Photosynthetic Apparatus

Photosynthetic Apparatus

In most photosynthetic bacteria the photosynthetic apparatus is localized in the ICM in structures known as chromatophores（Schachman et al.1952）.These double-membraned vesicles or tubules are clustered to form grainlike stacks and bundles.For R.capsulatus and R.sphaeroides the ICM is the result of invaginations of the cytoplasmic membrane in response to a lowering of the oxygen tension below 10 mmHg.The ICM contains three major pigment protein complexes that function in the capture of light energy and electrochemical gradient formation：LH-Ⅱ（B800-850），LH-Ⅰ（B870），and RC（see Figure 1）.

The LH-Ⅱ antenna complex consists of three polypeptides—α（12 kd），β（10 kd），and γ（14 kd）—three molecules of Bchl，and one molecule of Crt.Two molecules of Bchl，associated with the α subunit，and one molecule of Bchl，associated with theβ subunit，are believed to be responsible for the absorption maxima at 850 nm and 800 nm，respectively（Cogdell and Crofts 1978；Sauer and Austin I978）.The Crt is probably associated with theβ subunit（Webster et al.1980）.Cross-linking experiments support the idea that the LH-Ⅱcomplex in the membrane has an oligomeric（possibly a tetrameric）structure（Drews et al.1986）.An α and aβ polypeptide dimerize with α-helical structures crossing through the membrane.This dimer is considered to be the basic structural unit of the LH complexes.The oligomerization of the α-β dimers is proposed to optimize the efficiency of exciton migration by correctly orienting the chromophores.The LH-Ⅱ γ subunit is less hydrophobic and exposed on the cytoplasmic surface of the membrane.It does not bind Bchl（Drews et al.1986）.Although the function of the LH-Ⅱ γ subunit is unknown，it may be necessary for the assembly or stability of the complexes in R.capsulatus.However，the LH-Ⅱ γ subunit is absent in R.sphaeroides and other photosynthetic bacteria.The LH-Ⅱ complex from R.capsulatus has been crystallized，but its fine structure has not yet been resolved（Welte et al.1985）.

The LH-Ⅰ antenna complex（B870）from R.capsulatus consists of two polypeptides—α（12 kd）andβ（7 kd）—complexed with two molecules of Bchl and one molecule of Crt（Drews 1985）.They show very low homology to one another（7%—13%），but are very homologous（76%—78%）to the corresponding polypeptides in R.sphaeroides.As with the LH-11 complex，a pair of α andβ subunits span the membrane via a central，hydrophobic，αhelical domain.Conserved histidine residues in the hydrophobic regions are thought to bind to a pair of Bchl molecules.The polar C-terminal and N-terminal domains of each subunit are distributed toward the periplasm and cytosol，respectively.This basic structural unit is stabilized by forming oligomers，providing the specific binding and arrangement of the Bchl in the polypeptide-lipid environment for efficient energy transfer（Drews et al.1986；Zuber 1986）.

The RC is an integral membrane protein complex where charge separation occurs.It consists of three polypeptides designated RC-H，RC-M，and RC-L；four molecules of Bchl；two bacteriopheophytins（Bphs）；two ubioquinones；one non-heme Fe2+；and one molecule of the carotenoid spheroidene（Feher and Okamura 1978；Cogdell et al.1976）.The exact molecular weights of RC-H，RC-M，and RC-L from R.capsulatus based on DNA and deduced amino acid sequence data are 28，534，34，440，and 31，565 daltons，respectively.The RC-L and RC-M subunits have some similar sequences.Each polypeptide chain consists of five hydrophobic regions that are believed to span the membranes as α-helices.RC-L and RC-M bind Bchl and are involved in the primary photochemistry.The RC-H subunit has only one hydrophobic section near the amino terminal（Youvan et al.1984a and 1984b）and has its carboxyl terminal domain exposed on the cytoplasmic surface of the membrane.At least in R.sphaeroides and Rhodospillum rubrum the H subunit can be removed without loss of photosynthetic electron transport（Feher and Okamura 1978；Wiemken and Bachofen 1984）.The function of the H subunit is unknown.It has been suggested that it may play a role in initiating the assembly of the L-M complex（Chory et al.1984）or it may coordinate the interaction between RC and LH-Ⅰ（Takemoto et al.1982；Peters et al.1983）.It has been also reported that loss of the H subunit changes proper electron transport rates through QA and QB（Debus et al.1985）.(https://www.daowen.com)

