Photosynthetic bacteria cytochrome

By interesting coincidence, the old dogma that cytochromes are not present in anaerobes was demolished by discovery, at about the same time, of c-type cytochromes in Desulfovibrio and anoxygenic photosynthetic bacteria (Kamen and Vernon 1955). 

Crofts, A.R. Berry, E.A. (1998) Structure and function of the cytochrome bci complex of mitochondria and photosynthetic bacteria. Curr. Opin. Struct. Biol. 8, 501-509. 

Many cytochromes c are soluble but others are bound to membranes or to other proteins. A well-studied tetraheme protein binds to the reaction centers of many purple and green bacteria and transfers electrons to those photosynthetic centers.118 120 Cytochrome c2 plays a similar role in Rhodobacter, forming a complex of known three-dimensional structure.121 Additional cytochromes participate in both cyclic and noncyclic electron transport in photosynthetic bacteria and algae (see Chapter 23).120,122 124 Some bacterial membranes as well as those of mitochondria contain a cytochrome bct complex whose structure is shown in Fig. 18-8.125,126... 

In purple photosynthetic bacteria, electrons return to P870+ from the quinones QA and QB via a cyclic pathway. When QB is reduced with two electrons, it picks up protons from the cytosol and diffuses to the cytochrome bct complex. Here it transfers one electron to an iron-sulfur protein and the other to a 6-type cytochrome and releases protons to the extracellular medium. The electron-transfer steps catalyzed by the cytochrome 6c, complex probably include a Q cycle similar to that catalyzed by complex III of the mitochondrial respiratory chain (see fig. 14.11). The c-type cytochrome that is reduced by the iron-sulfur protein in the cytochrome be, complex diffuses to the reaction center, where it either reduces P870+ directly or provides an electron to a bound cytochrome that reacts with P870+. In the Q cycle, four protons probably are pumped out of the cell for every two electrons that return to P870. This proton translocation creates an electrochemical potential gradient across the membrane. Protons move back into the cell through an ATP-synthase, driving the formation of ATP. 

The chain of carriers between the two photosystems includes the cytochrome b6f complex and a copper protein, plastocyanin. Like the mitochondrial and bacterial cytochrome be i complexes, the cytochrome b(J complex contains a cytochrome with two b-type hemes (cytochrome b6), an iron-sulfur protein, and a c-type cytochrome (cytochrome /). As electrons move through the complex from reduced plastoquinone to cytochrome/, plastoquinone probably executes a Q cycle similar to the cycle we presented for UQ in mitochondria and photosynthetic bacteria (see figs. 14.11 and 15.13). The cytochrome bbf complex provides electrons to plastocyanin, which transfers them to P700 in the reaction center of photosystem I. The electron carriers between P700 and NADP+ and between H20 and P680 are... 

The traces shown here are measurements of optical absorbance changes at 870 and 550 nm when a suspension of membrane vesicles from photosynthetic bacteria was excited with a short flash of light. Downward deflection of the traces represent absorbance decreases. Explain the observations. (Absorption spectra of a c-type cytochrome in its reduced and oxidized forms are described in the previous chapter.)... 

Homologues of mitochondrial bc complex are found in photosynthetic bacteria and other prokaryotes. Some bacterial foC] complexes contain only the three redox-active subunits, cytochrome b, cytochrome C], and the Rieske protein. Another bc homologue, bgE complex in chloroplasts and cyanobacteria, has a shorter cytochrome bg, cytochrome E, Rieske protein and a subunit 4. The absence of the extra subunits in the bacterial complexes seems to indicate that the non-redox subunits present in the mitochondrial bc complex are generally not directly involved in electron transfer and proton translocation per se. 

Figure 18 Schematic structure of the cytochrome bc complex from mitochondria. The struemre of the complex from purple photosynthetic bacteria is thought to be similar. The pathway of electron and proton transfer (modified Q-cycle) is overlaid on the schematic structure. Movement of the Rieske FeS protein is shown by the semitransparent yellow areas ...

The intermediate electron transfer between the pool of quinones accepting electrons from the RC, and the water soluble proteins donating electrons to the RC (bacterial RC and the PSI-RC) is always promoted, at least in the systems studied so far in detail, by a multiprotein complex containing cytochromes and Fe-S proteins, the so called h/ci complex. The universal presence of this type of complex in many redox chains of respiration and photosynthesis has been recognized only very recently [109]. As far as photosynthesis is concerned, complexes of this kind have been characterized in facultative photosynthetic bacteria [110] in cyanobacteria [111], and in higher plant chloroplasts [112]. All these preparations share common characteristics and composition these properties are also very similar to those of analogous complexes isolated from mitochondria of mammals and fungi [109]. 

