"Autoreduction"--an unusual property of pure spinach cytochrome f

Importance of protein rearrangement in the electron-transfer reaction between the physiological partners cytochrome f and plastocyanin

Cytochrome f from turnip and plastocyanin from French bean were noninvasively cross-linked in the presence of the carbodiimide EDC so that the exposed heme edge in the former protein abuts the acidic patch remote from the copper site in the latter [Morand, L.Z., Frame, M.K., Colvert, K.K., Johnson, D.A., Krogmann, D.W., & Davis, D.J. (1989) Biochemistry 28, 8039]. The molecular mass, reduction potentials, and UV-visible and ESR spectra of the covalent complex were consistent with the composition cyt/pc and with a lack of noticeable structural perturbations of the protein molecules. Isoelectric focusing showed the presence of N-acylurea groups, byproducts of the cross-linking reaction [Zhou, J.S., Brothers, H.M. II, Neddersen, J.P., Peerey, L.M., Cotton, T.M., & Kostić, N.M. (1992) Bioconjugate Chem. 3, 382]. Laser flash spectroscopy, with riboflavin semiquinone as the reductant, showed that the electrontransfer reaction within the covalent complex cyt(II)/pc(II) is either undetectably slow or reversible. The question was resolved by monitoring, during redox titrations, the 1H NMR line widths of the heme methyl groups in free ferricytochrome f and in this protein cross-linked to plastocyanin.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of purified cytochrome c1 from Rhodobacter sphaeroides R-26

Cytochrome c1 from a photosynthetic bacterium Rhodobacter sphaeroides R-26 has been purified to homogeneity. The purified protein contains 30 nmol heme per mg protein, has an isoelectric point of 5.7, and is soluble in aqueous solution in the absence of detergents. The apparent molecular weight of this protein is about 150,000, determined by Bio Gel A-0.5 m column chromatography; a minimum molecular weight of 30,000 is obtained by sodium dodecylsulfate polyacrylamide gel electrophoresis. The absorption spectrum of this cytochrome is similar to that of mammalian cytochrome c1, but the amino acid composition and circular dichroism spectral characteristics are different. The heme moiety of cytochrome c1 is more exposed than is that of mammalian cytochrome c1, but less exposed than that of cytochrome c2. Ferricytochrome c1 undergoes photoreduction upon illumination with light under anaerobic conditions. Such photoreduction is completely abolished when p-chloromercuriphenylsulfonate is added to ferricytochrome c1, suggesting that the sulfhydryl groups of cytochrome c1 are the electron donors for photoreduction. Purified cytochrome c1 contains 3 +/- 0.1 mol of the p-chloromercuriphenylsulfonate titratable sulfhydryl groups per mol of protein. In contrast to mammalian cytochrome c1, the bacterial protein does not form a stable complex with cytochrome c2 or with mammalian cytochrome c at low ionic strength. Electron transfer between bacterial ferrocytochrome c1 and bacterial ferricytochrome c2, and between bacterial ferrocytochrome c1 and mammalian ferricytochrome c proceeds rapidly with equilibrium constants of 49 and 3.5, respectively. The midpoint potential of purified cytochrome c1 is calculated to be 228 mV, which is identical to that of mammalian cytochrome c1.

Isolation and characterization of soluble cytochrome c-553 and membrane-bound cytochrome f-553 from thylakoids of the green alga Scenedesmus acutus

Soluble cytochrome c-553 and membrane-bound cytochrome f-553 from the alga Scenedesmus acutus were purified to apparent homogeneity. The properties of cytochrome c-553 are comparable to preparations obtained from other eukaryotic algae, whereas the thylakoid-bound species resembles higher plant cytochrome f. Common characteristics are: 1. An asymmetrical alpha-band at 553 nm. 2. A midpoint redox potential of +38 MV (pH 7.0), with a pH dependency above pH 8.0 of -60mV/pH unit. 3. Formation of a pyridine hemochromogen with a maximum at 550 nm; no adducts with CN- or CO are observed. Distinguishing features are: 1. Cytochrome f-553 has a more complicated beta-band, with maxima at 531.5 and 524 nm, and hence a more complex low-temperature spectrum. Also the positions of the gamma- and delta-bank at 421.5 and 331 nm, respectively, distinguish cytochrome f-553 from cytochrome c-553, with gamma- and delta-bands at 416 and 318 nm. 2. The ferricytochrome c-553 spectrum exhibits a weak band at 692 nm, which is not observed with cytochrome f.