Cytochrome subunit

PCMH is a flavocytochrome c localized in the periplasmic space of several types of Pseudomonad (Hopper and Taylor, 1977). It catalyzes the oxidation of / -cresol first to / -hydroxybenzyl alcohol and then to / -hydroxybenzaldehyde. Electrons are passed sequentially to the endogenous cytochrome subunit and then to an exogenous secondary electron acceptor protein, possibly an azurin or another cytochrome (Causer et al., 1984). 

FIGURE 9. Stereo diagram of flavocytochrome c sulfide dehydrogenase. The flavoprotein subunit is at the top and the diheme cytochrome subunit is shown at the bottom. The flavin and two heme groups are shown as skeletal models. 

MR Gunner, B Homg. Electrostatic control of midpoint potentials m the cytochrome subunit of the Rhodopseudomonas viridis reaction center. Proc Natl Acad Sci USA 88 9151-9155, 1991. 

We now discuss kinetic evidence that supports the notion that a reduced cytochrome is the direct electron donor to the photooxidized P870. . In subsequent sections we discuss properties and reactions of the RC-associated cytochromes, i.e., those cytochromes that are firmly associated with the reaction centers. The topics to be discussed include the temperature-insensitive electron transfer from the cytochrome to the reaction center and the spatial arrangement of the hemes in the tetraheme cytochrome subunit. 

The center-to-center distance from either of the BChls of P to the nearest heme in the cytochrome subunit is about 21 A. A tyrosine residue of the protein sits squarely in the path from the heme to P [102]. Because the complete amino acid sequence of the cytochrome subunit has not yet been fitted to the crystallographic map of the reaction center, it is not clear which two of the four hemes are the low-potential hemes, and which two the high-potential, but information on this point should be available shortly. 

Gunner, M. R. and B. Honig. (1991). Electrostatic Control of Midpoint Potentials in the Cytochrome Subunit of the Rhodopseudomonas Viridis Reaction Center. Proc. Natl. Acad. Sci. USA. 88 9151-9155. 

FIGURE 8. stereo diagram of p-cresol methylhydroxylase. The flavoprotein subunit is on the left and the cytochrome subunit is on the right. The flavin-binding domain of the flavoprotein subunit is on the bottom and the catalytic domain is on the top. Skeletal models of the heme and FAD prosthetic groups are also shown. 

Osyczka, A., Nagashima, K. V. P., Sogabe, S., Miki, K., Yoshida, M., Shimada, K., and Matsuura, K., 1998, Interaction site for soluble cytochromes on the tetraheme cytochrome subunit bound to the bacterial photosynthetic reaction center mapped by site-directed mutagenesis Biochemistry 37 11732911744. 

U Feller, W Nitschke and FI Michel (1992) Characterization of an improved reaction center preparation from the photosynthetic green sulfur bacterium Chlorobium containing the FeS centers F and F and a bound cytochrome subunit. Biochemistry 31 2608-2614 

The quinone QA (the secondary acceptor) is next reduced by the BPh radical in 200 ps with development of a characteristic EPR signal321 330 at g = 1.82. Over a much longer period of time ( 320 ns) an electron passes from the tetraheme cytochrome subunit to the Chl+ radical in the special pair.323/323a y ie relatively slow rate of this reaction may be related to the fact that the bacteriochlorophyll of the special pair is 2.1 nm (center-to-center) from the nearest heme in the 

The third protein complex in this electron-transfer chain (complex 111) is ubiquinol cytochrome c oxidoreductase (E.C. 1.10.2.2), or commonly known as cytochrome be, complex named after the its b-type and c-type cytochrome subunits. Probably the best-understood one among the complexes, be, complex catalyses electron transfers between two mobile electron carriers the hydrophobic molecule ubiquinone (Q) and the small soluble haem-containing protein cytochrome c. Two protons are translocated across the membrane per quinol oxidized (Hinkel, 1991 Crofts, 1985 Mitchell, 1976). 

The L and M subunits form the structural and functional core of the bacterial photosynthetic reaction center (see Figure 19.9). Each of these homologous subunits contains five transmembrane helices. The H subunit, which has only one transmembrane helix, lies on the cytoplasmic side of the membrane. The cytochrome subunit, which contains four c-type hemes, lies on the opposite periplasmic side. Four bacteriochlorophyll b (BChl-Z>) molecules, two bacteriopheophytin b (BPh) molecules, two quinones (Q and Qg), and a ferrous ion are associated with the L and M subunits. 

Fig. 4. Arrangement of the prosthetic groups in the Rp. viridis. reaction center, redrawn from Ref. 101. Ob is shown at the site identified by Deisenhofcr et al. [102], but the orientation of the quinone in this site is drawn arbitrarily the exact orientation of Ob in the crystal structure has not been described. The four hemes at the top of the figure are in the cytochrome subunit the other components are in the L-M complex. As in Fig. 3, the normal to the chromatophore membrane is approximately vertical and the periplasmic side of the complex is at the top.

The other two subunits of the Rp. viridis reaction center are more globular in shape [102], The amino-terminal end of the H subunit has a hydrophobic (and presumably transmembrane) helix that runs parallel to the contact region of helices D and E of subunit M. Most of the rest of H forms a large globular domain at the cytoplasmic end of the L-M complex. The cytochrome subunit sits on the relatively flat surface at the periplasmic end of the L-M complex, in agreement with the observation that the cytochromes react with from this side of the membrane in chromatophores or whole cells. The cytochrome also has an internal axis of 2-fold rotational pseudosymmetry, which includes about 1/3 of its amino acid residues. Two of the four hemes lie on one side of this axis, and two on the other. 

In contrast to bacteria that do not contain the RC-associated cytochrome, the polypeptide chain of the M-subunit of bacteria of the second kind has been found to have 20 more residues at the C-terminus. It has been suggested that the extra twenty-odd amino-acid residues are used to hold, or snatch the (tetrajheme subunits. Bacteria such as Rb. sphaeroides, which lack this 20-residue extension into the periplasmic space, may not be able to snatch a tetraheme cytochrome subunit if it were available. 

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. 

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