Photosynthesis ferredoxins

During the 1960s, research on proteins containing iron—sulfur clusters was closely related to the field of photosynthesis. Whereas the first ferredoxin, a 2[4Fe-4S] protein, was obtained in 1962 from the nonphotosynthetic bacterium Clostridium pasteurianum (1), in the same year, a plant-type [2Fe-2S] ferredoxin was isolated from spinach chloroplasts (2). Despite the fact that members of this latter class of protein have been reported for eubacteria and even archaebacteria (for a review, see Ref. (3)), the name plant-type ferredoxin is often used to denote this family of iron—sulfur proteins. The two decades... 

Whereas the 2[4Fe-4S] ferredoxin may have been replaced by the [2Fe-2S] ferredoxins in oxygenic photosynthesis, another 2[4Fe-4S] protein, the so-called FA/FB-binding subunit (see Fig. 1), appears to be common to all RCI-type photosystems. 

At low irradiances, photosynthesis uses virtually 100% of the quanta, but in full sunlight, about 2000 imol quanta s , more quanta are available than can be used in photochemistry. Maximum rates of photosynthesis by Populus or Spinacia leaves of 15 and 70 jumol O2 m s , respectively, would require only 15 x 9 = 135 to 630 jumol quanta m s , or 10-40%. Leaves, therefore, need to be able to dissipate 60-90% of the quanta at high irradiance in an orderly manner such as non-radiative decay if they are to avoid the potentially damaging formation of oxygen radicals from reduced ferredoxin (Asada Takahashi, 1987). When plants are under a stress that restricts CO2 assimilation, excessive light will be reached at even lower irradiances. 

Iron-sulfur (Fe-S) proteins function as electron-transfer proteins in many living cells. They are involved in photosynthesis, cell respiration, as well as in nitrogen fixation. Most Fe-S proteins have single-iron (rubredoxins), or two-, three-, or four-iron (ferredoxins), or even seven/eight-iron (nitrogenases) centers. 

Peroxidase activity has long been associated with extracts of plant tissue and the crystalline enzyme from horse radish root has been studied in extenso, particularly in regard to its mechanism of action (11). Plants also contain ferredoxin and various specialized cytochromes, both of which substances play an essential role in photosynthesis (95, 96). Agavain, a crystalline proteolytic enzyme from the leaves of Agave,... 

Figure 10.3 Z-scheme of oxygenic photosynthesis in green algae and cyanobacteria, showing links to hydrogenase. Q (plastoquinone) and X (an iron-sulfur cluster) are electron acceptors from photosystems II and I, respectively.The two hydrogenases shown are the NADP-dependent bidirectional hydrogenase and a ferredoxin-dependent enzyme.

Fe Cytochrome oxidase reduction of oxygen to water Cytochrome P-450 0-insertion from O2, and detoxification Cytochromes b and c electron transport in respiration and photosynthesis Cytochrome f photosynthetic electron transport Ferredoxin electron transport in photosynthesis and nitrogen fixation Iron-sulfur proteins electron transport in respiration and photosynthesis Nitrate and nitrite reductases reduction to ammonium... 

The biological functions of chloroplast ferredoxins are to mediate electron transport in the photosynthetic reaction. These ferredoxins receive electrons from light-excited chlorophyll, and reduce NADP in the presence of ferredoxin-NADPH reductase (23). Another function of chloroplast ferredoxins is the formation oT" ATP in oxygen-evolving noncyclic photophosphorylation (24). With respect to the photoreduction of NADP, it is known that microbial ferredoxins from C. pasteurianum (16) are capable of replacing the spinach ferredoxin, indicating the functional similarities of ferredoxins from completely different sources. The functions of chloroplast ferredoxins in photosynthesis and the properties of these ferredoxin proteins have been reviewed in detail by Orme-Johnson (2), Buchanan and Arnon (3), Bishop (25), and Yocum et al. ( ). 

The cluster in the ferredoxin molecule associated with photosynthesis in higher plants is thought to have the bridged structure Fe2S2 as shown in figure. It is known as photo-synthetic ferredoxin. 

Tetranuclear iron-sulfur clusters are key relay stations in the electron flow in photosynthesis. Photosystem I comprises three subunits, PsaA, PsaB and PsaC. The latter contains two [Fe4S4] centres FA and FB. The core subunits PsaA and B, respectively, house a [Fe4S4] centre denoted FX in addition to other, organic cofactors. The role of this latter cluster was probed in preparations partially devoid of PsaC. It was concluded from the results that FX has a major role in controlling the electron transport through PS I.236 Since the final acceptor of the electrons in PS I is a ferredoxin with a [Fe2S2] cluster it was of interest to study a... 

Glucose 6-phosphate dehydrogenase, the first enzyme in the oxidative pentose phosphate pathway, is also regulated by this light-driven reduction mechanism, but in the opposite sense. During the day, when photosynthesis produces plenty of NADPH, this enzyme is not needed for NADPH production. Reduction of a critical disulfide bond by electrons from ferredoxin inactivates the enzyme. 

The NADP or TPN system, ubiquitous in living organisms and believed to play a major role in electron transport during photosynthesis, exhibits pE°(W) = —5.5. Moreover, various ferredoxins, now widely considered to be the primary electron receptors from excited chlorophylls (I), show pE°(W) values in the range —7.0 to —7.5. The coincidence... 

The second class of iron-containing proteins which have been well-studied by Mossbauer spectroscopy, and by other resonance techniques, are the iron-sulfur proteins. These molecules are also known by the name, ferredoxins. Iron-sulfur proteins in several varieties serve as electron-transport agents for processes in plants, bacteria, and mammals. Perhaps the most-studied physiological process involving the iron-sulfur proteins is the study of their role in photosynthesis. This subject has been extensively reviewed by Arnon 126,135), Hind and Olson 127), Hall and... 

The past four years have witnessed major changes in concepts of anaerobic fermentative metabolism and photosynthesis, due mainly to recognition of the key role of ferredoxin. Ferredoxin is a non-heme iron protein, containing no flavin, which carries the most energetic electrons in metabolism. The cellular function and chemistry of ferredoxin are described in this chapter. This is not intended to be an all-inclusive review of the literature pertaining to ferredoxin. Certain aspects of the subject are discussed in the earlier articles of Mortenson (71), Valentine (106), and... 

Malstrom and Neilands (68). The recent reviews of San Pietro and Black (86) and Arnon (6) are more specifically concerned with ferredoxin in photosynthesis. Also pertinent to the general subject of ferredoxin are the symposia on photosynthesis in higher plants (Photosynthetic Mechanisms of Green Plants (82)) and on the role of non-heme iron proteins in energy conversion (San Pietro (55)). 

Ferredoxin plays a key role in plant photosynthesis. As was discussed above, the green plant was the first source for the protein now known as ferredoxin, and it was here that the photosynthetic function of ferredoxin was first worked out. The functions of ferredoxin in chloroplasts of the green plant have been the topic of two different reviews San Pietro and Black 86) Arnon (6)) and, therefore, will be discussed only briefly here. These functions are summarized in Fig. 12. 

Tagawa, K. and D. I. Arnon Ferredoxins as electron carriers in photosynthesis and in the biological production and consumption of hydrogen gas. Nature 195, 537-543 (1962). 

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