Chromatophore vesicles of Rhodobacter capsulatus contain on average one F(O)F(1)-ATP synthase each.

ATP synthase is a unique rotary machine that uses the transmembrane electrochemical potential difference of proton (Delta(H(+))) to synthesize ATP from ADP and inorganic phosphate. Charge translocation by the enzyme can be most conveniently followed in chromatophores of phototrophic bacteria (vesicles derived from invaginations of the cytoplasmic membrane). Excitation of chromatophores by a short flash of light generates a step of the proton-motive force, and the charge transfer, which is coupled to ATP synthesis, can be spectrophotometrically monitored by electrochromic absorption transients of intrinsic carotenoids in the coupling membrane. We assessed the average number of functional enzyme molecules per chromatophore vesicle. Kinetic analysis of the electrochromic transients plus/minus specific ATP synthase inhibitors (efrapeptin and venturicidin) showed that the extent of the enzyme-related proton transfer dropped as a function of the inhibitor concentration, whereas the time constant of the proton transfer changed only marginally. Statistical analysis of the kinetic data revealed that the average number of proton-conducting F(O)F(1)-molecules per chromatophore was approximately one. Thereby chromatophores of Rhodobacter capsulatus provide a system where the coupling of proton transfer to ATP synthesis can be studied in a single enzyme/single vesicle mode

Mechanisms of Formation, Structure, and Dynamics of Lipoprotein Discs Stabilized by Amphiphilic Copolymers: A Comprehensive Review

Amphiphilic copolymers consisting of alternating hydrophilic and hydrophobic units account for a major recent methodical breakthrough in the investigations of membrane proteins. Styrene–maleic acid (SMA), diisobutylene–maleic acid (DIBMA), and related copolymers have been shown to extract membrane proteins directly from lipid membranes without the need for classical detergents. Within the particular experimental setup, they form disc-shaped nanoparticles with a narrow size distribution, which serve as a suitable platform for diverse kinds of spectroscopy and other biophysical techniques that require relatively small, homogeneous, water-soluble particles of separate membrane proteins in their native lipid environment. In recent years, copolymer-encased nanolipoparticles have been proven as suitable protein carriers for various structural biology applications, including cryo-electron microscopy (cryo-EM), small-angle scattering, and conventional and single-molecule X-ray diffraction experiments. Here, we review the current understanding of how such nanolipoparticles are formed and organized at the molecular level with an emphasis on their chemical diversity and factors affecting their size and solubilization efficiency