Quinone primary

In this context it should be mentioned that a magnetic interaction between 3P865 and the reduced primary quinone acceptor Pgg0 in bRC has been detected and theoretically analyzed (see review by Weber45). 

Another deviation (circle 10) is related to ET from that reduced primary quinone acceptor QA to the secondary quinone acceptor Qb- The process takes place at an edge-edge distance of about 14 A, but these centers are bridged with two hydrogen bonds and Fe atoms coordinated with two conducting imidazol groups (Rees et al 1989). The... 

Noguchi, T., Inoue, Y., and Tang, X-S. (1999) Hydrogen bonding interaction between primary quinone acceptor Qa and a histidine side chain in Photosystem II as revealed by Fourier transform infrared spectroscopy, Biochemistry 38, 399403. 

Figure 2. Paths of electron transfer in PSII P680, reaction-center chlorophyll that functions as the primary electron donor P680, first excited singlet state ofP680 Pheo, pheophytin QA, primary quinone electron acceptor QB, secondary quinone electron acceptor cyt b559, cytochrome b559 Chlz, redox-active chlorophyll that mediates electron transfer between cytochrome b559 and P680 YD, redox-active tyrosine that gives rise to the dark-stable tyrosine radical Yz, redox-active tyrosine that mediates electron transfer from the Mn complex to P680.

Fortunately, the electron-acceptor side of PSII can be exploited to allow turnover control of the S states in highly concentrated samples. A number of herbicides are known that bind tightly to the QB site and block electron transfer past the primary quinone electron acceptor (QA) (13). Some examples are shown in Figure 3. Equations 4 and 5 show the reactions of PSII in the presence of 3-(3,4-dichlorophenyl)-l,l-di-methylurea (DCMU, Figure 3). 

Fig. 1. Mechanism of NAD" photoreduction in purple and green sulfur bacteria. UQ-Fe represents the Fe "-quinone complex present at the primary quinone site of purple bacteria, although in some species menaquinone replaces ubiquinone. Fe/S represents the iron-sulfur center that functions as an early acceptor in green sulfur bacteria. The earliest electron acceptors have been omitted in the case of both green and purple bacteria. The involvement of Cyt b in S oxidation in green bacteria is speculative but is based on inhibition by antimycin A (Ref. 67).

The presence of a fast relaxing intermediate electron acceptor was first discovered when picosecond spectroscopy was applied to the study of photosynthetic systems. A laser flash-induced signal was detected when the primary quinone acceptor was prereduced [18,19], the lifetime of which was so short as to be observed only in the submicrosecond time range. The fast disappearance of the signal can be understood if it is considered that the rate of charge recombination in is very fast... 

X-ray crystallographic analysis shows that reaction centers ofRp. viridis md Rb. sphaeroides contain each two quinone molecules, designated as Qa and Qb- The latter, Qb, is a secondary quinone electron acceptor, in the sense that it receives electrons from the stable primary quinone electron acceptor Qa-It is also known that in native chromatophores there is a pool of quinone molecules, which are in redox equilibrium with Qb. The structure and reactions in the/ . reaction cenleris illustrated in... 

The Qa Qb complex may be looked upon as a two-electron gate, with the two quinones, one primary and the other secondary, acting in series. The two-electron reduction of the secondary quinone results from two one-electron reduction steps by the primary quinone and uptake of two protons. This series of reactions result in a binary oscillation as seen in the accompanying absorbance changes shown in figure 3. 

Chemical analysis of Cf. aurantiacus cells shows that the only quinone present is menaquinone (MQ), with menaquinone-10 (vitamin K-2) being the predominant form. It is therefore concluded that menaquinone serves as both the primary and secondary quinone acceptors, Qa and Qb, in Cf. aurantiacus. The presence of quinones as the electron acceptors in Cf. aurantiacus reinforces the notion that it is similar to the purple bacteria and green-plant PS II. Note that in the purple bacterium Rb. sphaeroides both acceptors are ubiquinone molecules, while in bothi p. viridis and Chromatium menaquinone is the primary quinone acceptor (Qf) and ubiquinone the secondary quinone acceptor (Qb). 

The involvement of Qb has been independently confirmed by Wydrzynski and Inoue from the effect observed upon selective removal of Qb by a heptane/isobutanol extraction procedure that does not disturb the primary quinone Qa- The flash-induced thermoluminescence glow curve in the extracted chloroplasts is identical to that in the DCMU-treated chloroplasts, namely, the B-band is absent and in its place there is a D-band arising from charge recombination in [S2/S3 -Qa ]. By reconstituting lyophilized chloroplasts with native plastoquinone, the B band was restored. Also of interest is the observation that when phenyl-p-benzoquinone or 2,5-dimethyl-/i-benzoquinone was added to reconstitute the extracted sample, the glow curves were not only different from each other, but also did not display the normal, DCMU-generated D-band. These results indicate that the role ofthe extracted Qb in photosystem II may... 

Qq - 2,3-dimethoxy-5-methyl-l,4-benzoquinone - primary quinone acceptor Qg - secondary quinone acceptor Q Fe - quinone-iron complex QHj - dihydroquinone or quinol Q - oxidizing quinone binding site also called Q, - reducing quinone binding site also called Q, Q - electronic transition moments... 

Figure 2. Arrangement of the electron transfer cofactors in the photosynthetic reaction center protein from the bacterium Rhodobacter sphaeroides. The figure shows the special pair of bacteriochlorophylls (top, in green and light blue), two accessory bacteriochlorophyll molecules (dark blue), two bacteriopheophytins (red), the primary quinone (Qa), the secondary quinone (Qb), and the non-heme iron.

