Sulfur photosynthetic reduction

Kiyonaga, T., T. Mitsui, M. Torikoshi, M. Takekawa, T. Soejima and H. Tada (2006). Ultrafast photosynthetic reduction of elemental sulfur by Au nanoparticle-loaded Ti02. Journal of Physical Chemistry B, 110(22), 10771-10778. 

The reductive TCA cycle uses CO2 for producing acetyl-CoA [28, 29]. Two CO2 units are converted in each cycle into acetyl-CoA using one ATP and four NAD(P) H units. The full cycle was first reported to be found in a green sulfur photosynthetic bacterium Chlorobium limicola) and was later also found to operate in Aquificales, Archeal Crenarcheota, and various types of proteobacteria. 

A study of photosynthetic organisms other than green plants has revealed that certain bacteria, such as the purple sulfur bacteria, utilize H2S instead of H20 as a reductant in photosynthesis. The product obtained is elemental sulfur instead of oxygen ... 

Fig. 5.2. The photosynthetic membrane of a green sulfur bacterium. The light-activated bacte-riochlorophyll molecule sends an electron through the electron-transport chain (as in respiration) creating a proton gradient and ATP synthesis. The electron eventually returns to the bacteri-ochlorophyll (cyclic photophosphorylation). If electrons are needed for C02 reduction (via reduction of NADP+), an external electron donor is required (sulfide that is oxidised to elemental sulfur). Note the use of Mg and Fe.

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. 

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 2Fe2S (S, acid-labile sulfur) ferredoxins have a redox active binuclear center, with each of the two iron atoms attached to the protein by two cysteinyl sulfur ligands and connected by two inorganic acid-labile sulfur ligands. At cty-ogenic temperatures these clusters are EPR detectable, with characteristic features in the vicinity of g = 1.94. Spinach ferredoxin has principal g values of 2.03, 1.96, and 1.88 and a broad absorbance spectrum with a weak maximum around 420 nm, giving these proteins a reddish brown color which bleaches on reduction. Ferredoxins are low potential electron carriers chloroplast ferredoxins function in photosynthetic electron transfer, but related proteins such as adrenal ferredoxin are involved in steroidogenic electron transfer in mitochondria in tissues which produce steroid hormones. 

If an enzyme binds a flavin radical much more tightly than the fully oxidized or reduced forms, reduction of the flavoprotein will take place in two one-electron steps. In such proteins the values of E° for the two steps may be widely separated. The best known examples are the small, low-potential electron-carrying proteins known as flavodoxins.266 269a These proteins, which carry electrons between pairs of other redox proteins, have a variety of functions in anaerobic and photosynthetic bacteria, cyanobacteria, and green algae. Their functions are similar to those of the ferredoxins, iron-sulfur proteins that are considered in Chapter 16. 

These are involved in a wide range of electron-transfer processes and in certain oxidation-reduction enzymes, whose function is central to such important processes as the nitrogen cycle, photosynthesis, electron transfer in mitochondria and carbon dioxide fixation. The iron-sulfur proteins display a wide range of redox potentials, from +350 mV in photosynthetic bacteria to —600 mV in chloroplasts. 

Table I also shows the great diversity of organisms in which iron—sulfur proteins have been detected. Thus far there is no organism which when appropriately examined has not contained an iron-sulfur protein, either in the soluble or membrane-bound form. Iron-sulfur proteins catalyze reactions of physiological importance in obligate anaerobic bacteria, such as hydrogen uptake and evolution, ATP formation, pyruvate metabolism, nitrogen fixation, and photosynthetic electron transport. These properties and reactions can be considered primitive and thus make iron-sulfur proteins a good place to start the study of evolution. These key reactions are also important in higher organisms. Other reactions catalyzed by iron-sulfur proteins can be added such as hydroxylation, nitrate and nitrite reduction, sulfite reduction, NADH oxidation, xanthine oxidation, and many other reactions (Table II).

Although hydrogenase linked H2 production does not require ATP utilization, normal aerobic fixation of atmospheric CO2 does. As will be discussed below, when CO2 fixation does not occur (as is the case under anaerobic, sulfur-deprived condi tions), the accumulation of ATP molecules in the stroma inhibits ATPase function. This results in the non dissipation of the proton gradient and causes the build-up of the proton motive force. It has been shown that, under these conditions, photosynthetic electron transport is down regulated917 and consequently reductant is not available for efficiently producing H2.140... 

Solubilization, uptake, and precipitation of Ca and Si are directly (or at least energetically) linked to the photosynthetic and respiratory cycling of C, H, and O. Acids from nitrification and sulfur oxidation aid phosphorus mobilization photosynthesis or respiration is required for the uptake and conversion of phosphorous into high-energy phosphate. Sulfur is oxidized (with the concomitant reduction of nitrate) by Thiobacillus denitrificans, likewise, some extremely thermophilic methanogens can transfer hydrogen not only to CO2, but also to S. These are a few examples of interrelations involved in biogeochemical cycles. 

As for chloroplast membranes, various compounds in mitochondrial membranes accept and donate electrons. These electrons originate from biochemical cycles in the cytosol as well as in the mitochondrial matrix (see Fig. 1-9) —most come from the tricarboxylic acid (Krebs) cycle, which leads to the oxidation of pyruvate and the reduction of NAD+ within mitochondria. Certain principal components for mitochondrial electron transfer and their midpoint redox potentials are indicated in Figure 6-8, in which the spontaneous electron flow to higher redox potentials is toward the bottom of the figure. As for photosynthetic electron flow, only a few types of compounds are involved in electron transfer in mitochondria—namely, pyridine nucleotides, flavoproteins, quinones, cytochromes, and the water-oxygen couple (plus some iron-plus-sulfur-containing centers or clusters). 

Those photosynthetic eubacteria with RC-2 centers (filamentous and purple bacteria) reduce NAD" for CO2 fixation by reverse electron flow from the quinone pool, whereas the green sulfur bacteria (RC-1 center) reduce ferredoxin and NAD directly from the secondary acceptor (Fe-S center) of the RC. In both cases an external reductant such as H2S is required. The mechanism of NAD reduction in the gram-positive line has not yet been investigated, but H. chlorum is a het-erotroph rather than an autotroph, and may not need to fix CO2. 

CO2 fixation is also found in many bacteria, both photosynthetic and non-photosynthetic. The purple sulfur and purple nonsulfur bacteria employ the RPP cycle as do plants. The photosynthetic green bacteria, however, use a group of ferre-doxin-linked carboxylases in a pathway known as the reductive carboxylic acid cycle [ ] ... 

Anabaena, is a 36 kDa basic protein having a noncovalently bound flavin (FAD) cofactor. Fd is a smaller (11 kDa) acidic [2Fe-2S] protein that is present in all photosynthetic organisms, and acts as a shuttle between larger proteins (in this case the iron-sulfur subunit of photosystem I and FNR), which are often anchored in membranes and have restricted mobility. Note that Fd is a one-electron carrier and NADP" " requires the simultaneous addition of two electrons for its reduction. 

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