The role of cytochrome c diffusion in mitochondrial electron transport

The interaction of horse heart cytochrome c with phospholipid bilayers. Structural and dynamic effects

The interaction of cytochrome c with phospholipid bilayers is reviewed. Special emphasis is given to the structural and dynamic perturbations induced, either in the membrane lipid component or protein itself, by the lipid-protein interaction. The lipid-induced perturbations in the structure of cytochrome c involve: i) conformational changes in and around the heme crevice, converting the heme iron to a high-spin state: and ii) a destabilisation/loosening of the overall tertiary and secondary structure. This highly mobile, partially unfolded intermediate of cytochrome c has a remarkable resemblance to partially folded membrane-bound intermediates of the precursor protein. The functional implications of lipid-protein intermediates for (apo) cytochrome c in (protein-translocation) electron-transfer are discussed.

Motional dynamics of functional cytochrome c delivered by low pH fusion into the intermembrane space of intact mitochondria

We have investigated the motional dynamics of cytochrome c in the intact, functional rat liver mitochondrion. To do this, functional, FITC-cytochrome c (fluorescein isothiocyanate monoderivatized cytochrome c) was incorporated into the intermembrane space (IMS) of intact mitochondria through encapsulation of cytochrome c into asolectin liposomes followed by low pH-induced fusion of the liposomes with the outer membranes of the mitochondria. A cytochrome c controlled enrichment of between 15%-50% (1800-7200 molecules incorporated per mitochondrion) was obtained. All cytochrome c incorporated, regardless of the quantity, participated in the function of electron transport, indicative of a functional, independent random diffusant. Resonance energy transfer was determined from the IMS-entrapped functional FITC-cytochrome c to octadecylrhodamine B incorporated into the mitochondrial membranes. Resonance energy transfer from FITC-cytochrome c to octadecylrhodamine B in isolated inner or outer mitochondrial membranes (IMM and OMM, respectively) was also measured. We found substantial differences in the effects of ionic strength (I) on the proximity of cytochrome c to isolated IMM and OMM. Interactions with isolated IMM were very dynamic, i.e., very I-dependent, and cytochrome c binding to IMM was significant only at very low I. I-dependent interactions of cytochrome c with isolated OMM were less I-dependent than those for the IMM. However, FITC-cytochrome c was essentially released from IMM and OMM at physiological I. The proximity of FITC-cytochrome c to each mitochondrial membrane after its incorporation into the IMS of intact mitochondria in the condensed configuration was estimated at different external, bulk I using: (a) resonance energy transfer from IMS-entrapped FITC-cytochrome c to octadecylrhodamine B-label evenly distributed in both mitochondrial membranes; and (b) resonance energy transfer from IMS-entrapped FITC-cytochrome c to octadecylrhodamine B-label concentrated in the OMM. Resonance energy transfer showed that the average distance between cytochrome c and the two IMS-membrane surfaces increased with increasing IMS-I, approaching a maximal measurable distance of 85 A at 150 mM I. This result is consistent with a dissociation of FITC-cytochrome c and both membranes of intact mitochondria at physiological I, i.e., when the activity of cytochrome c in electron transport is highest. Our findings reveal a primarily three-dimensional diffusion mode for IMS-cytochrome c during its function in electron transport in intact mitochondria at physiological I, and offer further evidence that mitochondrial electron transport is a process driven by random collisions between its independently diffusing electron transferring, redox components.

