniques. Langmuir-Blodgett and Langmuir-Schaeffer deposition and self-assembly techniques were used to form single monolayer films of the vectorially oriented peptide hbAP0 via both chemisorption and physisorption onto suitably alkylated solid substrates. The films were characterized by ultraviolet absorption, ellipsometry, circular dichroism, and polarized Fourier transform infrared spectroscopy. The alpha-helical secondary structure of the peptide was retained in the films. Under certain conditions, the average orientation of the helical axis was inclined relative to the plane of the substrate, approaching perpendicular in some cases. The halothane-binding affinity of the vectorially oriented hbAP0 peptide in the single monolayers, with the volatile anesthetic introduced into the moist vapor environment of the monolayer, was found to be similar to that for the detergent-solubilized peptide.     

Active site structure and dynamics of cytochrome c3 from Desulfovibrio gigas immobilized on electrodes

Cytochrome c3 from Desulfovibrio gigas is electrostatically adsorbed on Ag electrodes coated with self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid. The redox equilibria and electron transfer dynamics of the adsorbed four-heme protein are studied by surface enhanced resonance Raman spectroscopy. Immobilization on the coated electrodes does not cause any structural changes in the redox sites. The potential-dependent stationary experiments distinguish the redox potential of heme IV (-0.19 V versus normal hydrogen electrode) from those of the other hemes for which an average value of -0.3 V is determined. Taking into account the interfacial potential drops, these values are in good agreement with the redox potentials of the protein in solution. The heterogenous electron transfer between the electrode and heme IV of the adsorbed cytochrome c3 is analyzed on the basis of time-resolved experiments, leading to a formal electron transfer rate constant of 15 s(-1), which is a factor of 3 smaller than that of the monoheme protein cytochrome c.     

The cytochrome c oxidase-cytochrome c complex: spectroscopic analysis of conformational changes in the protein-protein interaction domain

Binding to cytochrome c oxidase induces a conformational change in the cytochrome c molecule. This conformational change has been characterized by comparing the binding of native cytochrome c and chemically modified cytochrome c derivatives to bovine cytochrome c oxidase by using absorption, circular dichroism (CD), and magnetic circular dichroism (MCD) spectroscopy. The following derivatives were analyzed: (i) cytochrome c modified at all 19 lysine residues to yield the (N epsilon-acetimidyl)19 cytochrome c, (N epsilon-isopropyl)19 cytochrome c, and (N epsilon,N epsilon-dimethyl)19 cytochrome c; (ii) cytochrome c in which Met65 and Met80 are converted to the methionine sulfoxide; (iii) cytochrome c with a single break in the polypeptide chain at Arg38 or Gly37. The derivatives bind to cytochrome c oxidase at a ratio of one heme c per heme aa3. The association constants are similar to that of native cytochrome c except for (N epsilon-isopropyl)19 and (N epsilon,N epsilon-dimethyl)19 cytochromes c, which bind respectively four times and six times less strongly. The derivatives are good substrates for the cytochrome c oxidase reaction. The spectral changes accompanying the binding of the modified cytochromes c to cytochrome c oxidase are quite different from the spectral changes observed with native cytochrome c. The different optical absorption and MCD changes are explained by a polarity change around the exposed heme edge in the cytochrome c-cytochrome c oxidase complex. The CD changes indicate a conformational rearrangement restricted to the surface area surrounding the exposed heme edge. The rearrangement may involve a movement of the evolutionarily conserved Phe82 out of the vicinity of the heme.(ABSTRACT TRUNCATED AT 250 WORDS)     

Complex formation between chlorophyll a and cytochrome c: surface properties at the air-water interface. Absorbance, fluorescence and fluorescence-lifetime in Langmuir-Blodgett films.

The binding of cytochrome c to an insoluble monolayer of chlorophyll a was studied. Surface pressure (II), surface potential (delta V) and [14C]cytochrome c surface-concentration (gamma) isotherms were measured versus molecular area (sigma) in mixed films. Compared to the successive-addition method, this procedure allows the formation of homogeneous mixed films. The cytochrome c is incorporated into a chlorophyll a monolayer, compressed at a surface pressure of 20 mN.m-1. On expansion, the quantity of protein incorporated into the monolayer gradually increases. Subsequent compression-expansion cycles result in similar isotherms, distinct from that measured during the first expansion. All surface properties measured, but more specifically the surface radioactivity of [14C]cytochrome c, indicate the irreversibility of protein incorporation into the chlorophyll a monolayer. In fact, surface properties of the binary film are completely different from the properties of either of the pure components. As a result, calculated values of surface potentials for mixed films using the additivity law deviate from experimentally measured potentials. The absorption and fluorescence spectra of mixed films transferred onto a solid substrate by the Langmuir-Blodgett technique, indicate a dilution effect of chlorophyll a by cytochrome c. However, the dilution effect cannot be detected by the fluorescence lifetimes of pure chlorophyll a and mixed chlorophyll a-cytochrome c films, both shorter than 0.2 ns. This provides support for the existence of an energy-transfer mechanism between chlorophyll a monomer and chlorophyll a aggregates which could serve as an energy trap. The role of the protein could be related to that of the matrix.