The photosynthetic reaction centre from the purple bacterium
Department of Biochemistry, Imperial College, London, SW7 2AY.
Christopher Leach and Henry S. Rzepa
Department of Chemistry, Imperial College, London, SW7 2AY.
The conversion of light energy into chemical energy is one of the most important processes in nature. By using light energy to oxidise organic or inorganic compounds, photosynthetic organisms obtain the electrons that are necessary for the fixation of carbon dioxide into carbohydrate which is needed for cell growth. The initial photochemistry occurs within membrane-bound pigment/protein complexes known as reaction centre complexes (RCs). The elucidation of the structure of the photosynthetic reaction centre complex from the purple non-sulphur photosynthetic bacterium Rhodopseudomonas viridis has formed a framework with which to understand the conversion of light energy to chemical energy.
The RC is composed of
. The L and M subunits each have 5 transmembrane alpha helices and contain all the cofactors required for charge separation within the complex. The H subunit has 1 transmembrane helix. On the perisplasmic side of the complex is a bound cytochrome subunit.
Each RC contains 4
b (Bchl) molecules, 2
b (Bpheo) molecules, a non-haem
, a bound
termed QA and in vivo another menaquinone termed QB. Although lost during the isolation of the complex the position of QB was estimated by soaking quinone analogues and competitive inhibitors into the crystals. The bound cytochrome contains 4
Organisation of the co-factors
molecules form two structurally equivalent,
that span the transmembrane portion of the complex. The
lies midway between the two quinones. Two of the four Bchl molecules form the primary electron donor of the RC (the 'special pair') which lies perpendicular to the membrane plane whereas the other two (accessory Bchl) lie between the special pair and the Bpheo molecules at an angle of approximately 30û to the membrane plane. Only the pigment branch more closely associated with the L subunit is used for light-driven electron transfer across the photosynthetic membrane.
' Bchls are located on the periplasmic side of the complex at the interface of the L and M subunits with each of the central Mg2+ ions ligated by histidine residues within the L
are ligated by histidine residues (
) that are present in alpha helical regions that lie parallel to the membrane. The C9 keto group of the active Bpheo molecule is hydrogen bonded to a glutamate residue within the L subunit (
). Loss of this H-bond results in a blue shift in the Qx absorption band of this molecule. The non-haem iron is ligated by four histidine residues (
L-H190, L-H230, M-H217, M-H264
) and a glutamate (
). The keto carbonyl groups of quinone QA form H-bonds with the peptide NH group of
and the N1 nitrogen atom of
which also ligates the
is coplanar to QA and may enhance binding of the quinone. Residues
H-bond to the carbonyl oxygen atoms of quinone QB.
Electron transfer through the complex
Light energy is used to excite the
to its lowest excited singlet state and within 3 ps the electron is transferred to the
molecule possibly after reduction of the
molecule on the active branch. The tyrosine residue at
is important for optimal primary electron transfer. The reduced Bpheo molecule reduces QA within 200 ps possibly aided by superexchange mechanism involving
. Reduction of QB by QA- occurs in xx. QA is a single electron carrier whereas after two turnovers of the RC QB is doubly reduced and becomes protonated to form the neutral quinol which leaves the RC to be replaced by a fresh quinone molecule. Protonation of reduced QB is thought to occur by a proton relay system involving amino acid side chains such as
. Herbicides of the s-triazine type inhibit electron flow through the complex by preventing the binding of quinone to the QB site. Mutant RCs that are unable to bind herbicide have mutations within the QB region at residues
. Reduction of the oxidised 'special pair' occurs by electron transfer from the bound cytochrome.