Rotational motion of cytochrome c derivatives bound to membranes measured by fluorescence and phosphorescence anisotropy

Order Parameters and Orientation Distributions of Solution Adsorbed and Microcontact Printed Cytochrome c Protein Films on Glass and ITO

The structure of solution adsorbed and microcontact printed (muCP) cytochrome c (cyt c) films on glass and indium tin oxide (ITO) was investigated using attenuated total reflectance (ATR) and total internal reflectance fluorescence (TIRF) spectroscopies to determine the orientation of the heme groups in the films. The second and fourth order parameters of the heme as well as information on the angle between the absorption and emission dipoles of the heme, gamma, were experimentally determined. The order parameters of the heme are related to the order parameters of the protein molecule using the known angle between the heme plane and the electrostatic dipole moment of the cyt c protein. The effect of the surface roughness of the substrates (glass and ITO) was also taken into account quantitatively using AFM data. Physically possible order parameters were obtained for the heme group in both solution adsorbed and muCP films, but not for the electrostatic dipole moment of the protein. In addition, the experimental values of {cos2 gamma} for immobilized zinc-substituted cyt c are greater than the values of {cos2 gamma} determined in viscous solutions, which could be an indication that the environment of the heme groups changes upon adsorption. The electron transfer behavior of solution adsorbed and muCP films on ITO, determined using electrochemical methods, is compared to their orientation distribution and surface coverage as determined by spectroscopic methods.

The binding of porphyrin cytochrome c to yeast cytochrome c peroxidase. A fluorescence study of the number of sites and their sensitivity to salt

Porphyrin c, the iron-free derivative of cytochrome c, is a reasonably good model for cytochrome c binding to cytochrome c peroxidase (CcP). It binds with the same stoichiometry but only one-quarter as tightly as cytochrome c. CcP (resting, FeIII) and CcP X CN can both bind up to two molecules of porphyrin c. The binding of the first porphyrin c is tight (kd = 1 X 10(-9) M, pH 6, ionic strength mu = 0, 4 degrees C) and results in quenching of the porphyrin c fluorescence. The binding is sensitive to ionic strength. The binding of the second porphyrin c is looser (Kd unknown) and does not result in quenching of the porphyrin fluorescence. The binding of porphyrin c to the cyano form and the resting forms of CcP cannot be distinguished by our methods. ES is the first acceptor of electrons from c(II) and can bind at least two molecules of porphyrin c. The binding of the first porphyrin c is extremely tight, results in substantial quenching and is insensitive to ionic strength. The binding of porphyrin c to the loose site (Kd = 2 X 10(-9) M, pH 6, 4 degrees C, mu = 0) results, unlike the resting and cyano forms, in quenching of fluorescence of the second porphyrin c. The binding of the second porphyrin c to ES is sensitive to ionic strength. The calculated distances between porphyrin c and the hemes of CcP(FeIII) and ES are approximately 2.5 nm.

Cytochrome c-mediated electron transfer between ubiquinol-cytochrome c reductase and cytochrome c oxidase. Kinetic evidence for a mobile cytochrome c pool

Ubiquinol oxidase has been reconstituted from ubiquinol-cytochrome c reductase (Complex III), cytochrome c and cytochrome c oxidase (Complex IV). The steady-state level of reduction of cytochrome c by ubiquinol-2 varies with the molar ratios of the complexes and with the presence of antimycin in a way that can be quantitatively accounted for by a model in which cytochrome c acts as a freely diffusible pool on the membrane. This model was based on that of Kröger & Klingenberg [(1973) Eur. J. Biochem. 34, 358-368] for ubiquinone-pool behaviour. Further confirmation of the pool model was provided by analysis of ubiquinol oxidase activity as a function of the molar ratio of the complexes and prediction of the degree of inhibition by antimycin.

Cytochrome c mediates electron transfer between ubiquinol-cytochrome c reductase and cytochrome c oxidase by free diffusion along the surface of the membrane

Ubiquinol oxidase can be reconstituted from ubiquinol-cytochrome c reductase (Complex III) and cytochrome c oxidase (Complex IV) whose endogenous phosphatidylcholine and phosphatidylethanolamine have been replaced by dimyristoylglycerophosphocholine. Phase transition of the lipid has no effect on Complex III and Complex IV activities assayed separately, but ubiquinol oxidase activity rapidly decreases as the temperature is lowered through the phase transition. A spin-labelled yeast cytochrome c derivative has been synthesized. Binding of the cytochrome c to liposomes demonstrates that only cardiolipin is involved under the conditions used for the ubiquinol oxidase experiments. In liposomes consisting of cardiolipin and dimyristoylglycerophosphocholine, e.s.r. (electron-spin-resonance) measurements show that rotational diffusion of cytochrome c is slowed in the gel phase of the latter lipid. We propose that the cytochrome c pool is bound to cardiolipin molecules, whose lateral and rotational diffusion in the bilayer is adequate to account for electron-transport rates.