FeS Centers Involved in Photosynthetic

A band with a molecular weight of 25 000 of the bacterial oxidoreductase has been identified with the high-potential Fe-S protein, by means of cross reaction with a monospecific antibody against the analogous electron carrier from Neurospora crassa mitochondria. The existence of this type of Fe-S center in photosynthetic bacteria was first discovered by ESR spectroscopy [125] and its involvement in photosynthetic electron transport was demonstrated. The midpoint potential in Rps. sphaeroides is 0.285 V, and is pH dependent above pH 8, with a decrease of 60 mV per pH unit [125]. 

Cytochromes, catalases, and peroxidases all contain iron-heme centers. Nitrite and sulfite reductases, involved in N-O and S-O reductive cleavage reactions to NH3 and HS-, contain iron-heme centers coupled to [Fe ] iron-sulfur clusters. Photosynthetic reaction center complexes contain porphyrins that are implicated in the photoinitiated electron transfers carried out by the complexes. 

The pathways involved in cyclic photophosphorylation in chloroplasts are not yet established. Electrons probably flow from the Fe-S centers Fdx, Fda, or Fdb back to cytochrome b563 or to the PQ pool as is indicated by the dashed line in Fig. 23-18. Cyclic flow around PSII is also possible. The photophosphorylation of inorganic phosphate to pyrophosphate (PP ) occurs in the chromatophores (vesicles derived from fragments of infolded photosynthetic membranes) from Rho-dospirillum rubrum. The PP formed in this way may be used in a variety of energy-requiring reactions in these bacteria.399 An example is formation of NADH by reverse electron transport. 

Iron is an essential micronutrient for all organisms. It is a required element in cytochromes and the Fe-S centers of redox proteins involved in key metabolic processes such as photosynthesis, respiration, and the reduction of nitrate. Given the importance of Fe in these major metabolic enzymes, microorganisms under Fe-deficient conditions reduce their cellular Fe quotas (Fe C content) (Sunda et al., 1991) and have reduced photosynthetic (Raven, 1988 Green et al., 1994) and bacterial growth (Tortell et al., 1996) efficiencies. Thus ecosystem C metabolism is regulated in part by the availability of Fe. 

Another nonheme Mn center is believed [172] to be present in photosynthetic green filamentous bacteria. The locus of the Mn ion in these bacteria is similar to that of the nonheme Fe11 in photosynthetic purple bacteria [173], i.e., between two quinones, along the pathway of electron transfer. Since the Fe center of purple bacteria does not seem to be involved directly in the electron transfer process (i.e., is not redox-active), the redox role of the Mn analog remains in question. This Mn may be redox-active, considering that (1) structural differences between the purple and green bacteria photosynthetic apparatus do exist [173] and (2) the green bacteria display different functionalities, such as C02 fixation, which does not occur via the classical Calvin or reverse Krebs cycle [174],... 

The presence of the two new chlorophyll molecules ( A and A ) is significant in that it points to the similarity between the photosystem-1 and the purple-bacterial reaction centers with regard to the electron-transport pathway and the kinds of pigment molecules involved, as well as their locations. While the involvement of an intermediary chlorophyll in electron transport in photosynthetic bacteria has gradually become clear (see Chapter 7), a similar involvement of an intermediary chlorophyll in photosystem 1 can only be surmised at present. With regard to the various cofactors involved, it is not known yet which ofthe two branches, primed or unprimed in Fig. 3, constitutes the photoactive electron-transport pathway. In any event, a unidirectional electron flow along a P700->(A[Chl] )->Ao->-A ->-FeS-X- FeS-(A/B) pathway is clearly indicated. 

The [4 Fe-4 S] cores have been one of the most intriguing inorganic structures involved in biological systems. Carter et al. (1977) 191 demonstrated that the same basic structure is present in the two [4 Fe—4 S] centers of the 8 Fe ferredoxin of Peptococcus aerogenes (E 0 = - 400 mV)2) and in the high potential iron protein (HiPIP) isolated from the purple photosynthetic bacterium Chromatium vinosum... 

Iron-sulfur proteins, Fe-S-proteins a group of proteins found in all organisms. They contain iron-sulfur centers (iron-sulfur clusters) and take part in electron transfer processes. They are involved In Hj metabolism, nitrogen and carbon dioxide fixation, oxidative and photosynthetic phosphorylation, mitochondrial hydroxylation and nitrite and sulfite reduction. The iron in the active centers is coordinated with the sulfur atoms of cysteine residues. In addition, all Fe-S-proteins except for Rubredoxins (see) contain the same number of labile or inorganic sulfur atoms as iron atoms, and both are covalently bound in iron-sulfur clusters. Since the iron is not bound in a porphyrin ring, this group of proteins is included in the Non-... 

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