The ionic strength of the intermembrane space of intact mitochondria is not affected by the pH or volume of the intermembrane space

Ionic strength affects the electron transport activity of cytochrome c through its electrostatic interactions with redox partners and membrane lipids. We previously reported (Cortese, J.D., Voglino, A.L. and Hackenbrock, C.R. (1991) J. Cell Biol. 113, 1331-1340) that the ionic strength (I) of the intermembrane space (IMS-I) in isolated, intact condensed mitochondria is similar to the external, bulk I, over a wide range of bulk I. We now consider the possible effects of IMS-pH and IMS-volume, both variable parameters of mitochondrial function in situ, on IMS-I. IMS-pH and IMS-I were measured with pH- and I-sensitive fluorescent probes (highly fluorescent FITC-dextran for IMS-pH and FITC-BSA for IMS-I). These probes were delivered into the IMS of intact mitochondria via probe encapsulation into asolectin vesicles, followed by low pH-induced fusion of the vesicles with the outer membranes of intact mitochondria. IMS-pH was found to be 0.4-0.5 units lower than bulk pH over the pH range 6.0-8.5 for mitochondria with a large IMS-volume separating the two mitochondrial membranes (condensed configuration), and 0-0.2 units lower for mitochondria with a small IMS-volume and membranes closely opposed (orthodox configuration). This small pH difference between IMS-pM and bulk pH did not influence the similarity between IMS-I and bulk I. When the IMS-volume was osmotically decreased, bringing the two mitochondrial membranes in close proximity as in the orthodox configuration, IMS-I followed the bulk I above 10 mM but did not respond to changes in bulk I below 10 mM. The lack of response of the IMS-I below 10 mM indicates that the close proximity of the two mitochondrial membranes excludes ions only at low, nonphysiological I. Since the similarity of IMS-I and bulk I is unaffected by either IMS-pH or IMS-volume above a bulk I of 10 mM, at cytosolic physiological I (i.e., 100-150 mM) cytochrome c can be expected to be a free, three-dimensional diffusant in the IMS irrespective of the pH or volume of the IMS.

Two-dimensional diffusion of F1F0-ATP synthase and ADP/ATP translocator. Testing a hypothesis for ATP synthesis in the mitochondrial inner membrane

We report here the first experimentally determined lateral diffusion coefficients of the F1F0-ATP synthase and the ADP/ATP translocator in isolated inner membranes of rat liver mitochondria. Rabbit IgG developed against the F1F0-ATP synthase isolated from rat liver mitochondria was determined to be immunospecific for the synthase subunits, notably the alpha-beta doublet, gamma and delta subunits of F1 and subunits two, three and four of F0. This IgG, conjugated with lissamine-rhodamine, was used as a fluorescent probe to monitor the diffusion of the synthase in the membrane. IgG to cytochrome bc1 complex, prepared and labeled similarly, was used as a fluorescent probe for diffusion of this redox component. Eosin maleimide was determined to specifically label the ADP/ATP translocator in the isolated inner membrane and was used as a specific probe for the diffusion of the translocator. Using fluorescence recovery after photobleaching, the experimental average lateral diffusion coefficient of the F1F0-ATP synthase was determined to be 8.4 x 10(-10) cm2/s or twice that of cytochrome bc1 complex while the diffusion coefficient of the ADP/ATP translocator was 1.7 x 10(-9) cm2/s or four times that of cytochrome bc1 complex suggesting that all three components are independent two-dimensional diffusants. Using these diffusion coefficients and applying a number of basic assumptions, we calculated the theoretical two-dimensional diffusion-controlled collision frequencies and derived collision efficiencies (protons transferred per collision) between each of the three proton-transferring redox complexes and both the F1F0-ATP synthase and ADP/ATP translocator by treating the redox components as proton donors and the synthase and translocator as proton acceptors. These collision efficiencies support the physical possibility of a diffusion-based, random collision process of proton transfer and ATP synthesis in the mitochondrial inner membrane.

The three-subunit cytochrome bc1 complex of Paracoccus denitrificans. Its physiological function, structure, and mechanism of electron transfer and energy transduction

The cytochrome bc1 complex purified from P. denitrificans has the same electron-transfer and energy-transducing activities, is sensitive to the same electron-transfer inhibitors, and contains cytochromes b, c1, iron-sulfur protein, and thermodynamically stable ubisemiquinone identical to the counterpart complexes from mitochondria. However, the bacterial bc1 complex consists of only three proteins, the obligate electron-transfer proteins, while the mitochondrial complexes contain six or more supernumerary polypeptides, which have no obvious electron-transfer function. The P. denitrificans complex is a paradigm for the bc1 complexes of all gram-negative bacteria. In addition, because of its simple polypeptide composition and apparently minimal damage during isolation, the P. denitrificans bc1 complex is an ideal system in which to study structure-function relationships requisite to energy transduction linked to electron transfer.

