Plants, higher Ferredoxin

In chloroplasts of higher plants the ferredoxin-thioredoxin system links light-triggered events in thylakoid membranes with the regulation of enzymes in the stroma (1,2). If the conformation of enzymes changes because of modulators action then the surface exposed to the solvent will be different from the native state (3). As a consequence, interactions of modified enzymes with supramolecular structures (membranes, protein complexes) will differ respect to native forms. Since thylakoid membranes are complex structures they are not adequate for uncovering molecular mechanisms that participate in protein interactions(4). In thfe respect, the well-defined structure of micelles of non-ionic detergents constitute model compounds for the analysis of hydrophobic interactions in proteins (5,6). We report herein that chloroplast fructose-1,6-bisphosphatase interacts with micelles of Triton X-114 in a pH-dependent process. 

The molecular weight of these proteins ranges from 14,000 to 23,000 as shown in Table 2. Organisms which have been reported to produce flavoproteins include several species of bacteria and alga. However, unlike the case with ferredoxins, these proteins have not yet been found in higher plants and animals. 

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. 

Fig. 1. The nitrate assimilation pathway in higher plants. The pathway of nitrate assimilation in the tobacco leaf is illustrated. In some other species an additional cytosolic GS is found in the leaf. The pathway in plant roots is more poorly documented and more variable GS in roots is mostly cytosolic, and some enzymes such as GOGAT are found as isoforms utilising alternate reducing substrates. T, expected nitrate carrier NR, nitrate reductase NiR, nitrite reductase GS, glutamine synthetase GOGAT, glutamate synthase Fd, ferredoxin Gin, glutamine Glu, glutamate.

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 is found in representatives of all major groups of organisms except animals (Table 2). It has been found in all photosynthetic bacteria, protozoa (Shapiro (89)), algae, and higher plants which have been exa-... 

Purification of ferredoxin from algae and higher plants is carried out similarly, but here an acetone fractionation procedure is used prior to collection of ferredoxin on DEAE-cellulose (Tagawa and Arnon (99),... 

Plant ferredoxin, therefore, appears to be twice the size of bacterial ferredoxin with a molecular weight of 13,000, although a higher value would seem possible from the available data. 

Fig. 4.7. The Z scheme of the higher plant photosynthetic chain visualized as an arrangement of multiprotein complexes. The two RC of PSII and PSI are arranged in parallel across the membrane and are intereonnected by the b /f complex. The electron transfer pathway within this complex follows a modified Q cycle scheme analogous to that proposed for bacterial photosynthesis. The oxygen-evolving complex is proposed to face the inner thylakoid lumen and to release protons in this compartment. The association of cytochrome 6-559 with PSII-RC and the cyclic role of ferredoxin are also depicted. Proton-binding and proton-releasing sites are illustrated (from Ref. 93).

Yamamoto, H., Kato, H., Shinzaki, Y., Horiguchi, S., Shikanai, T., Hase, T., Endo, T., Nishioka, M., Makino, A., Tomizawa, K., and Miyake, C. (2006). Ferredoxin hmits cyclic electron flow around PSI (CEF-PSI) in higher plants-stimulation of CEF-PSI enhances non-photochemical quenching of Chi fluorescence in transplastomic tobacco. Plant Cell Physiol. 47, 1355-1371. 

The [2Fe-2S]-ferredoxins serve as electron carriers in many systems such as the chloroplasts of higher plants. The iron atoms are linked by two sulfide... 

---