The photosynthetic reaction centre from the purple bacterium Rhodopseudomonas viridis

Peter Nixon

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.

Protein subunits

The RC is composed of four subunits

. 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.

Co-factors

Each RC contains 4 bacteriochlorophyll b (Bchl)

molecules, 2 bacteriopheophytin b (Bpheo)

molecules, a non-haem iron atom

, a bound menaquinone

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 haem molecules

Organisation of the co-factors

The Bchl

, Bpheo

and quinone

molecules form two structurally equivalent, C2-symmetry-related branches

that span the transmembrane portion of the complex. The non-haem iron

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 30° 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.

Cofactor/protein interactions

The 'special pair'

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 (L-H173)

and M Leach, P. Murray-Rust, R. Sayle, H. S. Rzepa and B. J. Whitaker, "Hyperactive Molecules and the World-Wide-Web Information System", J. Chem. Soc., Perkin Trans 2, 1995, 7.

We thank Tim Maffett and Bryan van Vliet of MDLI for much helpful advice, and for the implementation of CSML into Chemscape Chime.