Reaction center iron sulfur-type

Cluster Fx was also identified via its EPR spectral features in the RCI photosystem from green sulfur bacteria 31, 32) and the cluster binding motif was subsequently found in the gene sequence 34 ) of the (single) subunit of the homodimeric reaction center core (for a review, see 54, 55)). Whereas the same sequence motif is present in the RCI from heliobacteria (50), no EPR evidence for the presence of an iron-sulfur cluster related to Fx has been obtained. There are, however, indications from time-resolved optical spectroscopy for the involvement of an Fx-type center in electron transfer through the heliobacterial RC 56). 

Iron-sulfur proteins. In an iroinsulfiir protein, the metal center is surrounded by a group of sulfur donor atoms in a tetrahedral environment. Box 14-2 describes the roles that iron-sulfur proteins play in nitrogenase, and Figure 20-30 shows the structures about the metal in three different types of iron-sulfur redox centers. One type (Figure 20-30a l contains a single iron atom bound to four cysteine ligands. The electron transfer reactions at these centers... 

Photosynthetic bacteria have relatively simple phototransduction machinery, with one of two general types of reaction center. One type (found in purple bacteria) passes electrons through pheophytin (chlorophyll lacking the central Mg2+ ion) to a quinone. The other (in green sulfur bacteria) passes electrons through a quinone to an iron-sulfur center. Cyanobacteria and plants have two photosystems (PSI, PSII), one of each type, acting in tandem. Biochemical and biophysical... 

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... 

As is apparent in Fig. 3, considerable similarity exists in the arrangement of the electron transfer cofactors in PS I and PS n. The main differences between the two systems are as follows 1) PS I has three Pe4S4 iron-sulfur clusters. Ex, Ea, and Eb, located on the stromal side of the complex 2) In PS I the primary acceptor is a chlorophyll, not pheophytin and 3) the distance between the primary acceptor (Aqa3 ) and phylloquinone (Aia,b) in PS I is significantly shorter than the corresponding distance between PheoA,B and Qa.b in PS II and Type II reaction centers. These structural differences correlate with functional differences between the two types of reaction centers. In PS II, the mobile electron carrier on the stromal side of the complex is Qb, which is a lipid-soluble, two-electron acceptor. In contrast, the mobile electron carrier in PS I is ferredoxin, which is a water-soluble, one-electron acceptor. The three iron-sulfur clusters in PS I provide a chaimel by which electrons are funneled out of the reaction center to ferredoxin. On the donor side of the complex, plastocyanin, the reductant that replenishes electrons removed from P700, is also a water-soluble protein and is a one-electron donor. Thus, each photon absorbed by the PS I complex leads to the transfer of one electron from plastocyanin to ferredoxin. In Fig. 2, it is apparent that the midpoint potentials of the acceptors in PS I are about 500 to 700 mV more negative than those in PS II, and the... 

We have seen the Z-scheme for the two photosystems in green-plant photosynthesis and the electron carriers in these photosystems. We have also described how the photosystems of green plants and photosynthetic bacteria all appear to function with basically the same sort ofmechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, etc., yet the molecular constituents of the two reaction centers in green plants, in particular, are quite different from each other. Photosystem I contains iron-sulfur proteins as electron acceptors and may thus be called the iron-sulfur (FeS) type reaction center, while photosystem 11 contains pheophytin as the primary electron acceptor and quinones as the secondary acceptors and may thus be called the pheophytin-quinone (0 Q) type. These two types of reaction centers have also been called RCI and RCII types, respectively. 

As mentioned above, a 1 1 stoichiometric relationship between the photooxidized donor, P700 and the reduced terminal acceptors has not yet been established for the PS-I reaction center. As previously noted, the total amount of the recognized terminal acceptors reduced at 15 K is, on average, approximately 74% of the P700 photooxidized. Even more intriguing, in the reconstituted PS-I complexes from either the Cys-14->Asp or Cys-51->Asp mutant PsaC protein, the extent of photoreduction of each intact iron-sulfur cluster at 15 K remained nearly the same as in the wild-type preparation. The presence of the other cluster that was made photochemically inactive by site-directed mutagenesis apparently had no effect on the behavior of the unaltered cluster. 

Only in the reduced reaction centers, i.e., those in which the iron-sulfur-protein secondary acceptors are chemically pre-reduced and the lifetime ofthe photoinduced radical pair thereby increased to -25 ns, can the difference spectrum of the Chi a-type primary electron acceptor Aq be isolated from the chlorophyll antenna excitation bands. The [Aq -Aq] difference spectrum was thusobtainedwithamajorbleaching band at 693 nm in agreement with the earlier results of Shuvalov et as presented above in Fig. 5... 

Figure 1. Pheophytin-quinone and iron-sulfiir Reaction Centers. The dotted line represents the absorption of light by theprimary electron donor (Chl2 or BChl2).Thelineshows the energy transfers in the Reaction Center, from the PSII tyrosine residue (Yz), through the monomer bacttriochlorophyll (BChl), A) the monomer bacterio-pheophytin (BPhe), or B) pheophytin (Phe) and quinone transfer components, QA and QB, in the pheophytin-quinone type of Reaction Center, and Q through the monomer chlorophyll (Chi), quinone (Q) and F components in the iron-sulfur Reaction Centers.

Jagannathan B, Golbeck JH Unifying principles in homodimeric type I photosynthetic reaction centers Properties of PscB and the F-A, F-B and F-X iron-sulfin clusters in green sulfur bacteria. Biochim Biophys Acta 2008, 1777(12) 1535-1544. 

Xlld does not involve the chiral center, so if the reaction takes place by this pathway, the migration of the alkyl group from sulfur to palladium (with the concomitant or subsequent loss of sulfur dioxide) must take place with inversion of configuration at carbon. Inversion of configuration at carbon has been observed in the reverse-type reaction, the sulfur dioxide insertion into a carbon-iron sigma bond (49). Nucleophilic displacement at carbon in compounds of type Xld is unusually difficult, so the reaction via the sulfite intermediate Xlld would appear to be more likely. Conversion of the tosylate of l-phenyl-2,2,2-trifluoroethanol to the corresponding chloride, a reaction which takes place in the presence of tetra- (n-butyl) ajnmonium chloride with inversion of configuration at carbon, requires 100°C for 24 hrs in dimethylsulfoxide. 

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