Electrostatic effects on the kinetics of electron transfer reactions of cytochrome c caused by binding to negatively charged lipid bilayer vesicles

The effect of binding reduced tuna mitochondrial cytochrome c to negatively charged lipid bilayer vesicles at low ionic strength on the kinetics of electron transfer to various oxidants was studied by stopped-flow spectrophotometry. Binding strongly stimulated (up to 100-fold) the rate of reaction with the positively charged cobalt phenanthroline ion, whereas the rate of reaction with the negatively charged ferricyanide ion was greatly inhibited (up to 60-fold), as compared with the same systems either at high ionic strength or at low ionic strength either in the presence of electrically neutral vesicles or in the absence of vesicles. Reactions of tuna cytochrome c with uncharged or electrically neutral oxidants such as benzoquinone and Rhodospirillum rubrum cytochrome c2 were unaffected by binding to vesicles, suggesting little or no effect of membrane association on cytochrome structure or accessibility of the heme center. The kinetic effects were largest at lower cytochrome c to vesicle ratios, where there was a greater degree of exposure of negatively charged regions on the membrane. The reduction of cobalt phenanthroline and ferricyanide by bound cytochrome c proceeded by nonexponential kinetics, as compared with the monophasic kinetics observed in the absence of vesicles. This was probably due to the heterogeneous distribution of vesicle sizes which exists at a given lipid to protein ratio. Nonlinear oxidant concentration dependencies were observed for cobalt phenanthroline oxidation of membrane-bound cytochrome c, consistent with a (minimal) two-step kinetic mechanism involving association of the oxidant with the membrane followed by electron transfer. Based on a comparison of second-order rate constants as a function of lipid to protein mole ratio, binding of cytochrome c to the bilayer increased the efficiency of the cobalt phenanthroline reaction by a factor of approximately 500 at the highest lipid:protein ratio used. The results suggest a mechanism involving attractive and repulsive electrostatic interactions between the negatively charged bilayer and the electrically charged oxidants, which increase or decrease their effective concentrations at the membrane surface.

Cytochrome bc1 complexes of microorganisms

The cytochrome bc1 complex is the most widely occurring electron transfer complex capable of energy transduction. Cytochrome bc1 complexes are found in the plasma membranes of phylogenetically diverse photosynthetic and respiring bacteria, and in the inner mitochondrial membrane of all eucaryotic cells. In all of these species the bc1 complex transfers electrons from a low-potential quinol to a higher-potential c-type cytochrome and links this electron transfer to proton translocation. Most bacteria also possess alternative pathways of quinol oxidation capable of circumventing the bc1 complex, but these pathways generally lack the energy-transducing, protontranslocating activity of the bc1 complex. All cytochrome bc1 complexes contain three electron transfer proteins which contain four redox prosthetic groups. These are cytochrome b, which contains two b heme groups that differ in their optical and thermodynamic properties; cytochrome c1, which contains a covalently bound c-type heme; and a 2Fe-2S iron-sulfur protein. The mechanism which links proton translocation to electron transfer through these proteins is the proton motive Q cycle, and this mechanism appears to be universal to all bc1 complexes. Experimentation is currently focused on understanding selected structure-function relationships prerequisite for these redox proteins to participate in the Q-cycle mechanism. The cytochrome bc1 complexes of mitochondria differ from those of bacteria, in that the former contain six to eight supernumerary polypeptides, in addition to the three redox proteins common to bacteria and mitochondria. These extra polypeptides are encoded in the nucleus and do not contain redox prosthetic groups. The functions of the supernumerary polypeptides of the mitochondrial bc1 complexes are generally not known and are being actively explored by genetically manipulating these proteins in Saccharomyces cerevisiae.