Bacterial photosynthetic reaction center. The core of the reaction center from Rhodopseudomonas viridis consists of two similar chains: L (red) and M (blue). An H chain (white) and a cytochrome subunit (yellow) complete the structure. Notice that the L and M subunits are composed largely of α helices that span the membrane. Also notice that a chain of electron-carrying prosthetic groups, beginning with a special pair of bacteriochlorophylls and ending at a bound quinone, runs through the structure from bottom to top in this view.
[Drawn from 1PRC.pdb.]