Recently，a model of the arrangement of the protein backbone and the prosthetic groups of the RC of Rps.viridis has been reported（Deisenhofer et al.1984，1985a，and 1985b；Michel and Deisenhofer 1986）.The L and M polypeptides，274 and 320 amino acid residues，respectively，span the membrane with five helical segments similar to those hypothesized in R.capsulatus and R.sphaeroides.The N-terminals of both the L and M subunits are on the cytoplasmic side of the membrane，while the C-terminals of both proteins are on the periplasmic side of the membrane.The prosthetic groups are embedded in a central cylinder formed by the L and M proteins.The folding of L and M are similar，and they are related to one another by an axis of dyad symmetry oriented normal to the membrane and passing through the Bchl special pair and the Fe2+.The transmembrane helices are connected by segments that are partly helical，β-sheet，or irregular in conformation.These segments presumably lie on the membrane surface and mediate the contact to the H subunit.The H subunit（258 amino acid residues）is the cytoplasmic cover of the L-M complex.It is folded in a globular C-terminal part and a loosely folded N-terminal part containing a prominent transmembrane helix.Most of the H subunit is in contact with M and L polypeptides.One difference between the RCs of R.viridis and R.capsulatus is the presence of a tightly bound c-type cytochrome on the periplasmic surface of the membrane of R.viridis.The N-terminal transmembrane helix of the H subunit interacts with a segment of the cytochrome and acts as a transmembrane clamp.

In addition to the RC and LH complexes，the photosynthetic membrane contains a ubiquinol-cytochrome C2 oxidoreductase complex.The functional oxidoreductase that is part of the cyclic photosynthetic and respiratory electron chain of R.capsulatus and R.sphaeroides comprises three main subunits—cytochrome b，cytochrome C1，and a highpotential FeS proteincarrying a cluster of four redox centers（Gabellini et al.1982）.Furthermore，ATPase，another membrane component，couples proton flow across the membrane to the synthesis of ATP.This high-molecular-weight，multisubunit complex consists of two primary domains：F0 and F1.The F1 portion，consisting of five subunits，is hydrophilic and sits on the cytoplasmic surface of the membrane.The F0 part consists of three polypeptides—denoted a，b，and c—and is buried in the membrane.The c subunit from R.rubrum has been isolated and its sequence has been determined（Bachofen and Wiemken 1986）.

The proper assembly and packaging of photochemical pigments（Bchl，Crt），cofactors（quinone），and polypeptides in the LH and RC complexes are a prerequisite for efficient energy transfer from molecule to molecule.The detailed description of assembly and topology of the photosynthetic membranes of purple bacteria has recently been published（Drews et al.1983 and 1986；Bachofen and Wiemken 1986；Donohue and Kaplan 1986；Sprague and Varga 1986；Zuber et al.1986；Loach et al 1986）.Studies on the topography of RC polypeptides using proteolysis，site-specific labeling，and x-ray and neutron diffraction have shown that the RC is asymmetrically arranged（Drews 1985）.The RC-H subunit is exposed on the cytoplasmic surface of the membrane；the RC-M and RC-L subunits are buried in the membrane（Deisenhofer et al.1984；Youvan et al.1984b）.All three subunits of the RC interact.The LH-Ⅰ complexes are localized around the RC and are structurally and functionally interconnected to the RC.The LH-Ⅱ complexes are arranged peripherally，interconnecting many RC-LH-Ⅰ units（Monger and Parson 1977）.

The pathway of the photochemical reaction is depicted diagrammatically in Figure 1：Light energy absorbed by the antenna LH-Ⅱ-Bchl is transformed into mobile electronic singlet states，which migrate as excitons by a random walk through the LH-Ⅰ complex（Drews et al.1986）.These two LH complexes function to increase the cross-sectional area for absorption of light.The excitation energy is finally captured with high efficiency by RCBchl molecules，where the energy is transduced into a charge-separated state.A photon of light normally causes the oxidation of a Bchl dimer（the special pair）in the RC with the resultant transfer of an electron to a tightly bound quinone molecule（QA）.QA is then oxidized by a secondary quinone（QB）to form a stable semiquinone.A second photoreduction results in a second electron transfer to QB，resulting in production of the fully reduced quinol，which is released into the membrane.The reduced quinol is subsequently reoxidized by an adjacent membrane-bound cytochhrome ci oxidoreductase that pumps protons across the membrane from the cytoplasmic side to the periplasmic side.The electrons finally return to the RC through cytochrome c2，which is located on the periplasm of the ICM.The proton gradient across the membrane drives ATP synthesis via an ATPase in the membrane.