Bacteriochlorophyll protein structure

A view of the core of the reaction center of Rh. viridis69 is shown in Figure 2.36. It consists of three tetrapyrrolic cofactors the so-called special pair (SP), which is a dimer of bacteriochlorophylls, a monomeric bacteriochloro-phyll (BCh), and a bacteriopheophytin (BPh). As noted above, all these chro-mophores are arranged within the protein structure with oblique orientations to one another. In this bacterial triad, SP functions as the electron donor in... 

R Emerson and W Arnold (1932) The photochemical reaction in photosynthesis. J Gen Physiol 16 191-205 Govindjee (1999) On the requirement of minimum number of four versus eight quanta of light for the evolution of one molecule of oxygen in photosynthesis A historical note. Photosynthesis Res 59 249-254 RE Fenna, BW Mathews, JM Olson and EK Shaw (1974) Structure of a bacteriochlorophyll-protein from the green photosynthetic bacterium Chlorobium limicola crystallographic evidence fora trimer. J Mol Biol 84 231 -240... 

R5. BW Mathews, and RE Fenna (1980) Structure of a green bacteriochlorophyll protein. Accts Chem. Res. 13 309-317. 

RE Fenna, BW Mathews, JM Olson and EK Shaw (1974) Structure of a bacteriochlorophyll-protein from the green photosynthetic bacterium Chlorobium limicola-. Crystallographic evidence fora trimer. J Mol Biol 84 231 -240... 

Prince SM, Papiz, MZ, Freer AA, McDermott G, Hawthornthwaite-Lawless AM, Cogdell RJ and Isaacs NW (1977) Apoprotein structure in the LH2 complex from Rhodopseudomonas acidophila strain 10050 Modular assembly and protein pigment interactions. J Mol Biol 268 412-423 Pullerits T and Sundstrom V (1996) Photosynthetic lightharvesting pigment-protein complexes Toward understanding how and why. Acc Chem Res 29 381-389 Robert B and Lutz M (1985) Structure of anterma complexes of several Rhodospirillales from their resonance Raman spectra. Biochim Biophys Acta 807 10-23 Sauer K, Cogdell RJ, Prince SM, Freer AA, Isaacs NW and Scheer H (1996) Structure based calculations ofthe optical spectra ofthe LH2 bacteriochlorophyll-protein complex from Rhodopseudomonas acidophila. Photochem Photobiol 64 564-576... 

Reaction centers of purple bacteria. The exact composition varies, but the properties of reaction centers from several genera of purple bacteria are similar. In Rhodopseudomonas viridis there are three peptide chains designated H, M, and L (for heavy, medium and light) with molecular masses of 33,28, and 24 kDa, respectively. Together with a 38-kDa tetraheme cytochrome (which is absent from isolated reaction centers of other species) they form a 1 1 1 1 complex. This constitutes reaction center P870. The three-dimensional structure of this entire complex has been determined to 0.23-nm resolution288 319 323 (Fig. 23-31). In addition to the 1182 amino acid residues there are four molecules of bacteriochlorophyll (BChl), two of bacteriopheophytin (BPh), a molecule of menaquinone-9, an atom of nonheme iron, and four molecules of heme in the c type cytochrome. In 1984, when the structure was determined by Deisenhofer and Michel, this was the largest and most complex object whose atomic structure had been described. It was also one of the first known structures for a membrane protein. The accomplishment spurred an enormous rush of new photosynthesis research, only a tiny fraction of which can be mentioned here. 

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. 

Another antenna complex where high-resolution structural information is available is the bacteriochlorophyll a binding protein (also known as the Fenna-Matthews-Olson or FMO protein) from green sulfur bacteria. This complex serves as the bridge between the peripheral chlorosome complex and the membrane-bound reaction center complexes. In this... 

The basic structure of aU the component proteins making up the Light-harvesting (LH) complexes of purple photosynthetic bacteria is similar." " It consists of two polypeptides a- and jS-subunits) that fold into transmembrane Q -hehces and form a heterodimer that noncovalently binds bacteriochlorophyll... 

Figure 20.34 shows the reaction center of the related bacterium R. sphaeroides its structure is much more evident when the surrounding protein is stripped away. This complex consists of four bacteriochlorophyll molecules (the special pair and two others), two bacteriopheophytin molecules (which are bacteriochlorophyll molecules in which the central Mg ion is replaced by two hydrogen ions), two ubiquinone molecules (Fig. 20.35), and an iron(II) ion. Interestingly, these... 

---