Membrane lipid
Membrane lipids are a group of compounds (structurally similar to fats and oils) which form the
Biological roles
The bilayer formed by membrane lipids serves as a containment unit of a living cell. Membrane lipids also form a matrix in which membrane proteins reside. Historically lipids were thought to merely serve a structural role. Functional roles of lipids are in fact many: They serve as regulatory agents in cell growth and adhesion. They participate in the biosynthesis of other biomolecules. They can serve to increase enzymatic activities of enzymes.[1]
Non-bilayer forming lipid like monogalactosyl diglyceride (MGDG) predominates the bulk lipids in
Major classes
Phospholipids
Phospholipids and glycolipids consist of two long, nonpolar (
The heads of phospholipids are phosphorylated and they consist of either:
- Glycerol (and hence the name phosphoglycerides given to this group of lipids), or
- Sphingosine (e.g. sphingomyelin and ceramide).
Glycerol dialkyl glycerol tetraether (GDGT) is helping to study ancient environmental factors.[3]
Glycolipids
The heads of glycolipids (glyco- stands for sugar) contain a sphingosine with one or several sugar units attached to it. The hydrophobic chains belong either to:
- two fatty acids (FA) – in the case of the phosphoglycerides, or
- one FA and the hydrocarbon tail of sphingosine – in the case of sphingomyelin and the glycolipids.
Galactolipids – monogalactosyl diglyceride (MGDG) and digalactosyl diglycreride (DGDG) form the predominant lipids in higher plant chloroplast thylakoid membranes; liposomal structures formed by total lipid extract of thylakoid membranes have been found sensitive to sucrose as it turns bilayers into micellar structures.[4]
Fatty acids
The fatty acids in phospho- and glycolipids usually contain an even number, typically between 14 and 24, of carbon atoms, with 16- and 18-carbon being the most common. FAs may be saturated or unsaturated, with the configuration of the double bonds nearly always cis. The length and the degree of unsaturation of FAs chains have a profound effect on membranes' fluidity. Plant thylakoid membranes maintain high fluidity, even at relatively cold environmental temperatures, due to the abundance of 18-carbon fatty acyl chains with three double bonds, linolenic acid, as has been revealed by 13-C NMR studies.[5]
Phosphoglycerides
In
Sphingolipids
In glycolipids, the sugar component is attached to this group. The simplest glycolipid is cerebroside, in which there is only one sugar residue, either Glc or Gal. More complex glycolipids, such as gangliosides, contain a branched chain of as many as seven sugar residues.
Sterols
The best known
Cholesterol is bio-synthesised from
See also
- Homeoviscous adaptation
- Protein-lipid interaction
References
- ^ R. B. Gennis. Biomembranes - Molecular Structure and Function. Springer-Verlag, New York (1989).
- ^ YashRoy R.C. (1990) Lamellar dispersion and phase separation of chloroplast membrane lipids by negative staining electron microscopy. Journal of Biosciences, vol. 15(2), pp. 93-98.https://www.researchgate.net/publication/230820037_Lamellar_dispersion_and_phase_separation_of_chloroplast_membrane_lipids_by_negative_staining_electron_microscopy?ev=prf_pub
- .
- ^ YashRoy R.C. (1994) Destabilisation of lamellar dispersion of thylakoid membrane lipids by sucrose. Biochimica et Biophysica Acta, vol. 1212, pp. 129-133.https://www.researchgate.net/publication/15042978_Destabilisation_of_lamellar_dispersion_of_thylakoid_membrane_lipids_by_sucrose?ev=prf_pub
- ^ YashRoy R.C. (1987) 13-C NMR studies of lipid fatty acyl chains of chloroplast membranes. Indian Journal of Biochemistry and Biophysics, vol. 24(6), pp. 177-178.https://www.researchgate.net/publication/230822408_13-C_NMR_studies_of_lipid_fatty_acyl_chains_of_chloroplast_membranes?ev=prf_pub
- ^ de Meyer F, Smit B. Effect of cholesterol on the structure of a phospholipid bilayer. Proc Natl Acad Sci U S A 2009; 106: 3654-8.
- PMID 8301234.
- ^ Lang T, Bruns D, Wenzel D, Riedel D, Holroyd P, Thiele C, Jahn R. SNAREs are concentrated in cholesterol-dependent clusters that define docking and fusion sites for exocytosis EMBO J 2001;20:2202-13.
External links
- Membrane+lipids at the U.S. National Library of Medicine Medical Subject Headings (MeSH)