ATP Synthase – The Cell’s Energy Currency

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ATP synthase (EC 3. 6. 3. 14) is an important enzyme that provides energy for the cell to use through the synthesis of adenosine triphosphate (ATP). ATP is the most commonly used “energy currency” of cells from most organisms. It is formed from adenosine diphosphate (ADP) and inorganic phosphate (Pi), and needs energy. The overall reaction sequence is: ADP + Pi ? ATP, where ADP and Pi are joined together by ATPsynthase Energy is often released in the form of protium or H+, moving down an electrochemical gradient, such as from the lumen into the stroma of chloroplasts or from the inter-membrane space into the matrix in mitochondria.

Nicotinamide adenine dinucleotide phosphate, abbreviated NADP+ (or, in older notation, TPN (triphosphopyridine nucleotide)), is a coenzyme used in anabolic reactions, such as lipid and nucleic acid synthesis, which require NADPH as a reducing agent. NADPH is the reduced form of NADP+. NADP+ differs from NAD+ in the presence of an additional phosphate group on the 2′ position of the ribose ring that carries the adenine moiety. In photosynthetic organisms, NADPH is produced by ferredoxin-NADP+ reductase in the last step of the electron chain of the light reactions of photosynthesis.

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It is used as reducing power for the biosynthetic reactions in the Calvin cycle to assimilate carbon dioxide. Synthesis The oxidative phase of the pentose phosphate pathway is a major source of NADPH in cells,[1] and in cells without mitochondria, it is the only source. However there are several other lesser-known mechanisms of generating NADPH, all of which depend on the presence of mitochondria. The key enzymes in these processes are: NADP-linked malic enzyme, NADP-linked isocitrate dehydrogenase, and nicotinamide nucleotide transhydrogenase. 2]

The isocitrate dehydrogenase mechanism appears to be the major source of NADPH in fat and possibly also liver cells. [1] Also in mitochondria, NADH kinase produces NADPH and ADP using NADH and ATP as substrate. Ribulose-1,5-bisphosphate (RuBP) is an organic substance that is involved in photosynthesis. The anion is a double phosphate ester of the ketose (ketone-containing sugar) called ribulose. Salts of this species can be isolated, but its crucial biological function involves this colourless anion in solution. [1] To simplify the presentation, the image in the above table depicts the acid form of this anion.

Role in photosynthesis The Calvin cycle showing the role of ribulose-1,5-bisphosphate. The enzyme ribulose bisphosphate carboxylase oxygenase (RuBisCO) catalyzes the reaction between RuBP with carbon dioxide. The product is the highly unstable 6-carbon intermediate known as 3-keto-2-carboxyarabinitol 1,5-bisphosphate. This six-carbon intermediate decays virtually instantaneously into two molecules of glycerate 3-phosphate (G3P) (see figure). Less commonly (e. g. at high temperatures) RuBisCO catalyzes RuBP with oxygen (O2) in a process called photorespiration, a process that occurs at high temperatures in “C3 plants. In the Calvin Cycle, RuBP is a product of the phosphorylation of ribulose-5-phosphate by ATP.

Carboxylation in chemistry is a chemical reaction in which a carboxylic acid group is introduced in a substrate. The opposite reaction is decarboxylation. [edit] Carboxylation in organic chemistry In organic chemistry many different protocols exist for carboxylation. One general approach is by reaction of nucleophiles with dry ice (solid carbon dioxide)[1] or formic acid[2][3] Carboxylation in biochemistry Carboxyglutamic acid Further information: Carboxy-lyases

Carboxylation in biochemistry is a posttranslational modification of glutamate residues, to ?-carboxyglutamate, in proteins. It occurs primarily in proteins involved in the blood clotting cascade, specifically factors II, VII, IX, and X, protein C, and protein S, and also in some bone proteins. This modification is required for these proteins to function. Carboxylation occurs in the liver and is performed by ?-glutamyl carboxylase. [4] The carboxylase requires vitamin K as a cofactor and performs the reaction in a processive manner. [5] ?-carboxyglutamate binds calcium, which is essential for its activity. 6] For example, in prothrombin, calcium binding allows the protein to associate with the plasma membrane in platelets, bringing it into close proximity with the proteins that cleave prothrombin to active thrombin after injury. [7] An oxygenase is any enzyme that oxidizes a substrate by transferring the oxygen from molecular oxygen O2 (as in air) to it. The oxygenases form a class of oxidoreductases; their EC number is EC 1. 13 or EC 1. 14. Oxygenases were discovered in 1955 simultaneously by two groups, Osamu Hayaishi from Japan[1][2][3] and Howard S.

Mason from the US. [4][5] There are two types of oxygenases: •Monooxygenases, or mixed function oxidase, transfer one oxygen atom to the substrate, and reduce the other oxygen atom to water. •Dioxygenases, or oxygen transferases, incorporate both atoms of molecular oxygen (O2) into the product(s) of the reaction. [6] Among the most important monooxygenases are the cytochrome P450 oxidases, responsible for breaking down numerous chemicals in the body. Photolysis is part of the light-dependent reactions of photosynthesis.

The general reaction of photosynthetic photolysis can be given as H2A + 2 photons (light) ? 2 e- + 2 H+ + A The chemical nature of “A” depends on the type of organism. In purple sulfur bacteria, hydrogen sulfide (H2S) is oxidized to sulfur (S). In oxygenic photosynthesis, water (H2O) serves as a substrate for photolysis resulting in the generation of diatomic oxygen (O2) from carbon dioxide (CO2). This is the process which returns oxygen to earth’s atmosphere. Photolysis of water occurs in the thylakoids of cyanobacteria and the chloroplasts of green algae and plants.

