Electron transfer cofactors

In cases where an oxidizing enzyme is coupled with a second, non-redox enzyme, obviously, only the oxidizing component needs to be considered in terms of electron transfer cofactor regeneration. 

This review is intended to summarize the PS II/OEC unit in terms of its polypeptide and electron transfer cofactor composition, its electron transfer pathways and its mode of operation in producing oxygen. The photochemical aspects of its operation wil be dealt with only cursorily as these are treated in detail in Chapter 4 of this volume. There has been considerable review activity recently on specific aspects of PS II/OEC function, including articles on polypeptide composition [7-9], manganese function [10-12], electron transfer and Oj-evolving properties [13-18] and the chloride requirement [19]. 

Fig. 3 shows the arrangement of electron transfer cofactors in PS I and PS II as given by their X-ray crystal structures (2, 3). The two structures are similar, and each has a so-called special pair of chlorophylls located on the stromal side of the complex and shown in green in Fig. 2. Extending across the membrane from the respective special pairs are two branches of cofactors that act as the electron acceptors.

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... 

After initial tests for tyrosyl radical, -benzoquinone, and p-benzosemiquinone radical anion showed the power of DF-based methods for calculating accurate structures, vibrations, and spin properties, their use to predict the properties of other important radicals exploded. Published HF/DF calculations to characterize the photosynthetic electron transfer cofactors plastoquinone, menaquinone, and ubiquinone are described next. 

A) Yd as electron-transfer cofactor (B) Y as hydrogen-atom abstractor... 

Fig. 3. (A) Arrangement of pigment molecules and electron-transfer cofactors in the PS-1 reaction center, viewed along the membrane plane. Numerical values are distances in A. (B) stereo view ofthe same pigment and cofactor molecules as in (A). Both figures adapted from Schubert, Klukas, KrauB, Saenger, Fromme and Witt (A) (1995) Present state of the crystal structure analysis of photosystem I at 4.5 A resolution. In P Mathis (ed) Photosynthesis From Light to Biosphere, II 5. Kluwer (B) (1997) Photosystem I of Synechococcus elongatus at 4 A resolution comprehensive structure analysis. J Mol Biol 272 p 756. Also see Color Plate 10 for a color rendition ofthe electron-density map of (A).

The PS-I core complex (CC 1) used by these workers for crystallization and X-ray crystallographic analysis was purified from Synechococcus elongatus and found to exist in the trimeric form. Each PS-1 monomer unit consisted of the major PsaA and PsaB polypeptides, plus several other smaller subunits, and included a full complement of electron-transfer cofactors, P700, (A), Ao, A, FeS-X and FeS-A/B. It was also known to contain 90 Chl-a molecules per P700. The amount of core-antenna chlorophyll-a in this preparation was comparable to that found in the core complexes of spinach and other cyanobacteria. To simplify the designation, the core-antenna Chi a will now be abbreviated as CA-Chl a. ... 

In the 1992 report of Witt et a preliminary model of the PS-I reaction-center core consisting of polypeptide helices and antenna Chl-a molecules, depicted as rods and disks, respectively, as shown in Fig. 5 (A). This model is viewed from the stroma side in a direction perpendicular to the membrane normal, in the same manner as that shown previously in Fig. 6 (B) ofChapter 25. Some ofthe polypeptide rods ofthe reaction-center core are shown shaded, while the iron-sulfur clusters at the top are depicted as cubes and the chlorophyll molecules as disks. The organization of the polypeptide helices of the PS-I reaction-center has already been discussed in Chapter 25. These workers could identify 45 CA-Chl-a molecules out of a possible 90 and, since six chlorophyll-a molecules are required to account for the electron-transfer cofactors, 45 CA-Chl a molecules remain to be identified. Fig. 5 (B) shows the same model after rotation about a horizontal line through the middle so that the FeS cubes are nearest the viewer. The entire model is also rotated 90° about the membrane normal relative to that in Fig. 5 (A). The polypeptide helices represented by the rods and three iron-sulfur clusters have been omitted in this figure for the sake of clarity. The arrangement of the chlorophyll molecules was described by the authors as... 

In the core-antenna assembly, each CA-Chl a has at least one neighbor spaced at a center-to-center distance of <16 A. Relative to the Chl-a electron-transfer cofactors, namely, P700, the accessory chlorophyll (A), and the primary electron acceptor (Aq), however, the CA-Chl a molecules are at various distances, as can be seen in Table 11 which shows various distances relative to the three electron-transfer Chls a, the number of such core-antenna chlorophylls. 

The photosynthetic reaction center stores light energy by effecting electron transfer to reduce an electron transfer cofactor and form a proton gradient across the membrane. The arrangement of electron transfer cofactors is indicated in Figure 2 and includes a special pair of bacteriochlorophyll molecules, two accessory bacteriochloroophylls, two bacteriopheophytins, two quinone electron acceptors, and a non-henae iron. The reaction center functions... 

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.

Wise, K. E. Computational Studies of Biological Electron Transfer Cofactors and Electron Donor-Acceptor Complexes PhD Dissertation University of Oklahoma Norman, OK, 1999, pp 203. 

Enzymes from the xanthine oxidase family are found in all forms of life. They are complex, multi-cofactor en mes comprising a Mo active site linked with several electron transfer cofactors. 

Where S = fatty acid substrate and X = electron transfer cofactor... 

Iron-sulfur clusters are versatile electron transfer cofactors, which are ubiquitous in many metalloenzymes. In the Bacillus subtilis, redox regulator (Fnr) that controls genes of the anaerobic metabolism in response to low oxygen tension, an unusual structure for the oxygen-sensing [4Fe-4S] " cluster was detected by a combination of genetic experiments with UV-visible and Mossbauer spectroscopy [79]. Asp-141 was identified as the fourth iron-sulfur cluster ligand besides three Cys residues. 

The qualitative and quantitative difference between the two membranes could well result from the presence of various aqueous components (enzymes, other proteins and electron transfer cofactors) which specifically direct the electron flow in photo-... 

Lipoic acid (l,2-dithiolane-3-valeric acid) is widely distributed among microorganisms, plants, and animals. It belongs to the group of cofactors containing sulfur and in nature it is coupled to thiamine pyrophosphate (see Section 7.3). However, lipoic acid basically belongs to another class of electron transfer cofactors where the net oxidoreduction function is to produce ATP. The cofactor is needed in fatty acid synthesis and in the metabolism of carbohydrates. 

Figure 1. Location of the electron transfer cofactors in the photosynthetic reaction center of Rb. sphaeroides.

The only known function of PhQ in cyanobacteria and plants is to function as an electron transfer cofactor in PS I. In spite of its importance in cyanobacteria, the biosynthetic route of PhQ was not previously elucidated. Many prokaryotes contain the metabolic pathway for the biosynthesis of menaquinone (MQ), a PhQ-Hke molecule (Figure 119.1). In certain bacteria, MQ is used during fumarate reduction in anaerobic respiration. - In green sulfur bacteria and in heliobacteria, MQ may function as a loosely bound secondary electron acceptor in the photosynthetic reaction center. The genes encoding enzymes involved in the conversion of chorismate to MQ were cloned in a variety of organisms. MQ differs from PhQ only in the tail portion of the molecule an unsaturated C-40 side chain is present, rather than a mostly saturated C-20 phytyl side chain. Therefore, the synthesis of the naphthalene rings in PhQ and MQ involves similar steps in both pathways. 

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