title: Chloroplast cytochrome b-6. Molecular composition as a lipoprotein.
abstract: Disc electrophoretically homogeneous spinach-chloroplast cytochrome b-6 was found to be a lipoprotein whose redox potential was essentially unchanged during isolation. These results further support the hypothesis of Triton X-100/4 M urea, pH 8, as a useful extracting medium for membrane lipoproteins. Cytochrome b-6 was found to have a heme equivalent dry weight of 1 mol of heme per 60000 g. Of this, 20000 g was lipid-extractable. The molecular weight was 60000 with a partial specific volume of 0.84 ml/g. The protein portion of the molecule (40000) consisted of 1 polypeptide chain of 20000 daltons, 1 of 9600 daltons and 2 of 6600 daltons. A simple lipid composition (relative to the original membrane) was found consisting of 7 mol of chlorophyll a and 6 mol of cardiolipin per mol of cytochrome; these two lipids thus account for about 75-80% of the lipid content. An unidentified minor neutral lipid and minor polar lipid were also detected. At pH 7.0 in the presence of 0.5% Triton X-100, E'-o was -0.080 V, and in the absence of Triton X-100, E'-o was -0.120 V. At pH 8 in 0.5% Triton X-100, E'-o was -0.084 V, thus indicating that the redox potential is independent of pH in the region 7-8. The redox reaction proceeded via a one-electron-transfer.
mesh_terms: D002308:Cardiolipins / Q000032:analysis; D002734:Chlorophyll / Q000032:analysis; D002736:Chloroplasts / Q000032:analysis*; D003580:Cytochromes; D004589:Electrophoresis, Disc; D006863:Hydrogen-Ion Concentration; D008055:Lipids / Q000032:analysis; D008074:Lipoproteins; D008970:Molecular Weight; D010084:Oxidation-Reduction; D010940:Plant Proteins; D010944:Plants; D011092:Polyethylene Glycols; D014508:Urea
chemical_list: D002308:Cardiolipins; D003580:Cytochromes; D008055:Lipids; D008074:Lipoproteins; D010940:Plant Proteins; D002734:Chlorophyll; D011092:Polyethylene Glycols; D014508:Urea