In biology, carbon fixation is the reduction of inorganic carbon (carbon dioxide) to organic compounds by living organisms. The most prominent example is photosynthesis. Organisms that grow by fixing carbon are called autotrophs—plants for example. Heterotrophs, like animals, are organisms that grow using the carbon fixed by autotrophs. Fixed carbon, reduced carbon, and organic carbon all mean organic compounds. Oxygenic photosynthesis Oxygenic photosynthesis is used by the chief primary producers—plants, algae, and cyanobacteria.

They contain the pigment chlorophyll, and use the Calvin cycle to fix carbon autotrophically. Somewhere between 3. 5 and 2. 3 billion years ago, cyanobacteria evolved oxygenic photosynthesis. [1][2][3] The process works like this: 2H2O ? 4e- + 4H+ + O2 CO2 + 4e- + 4H+ ? CH2O + H2O The essential innovation is the first step, the dissociation of water into electrons, protons, and free oxygen. This allows the use of water, one of the most abundant substances on Earth, as an electron donor—as a source of reducing power. The release of free oxygen is a side-effect of enormous consequence.

The first step uses the energy of sunlight to oxidize water to O2, and, ultimately, to produce ATP ADP + Pi ATP + H2O and the reductant, NADPH NADP+ + 2e- + 2H+ NADPH + H+ The second step, the actual fixation of carbon dioxide, is carried out in the Calvin cycle, which consumes ATP and NADPH. Although redox is thought of as electron transfer, fixing carbon dioxide requires transfer of hydrogen as well. Of course, NADPH can be used to further reduce CH2O. Energy is not stored by fixed carbon alone, but by fixed carbon and free oxygen together.

The light-independent reactions of photosynthesis are chemical reactions that convert carbon dioxide and other compounds into glucose. These reactions occur in the stroma, the fluid-filled area of a chloroplast outside of the thylakoid membranes. These reactions take the light-dependent reactions and perform further chemical processes on them. There are three phases to the light-independent reactions, collectively called the Calvin cycle: carbon fixation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration.

Despite its name, this process occurs only when light is available. Plants do not carry out the Calvin cycle by night. They, instead, release sucrose into the phloem from their starch reserves. This process happens when light is available independent of the kind of photosynthesis (C3 carbon fixation, C4 carbon fixation, and Crassulacean Acid Metabolism); CAM plants store malic acid in their vacuoles every night and release it by day in order to make this process work. 1] Photosystems are functional and structural units of protein complexes involved in photosynthesis that together carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons.

They are found in the thylakoid membranes of plants, algae and cyanobacteria (in plants and algae these are located in the chloroplasts), or in the cytoplasmic membrane of photosynthetic bacteria. At the heart of a photosystem lies the Reaction Center, which is an enzyme that uses light to reduce molecules. In a photosystem, this Reaction Center is surrounded by ight-harvesting complexes that enhance the absorption of light and transfer the energy to the Reaction Centers. Light-Harvesting and Reaction Center complexes are membrane protein complexes that are made of several protein-subunits and contain numerous cofactors. In the photosynthetic membranes, reaction centers provide the driving force for the bioenergetic electron and proton transfer chain. When light is absorbed by a reaction center (either directly or passed by neighbouring pigment-antennae), a series of oxido-reduction reactions is initiated, leading to the reduction of a terminal acceptor.

Two families of reaction centers in photosystems exist: type I reaction centers (such as photosystem I (P700) in chloroplasts and in green-sulphur bacteria) and type II reaction centers (such as photosystem II (P680) in chloroplasts and in non-sulphur purple bacteria). Each photosystem can be identified by the wavelength of light to which it is most reactive (700 and 680 nanometers, respectively for PSI and PSII in chloroplasts), the amount and type of light-harvesting complexes present and the type of terminal electron acceptor used.

Type I photosystems use ferredoxin-like iron-sulfur cluster proteins as terminal electron acceptors, while type II photosystems ultimately shuttle electrons to a quinone terminal electron acceptor. One has to note that both reaction center types are present in chloroplasts and cyanobacteria, working together to form a unique photosynthetic chain able to extract electrons from water, creating oxygen as a byproduct. An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound.

It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process. [1] Typical oxidizing agents undergo permanent chemical alteration through covalent or ionic reaction chemistry, resulting in the complete and irreversible transfer of one or more electrons. In many chemical circumstances, however, the transfer of electronic charge from an electron donor may be only fractional, meaning an electron is not completely transferred, but results in an electron resonance between the donor and acceptor.

This leads to the formation of charge transfer complexes in which the components largely retain their chemical identities. The electron accepting power of an acceptor molecule is measured by its electron affinity which is the energy released when filling the lowest unoccupied molecular orbital (LUMO). The overall energy balance (?E), i. e. , energy gained or lost, in an electron donor-acceptor transfer is determined by the difference between the acceptor’s electron affinity (A) and the ionization potential (I) of the electron donor: .

In chemistry, a class of electron acceptors that acquire not just one, but a set of two paired electrons that form a covalent bond with an electron donor molecule, is known as a Lewis acid. This phenomenon gives rise to the wide field of Lewis acid-base chemistry. [2] The driving forces for electron donor and acceptor behavior in chemistry is based on the concepts of electropositivity (for donors) and electronegativity (for acceptors) of atomic or molecular entities.

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