Sulfoquinovosyl diacylglycerols, abbreviated SQDG, are a class of sulfur-containing but phosphorus-free lipids (sulfolipids) found in many photosynthetic organisms.
In 1959 A. A. Benson and coworkers discovered a new sulfur-containing lipid in plants and identified it as sulfoquinovosyl diacylglycerol (SQDG). [1] The sulfolipid structure was defined as 1,2-di-O-acyl-3-O-(6-deoxy-6-sulfo-α-D-glucopyranosyl)-sn-glycerol (SQDG). The distinctive feature of this substance is carbon bonded directly to sulfur as C-SO3. Sulfonic acids of this type are chemically stable and strong acids. [2]
SQDGs have been found in all photosynthetic plants, algae, cyanobacteria, purple sulfur and non-sulfur bacteria and is localised in the thylakoid membranes, being the most saturated glycolipid. [3]
SQDGs have been found to be closely associated with certain membrane proteins. In some cases the (electrostatic) interactions may be very strong, as suggested by the inability of saturated SQDG molecules associated with purified chloroplast CF0-CF1 ATPase to exchange with other acidic lipids. [4] It was shown also that SQDGs protect CF1 against cold inactivation in the presence of some ATP. CF1 bound to membranes was found to be much more resistant to heat and cold than solubilised protein. Mitochondrial coupling factor F1 is similarly protected by phospholipids and SQDGs although, in that case, both were equally effective. [5] [6]
Information about SQDG and the Rieske protein interaction in the cyt b6f structures is also present. SQDGs seem to be involved in the turnover of cyt f in a similar manner like D1 and raises the question of whether a similar mechanism underlies the role of SQDG in the assembly of both subunits. [7]
Extensive SQDG accumulation was observed in apple shoot bark and wood (Okanenko, 1977) and in pine thylakoid during the autumn hardening, [8] while heat and drought action upon wheat, [9] at NaCl action in the halophyte Aster tripolium . [10]
SQDG also inhibits viral development by interfering with DNA-polymerase and reverse transcriptase activity. [11]
In cyanobacteria and plants, SQDG is synthesized in two steps. First, UDP-glucose and sulfite are combined by UDP-sulfoquinovose synthase (SQD1) to produce UDP-sulfoquinovose. Second, the sulfoquinovose portion of UDP-sulfoquinovose is transferred to diacylglycerol by the glycosyltransferase SQDG synthase, to form SQDG. [12]
SQDGs degrade during sulfur starvation in some species such as Chlamydomonas reinhardtii . This response is for the redistribution of its sulfur to make new protein. [13] A wide range of bacteria cleave SQDG to form sulfoquinovose, and then metabolize the sulfoquinovose through a process termed sulfoglycolysis. SQDG are cleaved by enzymes termed sulfoquinovosidases. [14]
Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy-storage molecules ATP and NADPH while freeing oxygen from water in plant and algal cells. They then use the ATP and NADPH to make organic molecules from carbon dioxide in a process known as the Calvin cycle. Chloroplasts carry out a number of other functions, including fatty acid synthesis, much amino acid synthesis, and the immune response in plants. The number of chloroplasts per cell varies from one, in unicellular algae, up to 100 in plants like Arabidopsis and wheat.
Metabolism is the set of life-sustaining chemical reactions in organisms. The three main purposes of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the conversion of food to building blocks for proteins, lipids, nucleic acids, and some carbohydrates; and the elimination of metabolic wastes. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. The word metabolism can also refer to the sum of all chemical reactions that occur in living organisms, including digestion and the transport of substances into and between different cells, in which case the above described set of reactions within the cells is called intermediary metabolism or intermediate metabolism. In various diseases, such as type II diabetes, metabolic syndrome, and cancer, normal metabolism is disrupted.
In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name organelle comes from the idea that these structures are parts of cells, as organs are to the body, hence organelle, the suffix -elle being a diminutive. Organelles are either separately enclosed within their own lipid bilayers or are spatially distinct functional units without a surrounding lipid bilayer. Although most organelles are functional units within cells, some functional units that extend outside of cells are often termed organelles, such as cilia, the flagellum and archaellum, and the trichocyst.
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. This chemical energy is stored in carbohydrate molecules, such as sugars and starches, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek phōs, "light", and sunthesis, "putting together". In most cases, oxygen is also released as a waste product. Most plants, algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the energy necessary for life on Earth.
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Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as grana. Grana are connected by intergranal/stromal thylakoids, which join granum stacks together as a single functional compartment.
Chloroplasts contain several important membranes, vital for their function. Like mitochondria, chloroplasts have a double-membrane envelope, called the chloroplast envelope, but unlike mitochondria, chloroplasts also have internal membrane structures called thylakoids. Furthermore, one or two additional membranes may enclose chloroplasts in organisms that underwent secondary endosymbiosis, such as the euglenids and chlorarachniophytes.
Chloroflexus aurantiacus is a photosynthetic bacterium isolated from hot springs, belonging to the green non-sulfur bacteria. This organism is thermophilic and can grow at temperatures from 35 °C to 70 °C. Chloroflexus aurantiacus can survive in the dark if oxygen is available. When grown in the dark, Chloroflexus aurantiacus has a dark orange color. When grown in sunlight it is dark green. The individual bacteria tend to form filamentous colonies enclosed in sheaths, which are known as trichomes.
