The structure of a photosynthetic reaction center

In 1984, after having analyzed the X-ray diffraction pattern from the reaction center crystals, Johann Deisenhofer, Robert Huber and Hartmut Michel could present the 3-dimensional structure of the reaction center, the first high-resolution structure of a membrane protein and also the most complex molecular structure which had been solved.
    The reaction center is composed of four protein subunits. Two of these, the L and M subunits, each form five membrane-spanning helices. The structure shows the precise arrangement in the L and M subunits of the photochemically active groups – two chlorophyll molecules forming a dimer, two monomeric chlorophylls, two pheophytin molecules (these lack the central magnesium ion of chlorophyll), one quinone molecule, called Q_A (a second quinone molecule, Q_B, is lost during the preparation of the reaction center) and one iron ion (Fe). The L and M subunits and their chromophores are related by a twofold symmetry axis that passes through the chlorophyll dimer and the iron.
    A third subunit, H, without active groups and located on the membrane inner surface, is anchored to the membrane by a protein helix.
    The remaining subunit, a cytochrome with four heme groups (related to the blood pigment hemoglobin), binds at the outer surface of the membrane.