Sulfoquinovosyl diacylglycerol

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Sulfoquinovosyl diacylglycerol (SQDG) is a sulfur-containing, phosphorus-free lipid (a sulfolipid) found in many photosynthetic organisms. It was discovered in 1959 by A. A. Benson and colleagues. Its key feature is a sulfoquinovose sugar attached to a glycerol backbone with two fatty acids, connected by a stable C-SO3 bond.

Where it’s found and what it does
- SQDG is present in all photosynthetic plants, algae, cyanobacteria, and some bacteria. It sits in the thylakoid membranes of chloroplasts and is the most saturated glycolipid there.
- It often sits close to certain membrane proteins and can interact strongly with them, helping stabilize energy-making machines in the cell, especially under temperature stress.

Interactions with energy machinery
- SQDG can protect the chloroplast ATP synthase (CF1) from cold damage and helps membrane-bound CF1 be more heat- and cold-resistant than when it’s not bound to membranes.
- It also appears to interact with other parts of the photosynthetic system, such as the cyt b6f complex and components involved in building or stabilizing protein subunits.

Natural variations and stress
- Scientists have observed high SQDG levels in some plants during stress or seasonal changes, such as in apple bark/wood and pine leaves in autumn, and in wheat under heat or drought, as well as in salt-tolerant plants like Aster tripolium.

Antiviral activity
- SQDG can interfere with certain viral enzymes, like DNA polymerase and reverse transcriptase, showing some antiviral effects.

How SQDG is made
- In cyanobacteria and plants, SQDG is built in two steps:
1) UDP-glucose reacts with sulfite via the enzyme SQD1 to make UDP-sulfoquinovose.
2) The sulfoquinovose part is transferred to diacylglycerol by SQDG synthase, forming SQDG.

How it’s broken down and reused
- When sulfur is scarce, some organisms break down SQDG to release sulfur for other uses, such as making proteins.
- In bacteria, SQDG can be cleaved to release sulfoquinovose, which is then fed into a sulfur-containing metabolic pathway called sulfoglycolysis. Enzymes called sulfoquinovosidases perform the initial cut.