The Reactive Chlorophyll Is Bound to Proteins in Reaction Centers

The Reactive Chlorophyll Is Bound to Proteins in Reaction Centers 

The chlorophyll or bacteriochlorophyll that undergoes pho-tooxriiation is bound to a protein in a complex caffed a reaction center. Reaction centers have been purified by disrupt- 

Reaction centers of purple bacteria typically contain three polypeptides, four molecules of bacteriochlorophyll, two bacteriopheophytins, two quinones, and one nonheme iron atom. In some bacterial species, both quinones are ubiquinone. In others, one of the quinones is menaquinone (vitamin K2), a naphthoquinone that resembles ubiquinone in having a long side chain (fig. 15.10). Reaction centers of some species, such as Rhodopseudomonas viridis, also have a cytochrome subunit with four c-type hemes. 

The crystal structure of reaction centers from R. viridis was determined by Hartmut Michel, Johann Deisenhofer, Robert Huber, and their colleagues in 1984. This was the first high-resolution crystal structure to be obtained for an integral membrane protein. Reaction centers from another species, Rhodobacter sphaeroides, subsequently proved to have a similar structure. In both species, the bacteriochlorophyll and bacteriopheophytin, the iron atom and the quinones are all on two of the polypeptides, which are folded into a series of a helices that pass back and forth across the cell membrane (fig. 15.1 la). The third polypeptide resides largely on the cytoplasmic side of the membrane, but it also has one transmembrane a helix. The cytochrome subunit of the reaction center in R. viridis sits on the external (periplasmic) surface of the membrane. 

The structures of ubiquinone, menaquinone (vitamin K2), plastoquinone, and phylloquinone (vitamin K ). Purple photosynthetic bacteria contain ubiquinone, menaquinone, or both, depending on the bacterial species chloroplasts contain plastoquinone and phylloquinone. 

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