Sulfoquinovosyl diacylglycerols, abbreviated SQDG, are a class of

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

-containing

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

-free

lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...

s (

sulfolipid Sulfolipids are a class of lipids which possess a sulfur-containing functional group. An abundant sulfolipid is sulfoquinovosyl diacylglycerol, which is composed of a glycoside of sulfoquinovose and diacylglycerol. In plants, sulfoquinovosyl d ...

s) found in many photosynthetic organisms.

Discovery and properties

In 1959 A. A. Benson and coworkers discovered a new

-containing

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 In organic chemistry, sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula , where R is an organic alkyl or aryl group and the group a sulfonyl hydroxide. As a substituent, it is kn ...

s of this type are chemically stable and strong acids.

Biological occurrence and functions

SQDGs have been found in all

photosynthetic Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...

purple sulfur The purple sulfur bacteria (PSB) are part of a group of Pseudomonadota capable of photosynthesis, collectively referred to as purple bacteria. They are Anaerobic organism, anaerobic or microaerophilic, and are often found in stratified water env ...

and non-sulfur bacteria and is localised in the

thylakoid membranes 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 thylak ...

, being the most saturated

glycolipid 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 c ...

. SQDGs have been found to be closely associated with certain membrane

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

s. 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 ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, ATP hydrolase, adenosine triphosphatase) are a class of enzymes that catalyze the decomposition of ATP into ADP and a free phosphate ion or ...

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 A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

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 ''Tripolium pannonicum'', called sea aster or seashore aster and often known by the synonyms ''Aster tripolium'' or ''Aster pannonicus'', is a flowering plant, native to Eurasia and northern Africa, that is confined in its distribution to salt ma ...

''. SQDG also inhibits viral development by interfering with DNA-polymerase and reverse transcriptase activity.

Biosynthesis

In cyanobacteria and plants, SQDG is synthesized in two steps. First,

UDP-glucose Uridine diphosphate glucose (uracil-diphosphate glucose, UDP-glucose) is a nucleotide sugar. It is involved in glycosyltransferase reactions in metabolism. Functions UDP-glucose is used in nucleotide sugar metabolism as an activated form of g ...

and

sulfite Sulfites or sulphites are compounds that contain the sulfite ion (systematic name: sulfate(IV) ion), . The sulfite ion is the conjugate base of bisulfite. Although its acid (sulfurous acid) is elusive, its salts are widely used. Sulfites are ...

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 Glycosyltransferases (GTFs, Gtfs) are enzymes ( EC 2.4) that establish natural glycosidic linkages. They catalyze the transfer of saccharide moieties from an activated nucleotide sugar (also known as the "glycosyl donor") to a nucleophilic gl ...

SQDG synthase, to form SQDG.

Degradation

SQDGs degrade during sulfur starvation in some species such as ''

Chlamydomonas reinhardtii ''Chlamydomonas reinhardtii'' is a single-cell green alga about 10 micrometres in diameter that swims with two flagella. It has a cell wall made of hydroxyproline-rich glycoproteins, a large cup-shaped chloroplast, a large pyrenoid, and a ...

''. This response is for the redistribution of its sulfur to make new protein. 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.

References

{{reflist Lipids Sulfonic acids Sulfate esters

Discovery and properties

Biological occurrence and functions

Biosynthesis

Degradation

See also

References