SQDG
Encyclopedia
Sulfoquinovosyl diacylglycerols, abbreviated SQDG, are a class of sulfur
-containing but phosphorus
-free lipid
s (sulfolipid
s) found in many photosynthetic organisms.
-containing lipid
in plants and identified it as sulfoquinovosyl diacylglycerol (SQDG). 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 acid
s of this type are chemically stable and strong acids over a wide pH
range.
s, algae
, cyanobacteria, purple sulfur
and non-sulfur bacteria and is localised in the thylakoid membranes, being the most saturated glycolipid
.
SQDGs have been found to be closely associated with certain membrane protein
s. In some cases the (electostratic) 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. 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.
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.
Extensive SQDG accumulation was observed in apple shoot bark and wood (Okanenko, 1977) and in pine thylakoid during the autumn hardening, while heat and drought action upon wheat, at NaCl action in the halophyte Aster tripolium
.
SQDG also inhibits viral development by interfering with DNA-polymerase and reverse transcriptase activity.
In cyanobacteria, SQDGs are synthesized in two stages. 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 SQDG synthase to form SQDG.
. This response is for the redistribution of its sulfur to make new protein.
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
-containing but phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
-free lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
s (sulfolipid
Sulfolipid
Sulfolipids are a class of lipids which possess a sulfur-containing functional group. One of the most common consituents of sulfolipids is sulfoquinovose, which is acylated to form sulfoquinovosyl diacylglycerols. In plants, sulfolipids are important intermediates in the sulfur cycle....
s) found in many photosynthetic organisms.
Discovery, structure and chemical properties
In 1959 A. A. Benson and coworkers discovered a new sulfurSulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
-containing lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
in plants and identified it as sulfoquinovosyl diacylglycerol (SQDG). 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 acid
Sulfonic acid
Sulfonic acid usually refers to a member of the class of organosulfur compounds with the general formula RS2–OH, where R is an alkyl or aryl. The formal part of acid, HS2–OH, are formally derivatives of the "parent" inorganic compound with the formula HSO2.-Preparation:Sulfonic acid is...
s of this type are chemically stable and strong acids over a wide pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
range.
Biological occurrence and functions
SQDGs have been found in all photosynthetic plantPlant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s, algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
, cyanobacteria, purple sulfur
Purple sulfur bacteria
The purple sulfur bacteria are a group of Proteobacteria capable of photosynthesis, collectively referred to as purple bacteria. They are anaerobic or microaerophilic, and are often found in hot springs or stagnant water. Unlike plants, algae, and cyanobacteria, they do not use water as their...
and non-sulfur bacteria and is localised in the thylakoid membranes, being the most saturated glycolipid
Glycolipid
Glycolipids are lipids with a carbohydrate attached. Their role is to provide energy and also serve as markers for cellular recognition.-Metabolism:...
.
SQDGs have been found to be closely associated with certain membrane protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s. In some cases the (electostratic) interactions may be very strong, as suggested by the inability of saturated SQDG molecules associated with purified chloroplast CF0-CF1 ATPase
ATPase
ATPases are a class of enzymes that catalyze the decomposition of adenosine triphosphate into adenosine diphosphate and a free phosphate ion. This dephosphorylation reaction releases energy, which the enzyme harnesses to drive other chemical reactions that would not otherwise occur...
to exchange with other acidic lipids. 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
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
coupling factor F1 is similarly protected by phospholipids and SQDGs although, in that case, both were equally effective.
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.
Extensive SQDG accumulation was observed in apple shoot bark and wood (Okanenko, 1977) and in pine thylakoid during the autumn hardening, while heat and drought action upon wheat, at NaCl action in the halophyte Aster tripolium
Aster tripolium
Aster tripolium is a plant of northern Europe that is confined in its distribution to salt marshes, estuaries and occasionally to inland salt works....
.
SQDG also inhibits viral development by interfering with DNA-polymerase and reverse transcriptase activity.
Synthesis in cyanobacteria
SQDGs are synthesized from UDP-sulfoquinovose and diacylglycerols.In cyanobacteria, SQDGs are synthesized in two stages. First, UDP-glucose and sulfite
Sulfite
Sulfites are compounds that contain the sulfite ion SO. The sulfite ion is the conjugate base of bisulfite. Although the acid itself is elusive, its salts are widely used.-Structure:...
are combined by UDP-sulfoquinovose synthase
UDP-sulfoquinovose synthase
UDP-sulfoquinovose synthase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are UDP-glucose and sulfite, whereas its two products are UDP-6-sulfoquinovose and H2O....
(SQD1) to produce UDP-sulfoquinovose. Second, the sulfoquinovose portion of UDP-sulfoquinovose is transferred to diacylglycerol by SQDG synthase to form SQDG.
Degradation during sulfur starvation
SQDGs degrade during sulfur starvation in some species such as Chlamydomonas reinhardtiiChlamydomonas reinhardtii
Chlamydomonas reinhardtii is a single celled green alga about 10 micrometres in diameter that swims with two flagella. They have a cell wall made of hydroxyproline-rich glycoproteins, a large cup-shaped chloroplast, a large pyrenoid, and an "eyespot" that senses light.Although widely distributed...
. This response is for the redistribution of its sulfur to make new protein.