Reduction photosynthetic organism

Chemoheterotrophs Organic compounds Oxidation-reduction reactions Organic compounds, e.g., glucose All animals, most microorganisms, nonphotosynthetic plant tissue such as roots, photosynthetic cells in the dark... 

The reduction of ketones, aldehydes, and olefins has been extensively explored using chemical and biological methods. As the latter method, reduction by heterotrophic microbes has been widely used for the synthesis of chiral alcohols. On the contrary, the use of autotrophic photosynthetic organisms such as plant cell and algae is relatively rare and has not been explored because the method for cultivation is different from that of heterotrophic microbes. Therefore, the investigation using photosynthetic organisms may lead to novel biotransformations. 

Development of new reduction systems that reduce sterically hindered compounds The reported examples of reduction of carbonyl compounds are usually for the substrates that can be easily reduced such as methyl ketones. Since the demand for reduction of various types of compounds is increasing, investigation of new biocatalytic reductions is required. Photosynthetic organisms are not investigated yet, and they may have new type of enzymes, which can reduce sterically hindered compounds. 

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 ... 

In this reaction, A is an electron-accepting species, which varies with the type of photosynthetic organism, and water serves as the electron donor in an oxidation-reduction sequence (see Fig. 19-XX) that is fundamental to all life. 

The transfer of phosphoryl groups is a central feature of metabolism. Equally important is another kind of transfer, electron transfer in oxidation-reduction reactions. These reactions involve the loss of electrons by one chemical species, which is thereby oxidized, and the gain of electrons by another, which is reduced. The flow of electrons in oxidation-reduction reactions is responsible, directly or indirectly, for all work done by living organisms. In nonphotosynthetic organisms, the sources of electrons are reduced compounds (foods) in photosynthetic organisms, the initial electron donor is a chemical species excited by the absorption of light. The path of electron flow in metabolism is complex. Electrons move from various metabolic intermediates to specialized electron carriers in enzyme-catalyzed reactions. 

FIGURE 20-4 The three stages of C02 assimilation in photosynthetic organisms. Stoichiometries of three key intermediates (numbers in parentheses) reveal the fate of carbon atoms entering and leaving the cycle. As shown here, three C02 are fixed for the net synthesis of one molecule of glyceraldehyde 3-phosphate. This cycle is the photosynthetic carbon reduction cycle, or the Calvin cycle. 

The enzymes from green plants and fungi are large multifunctional proteins,80 which may resemble assimilatory sulfite reductases (Fig. 16-19). These contain siroheme (Fig. 16-6), which accepts electrons from either reduced ferredoxin (in photosynthetic organisms) or from NADH or NADPH. FAD acts as an intermediate carrier. It seems likely that the nitrite N binds to Fe of the siroheme and remains there during the entire six-electron reduction to NH3. Nitroxyl (NOH) and hydroxylamine (NH2OH) may be bound intermediates as is suggested in steps a-c of Eq. 24-14. 

Photosynthetic organisms take advantage of the fact that chlorophyll becomes a strong reductant when it is excited with light. Photooxidation of chlorophyll or bacteriochlorophyll occurs in pigment-protein reaction centers. Chloroplasts have two types of reaction cen-... 

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. 

To provide an environmentally friendly system, photochemical methods have been developed, which utilize light energy for the regeneration of NAD(P)H[1, 32, 33l Recently, the use of cyanobacterium, a photosynthetic biocatalyst, for the reduction was reported where the effective reduction occurred under illumination (Fig. 15-5)[321. When a photosynthetic organisms is omitted, the addition of a photosensitizer is necessary. The methods utilize light energy to promote the transfer of an electron from a photosensitizer via an electron transport reagent to NAD(P)+[1]. 

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