Glycolipids are lipids with a carbohydrate attached by a glycosidic (covalent) bond. Their role is to maintain the stability of the cell membrane and to facilitate cellular recognition, which is crucial to the immune response and in the connections that allow cells to connect to one another to form tissues. Glycolipids are found on the surface of all eukaryotic cell membranes, where they extend from the phospholipid bilayer into the extracellular environment.
Photosystems are functional and structural units of protein complexes involved in photosynthesis. Together they carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. Photosystems are found in the thylakoid membranes of plants, algae, and cyanobacteria. These membranes are located inside the chloroplasts of plants and algae, and in the cytoplasmic membrane of photosynthetic bacteria. There are two kinds of photosystems: PSI and PSII.
Photosystem I is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane protein complex that uses light energy to catalyze the transfer of electrons across the thylakoid membrane from plastocyanin to ferredoxin. Ultimately, the electrons that are transferred by Photosystem I are used to produce the high energy carrier NADPH. The combined action of the entire photosynthetic electron transport chain also produces a proton-motive force that is used to generate ATP. PSI is composed of more than 110 cofactors, significantly more than Photosystem II.
Chlorophyll a is a specific form of chlorophyll used in oxygenic photosynthesis. It absorbs most energy from wavelengths of violet-blue and orange-red light, and it is a poor absorber of green and near-green portions of the spectrum. Chlorophyll does not reflect light but chlorophyll-containing tissues appear green because green light, diffusively reflected by structures like cell walls, becomes enriched in the reflected light. This photosynthetic pigment is essential for photosynthesis in eukaryotes, cyanobacteria and prochlorophytes because of its role as primary electron donor in the electron transport chain. Chlorophyll a also transfers resonance energy in the antenna complex, ending in the reaction center where specific chlorophylls P680 and P700 are located.
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Membrane lipids are a group of compounds which form the double-layered surface of all cells. The three major classes of membrane lipids are phospholipids, glycolipids, and cholesterol. Lipids are amphiphilic: they have one end that is soluble in water ('polar') and an ending that is soluble in fat ('nonpolar'). By forming a double layer with the polar ends pointing outwards and the nonpolar ends pointing inwards membrane lipids can form a 'lipid bilayer' which keeps the watery interior of the cell separate from the watery exterior. The arrangements of lipids and various proteins, acting as receptors and channel pores in the membrane, control the entry and exit of other molecules and ions as part of the cell's metabolism. In order to perform physiological functions, membrane proteins are facilitated to rotate and diffuse laterally in two dimensional expanse of lipid bilayer by the presence of a shell of lipids closely attached to protein surface, called annular lipid shell.
Chlorophyllase (klawr-uh-fil-eys) is the key enzyme in chlorophyll metabolism. It is a membrane protein that is commonly known as chlase and systematically known as chlorophyll chlorophyllidohydrolase. Chlorophyllase can be found in the chloroplast, thylakoid membrane and etioplast of at least higher plants such as ferns, mosses, brown and red algae and diatoms. Chlase is the catalyst for the hydrolysis of chlorophyll to produce chlorophyllide and phytol. It is also known to function in the esterification of Chlide and transesterification. The enzyme functions optimally at pH 8.5 and 50 °C.
UDP-sulfoquinovose synthase (EC 3.13.1.1) is an enzyme that catalyzes the chemical reaction
In enzymology, a long-chain-alcohol O-fatty-acyltransferase is an enzyme that catalyzes the chemical reaction
Sulfoquinovose, also known as 6-sulfoquinovose and 6-deoxy-6-sulfo-D-glucopyranose, is a monosaccharide sugar that is found as a building block in the sulfolipid sulfoquinovosyl diacylglycerol (SQDG). Sulfoquinovose is a sulfonic acid derivative of glucose, the sulfonic acid group is introduced into the sugar by the enzyme UDP-sulfoquinovose synthase (SQD1). Sulfoquinovose is degraded through a metabolic process termed sulfoglycolysis. The half-life for mutarotation of sulfoquinovose at pD 7.5 and 26C is 299 minutes.
In photosynthesis, the light-dependent reactions take place on the thylakoid membranes. The inside of the thylakoid membrane is called the lumen, and outside the thylakoid membrane is the stroma, where the light-independent reactions take place. The thylakoid membrane contains some integral membrane protein complexes that catalyze the light reactions. There are four major protein complexes in the thylakoid membrane: Photosystem II (PSII), Cytochrome b6f complex, Photosystem I (PSI), and ATP synthase. These four complexes work together to ultimately create the products ATP and NADPH.
Sulfoglycolysis is a catabolic process in primary metabolism in which sulfoquinovose (6-deoxy-6-sulfonato-glucose) is metabolized to produce energy and carbon-building blocks. Sulfoglycolysis pathways occur in a wide variety of organisms, and enable key steps in the degradation of sulfoquinovosyl diacylglycerol (SQDG), a sulfolipid found in plants and cyanobacteria into sulfite and sulfate. Sulfoglycolysis converts sulfoquinovose (C6H12O8S−) into pyruvate CH3COCOO− + H+. The free energy is used to form the high-energy molecules ATP (adenosine triphosphate) and NADH (reduced nicotinamide adenine dinucleotide). Unlike glycolysis, all known sulfoglycolysis pathways convert only half the carbon content of sulfoquinovose into pyruvate; the remained is excreted as a C3-sulfonate: 2,3-dihydroxypropanesulfonate (DHPS) or sulfolactate (SL).