Sulfoquinovosyl diacylglycerols, abbreviated SQDG, are a class of

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

-containing but

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

-free

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

s ( sulfolipids) found in many photosynthetic organisms.

Discovery, structure and chemical 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 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. Some of this chemical energy is stored in c ...

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

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 or microaerophilic, and are often found in stratified water environments including ...

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 thyl ...

, 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 connec ...

. 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 residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

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, SV40 T-antigen, ATP hydrolase, complex V (mitochondrial electron transport), (Ca2+ + Mg2+)-ATPase, HCO3−-ATPase, adenosine triphosphatase) are ...

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 use ...

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 gl ...

and

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

are combined by

UDP-sulfoquinovose synthase UDP-sulfoquinovose synthase () is an enzyme that catalyzes the chemical reaction :UDP-glucose + sulfite \rightleftharpoons UDP-6-sulfoquinovose + H2O Thus, the two substrates of this enzyme are UDP-glucose and sulfite, whereas its two products ...

(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 glycos ...

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 an ...

''. 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, structure and chemical properties

Biological occurrence and functions

Biosynthesis

Degradation

See also

References