The components of the quinol-cytochrome c (plastocyanin) oxidoreductase of chloroplasts, cyanobacteria and photosynthetic bacteria have been demonstrated to be very similar. This analogy proves the substantial unity of the mechanism of electron flow in all photosynthetic systems. For this reason the different components of the complexes will be discussed unitarily in the following sections in order to emphasize the functional and structural similarities between them. 

Cytochromes of b type are invariably involved in electron transfer in photosynthetic bacteria and in plant chloroplasts and cyanobacteria, and take active part in the mechanism of the quinol-cytochrome c (plastocyanin) oxidoreductase complex. 

In purple photosynthetic bacteria, and specifically in Rps. sphaeroides and Rps. capsulata, three cytochromes of b type have been identified by means of redox titration, in the dark, of isolated chromatophores [116]. They are characterized by midpoint potentials at pH = 7.0 equal to 0.155, 0.050 and -0.090 V (in Rps. sphaeroides)-, the of the 0.050 V species is pH dependent ( — 60 mV per pH unit) [116,117]. The presence of a cytochrome cc in these organisms, interfering spectrally with cytochrome b, makes the situation unclear as far as the existence of cyt. b E j = 0.155 V) is concerned [118]. The two other cytochromes E = 0.050 and — 0.090 V) have also been resolved kinetically in studies on the photosynthetic electron transport and on the basis of their spectral characteristics (band at 561 nm and a spht bands at 558 and 556 nm, respectively these two cytochromes will be referred to as 6-561 and 6-566 in the following) [119]. 

A second bound form of cytochrome c is an integral part of the oxidoreductase complexes. Cytochrome c, present in photosynthetic bacteria has been distinguished from cyt. C2 (the soluble electron carrier) both thermodynamically and kinetically [121,122]. It is present in the isolated oxidoreductase with a stoicheiometry of one per two cytochromes of b type, and it is associated with the 34000 Da subunit. According to kinetic evidence this cytochrome acts as immediate electron donor to cyt. C2 and electron acceptor from the high potential Fe-S protein [122]. The midpoint potential of cyt. c, is 0.285 V at pH 7 [121,122]. 

Several observations made in whole membranes or in the isolated complexes are in line with these concepts the shifts induced by antimycin A [110,137] and myxothiazol on the absorption spectra of cytochromes b and the alterations of the ESR spectrum of the FeS protein by UHDBT or DBMIB [131]. Moreover, the oxidant-induced reduction of cytochromes b, the key observation for accepting these electron transfer schemes, has been demonstrated in all h/c, complexes isolated so far from mitochondria [134], chloroplasts [111], cyanobacteria [112] and photosynthetic bacteria [110]. In the chloroplast b /f complex this reaction has been demonstrated also in the absence of any exogenously added quinol, indicating that possibly a structurally bound quinone (quinone is always present in the isolated complexes with a stoicheiometry of about 0.5-0.7 mol/mol of cyt. c, [110,111]) is sufficient to drive the reduction of cytochromes [138]. Since a detailed treatment of the genera] mechanism, as well as of the more specific problems of the mitochondrial respiratory chain, are reported in Chapter 3 of this volume, the following discussion will deal only with the specific features of the electron transfer chains in photosynthetic membranes. 

The location of cytochrome C2 in the periplasmic space of purple photosynthetic bacteria has been demonstrated directly by its prompt release following the preparation of sphaeroplasts, and by its accessibility to antibodies in these preparations [220]. Cytochromes c are oxidized in single turnover experiments with a biphasic kinetics (<1 2 and 200-400 /is) this pattern has been interpreted as due to the presence in chromatophores of both cyt. Cj and C2, which are oxidized in series [122]. 

Many cytochromes c are soluble but others are bound to membranes or to other proteins. A well-studied tetraheme protein binds to the reaction centers of many purple and green bacteria and transfers electrons to those photosynthetic centers. Cytochrome... 

C2 plays a similar role in Rhodobacter, forming a complex of known three-dimensional structure. Additional cytochromes participate in both cyclic and noncyclic electron transport in photosynthetic bacteria and algae (see Chapter Some bacterial mem-... 

Cytochromes b of mitochondrial membranes are involved in passing electrons from succinate to ubiquinone in complex and also from reduced ubiquinone to cytochrome Cj in the 248-kDa complex III (Fig. 18-8). A similar complex is present in photosynthetic purple bacteria. Cytochrome ftmctions in the transport of electrons from succinate dehydrogenase to ubiquinone, and cytochrome of secretory vesicle membranes has a specific role in reducing ascorbic acid radicals. ... 

---