Molecular dynamics simulations of the RC of Rps. viridis have provided additional evidence supporting the movement of Qb between the distal site and the proximal site (23). This work showed that the equilibrium between the two binding sites is not displaced by the reduction of Qb to the semiquinone, by the preceding reduction of the primary quinone Qa and by accompanying protonation changes in the protein. 

Although the core of the protein displays a striking symmetry, the pathway of ET is asymmetric (Kirmaier et al., 1985 Zinth et al., 1985). Upon either transfer of excitation energy from the LH complexes or direct illumination, the special pair D is excited to D and an electron is transferred exclusively along branch A to The time constant is 3.5 ps for both RCs in a second step, the electron interacts with the electron acceptor, O. with a time constant of 0.9 and 0.65 ps for the RCs of Rb. sphaeroides and Rp. viridis, respectively (Zinth et al., 1996). The electron from is transferred to the primary quinone in about 200 ps. is a so-called one-electron gate which releases the electron within 10 to 100 jus to the secondary quinone Qg. Before this step occurs, the oxidized primary donor (D" ) is reduced by a secondary electron donor which is heme 3 of the cytochrome-c subunit in the RC of Rp. viridis (about 200 ns)... 

Breton J (1997) Efficient exchange of the primary quinone acceptor Q-A in isolated reaction centers of Rhodopseudomonas viridis. Proc Natl Acad Sd USA 94 11318-11323 Breton J and VermegUo A (eds) (1988) The Photosynthetic Bacterial Reaction Center Structure and Dynamics. Plenum Press, New York... 

The primary photochemistry in bacterial reaction centers (RCs) involves the light-induced electron transfer from a primary electron donor, D (a bacteriochlorophyll dimer), through a series of electron acceptors (a bacteriopheophytin, ( )a, and a primary quinone, Q ) to a secondary quinone acceptor, Qb (reviewed in ref. 1). The charge transfer is accompanied by protonation of the quinones which is the first step in proton translocation across the bacterial membrane. The electrochemical proton gradient formed across the membrane provides the driving force for ATP synthesis (reviewed in ref. 2). Thus, electron transfer can be viewed as a mechanism for setting up the system to carry out the physiologically important function of proton translocation (2). 

Heterodimer-containing His - 73 Leu RCs are photochemical active. These RCs exhibit reversible bleaching of their long wavelength absorption band by actinic light. Both the oxidized-minus-reduced difference spectrum and the rate of charge recombination between the oxidized heterodimer and the primary quinone anion are similar to that observed in His OO Leu RCs (data not shown). 

The three dimensional structure of the reaction center (RC) from Rhodobacter sphaeroides R-26 and 2.4.1 has been reported at a resolution of 2.8 A and 3.0 A respectively (1-4). To improve the accuracy of these models we have futher refined the R-26 data using molecular dynamics methods. We have also collected diffraction data at higher resolution for RCs from the 2.4.1 strain. To relate the three dimensional structure to its function, we are performing various studies on RCs with modified structures (altered amino acid composition, cofactors removed, with herbicide bound). We describe the structure of RCs containing only the primary quinone and the structure of RCs with the herbicide, terbutryn, bound. Progress in crystallizing and determining the structures of other modified RCs is also reported. 

RCs with the secondary quinone removed were prepared as previously described (5). For the herbicide studies, terbutryn was added to the crystallization solutions of RCs, polyethylene glycol, NaCl, and lau l dimethyl amineoxide. The RCs contained either both the primary and secondary quinones or only the primary quinone before the terbutryn was added. The terbutryn concentration was -1000 times higher than the inhibition constant of 0.1 pM (6). All crystals of RCs had the space group P2i2i2i with cell parameters similiar to those of native RCs (1). 

RECONSTITUTION OF PHOTOCHEMICAL ACTIVITY IN Rhodobacter capsulatus REACTION CENTERS CONTAINING MUTATIONS AT TRYPTOPHAN M-250 IN THE PRIMARY QUINONE BINDING SITE. 

Tryptophan M-250 (Trp °) in the reaction center (RC) of the purple non-sulfur bacteria occupies a key position between the photoactive pheophytin (H ) and the primary quinone Qf), according to the X-ray crystal structures obtained for Rps. viridis and Rb. sphaeroides (refs. 1,2 Fig. 1). 

The absorption of light in the reaction center (RC) of photosynthetic bacteria induces electron transfer from the special bacteriochlorophyll pair (P) through a series of one-electron acceptors (bacteriopheophytin, and a primary quinone, Q ) to a two-electron acceptor quinone, Qg [1], In RCs from sphaeroides, both and Qg are ubiquinone-10. It is generally believed that the doubly reduced secondary quinone (hydroquinone dianion) will form quinol (hydroquinone) by taking up two protons before being released from the RC and replaced by another quinone from the quinone-pool. The rate of quinol formation can be limi ted by either of these processes the second electron transfer from Qb to Q/vQb the... 

The primary electron transfer event in photosynthesis is the transfer of an electron from the excited state reaction centre chlorophyll [P]to pheophytin [I]. This charge separation is then stabilised by transfer of the electron to a chain of acceptors and the rereduction of the reaction centre chlorophyll by an electron donor. In the purple photosynthetic bacterium Rhodopseudomonas viridis the electron acceptors are quinones and the electron donors are cytochrome haems. The acceptor complex is thought to consist of a primary quinone [Qa], which is a menaquinone, and a secondary quinone [Qb], which is ubiquinone. Qa is tightly bound to the reaction centre and undergoes... 

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