02 - Lipids & Membranes
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What are the major membrane lipids?
Glycerophospholipids (phosphoglycerides), sphingolipids, and cholesterol
What does a phosphoglyceride look like?
Glycerol backbone, with two hydroxyl groups being esterified with fatty acids (typically the FA on C-1 is saturated and C-2 is unsaturated) and the third hydroxyl group is attached to a polar alcohol via a phosphodiester linkage
Why do the presence of double bonds decrease the melting point?
It prevents the formation of closely packed, well-ordered crystals during solidification
What is a sphingolipid?
Instead of having a glycerol, there is an amino alcohol; when there is a FA attached to the amino group via an amide linkage, the compound is called ceramide
What is the most abundant sphingolipid?
Sphingomyelin; formed by esterification of the terminal hydroxyl group of ceramide with phosphoryl choline
What are glycosphingolipids?
Do not contain a phosphate and are formed by the attachment of a sugar residue to the terminal hydroxyl group of ceramide by a β glycosidic linkage
What are the two important subgroups of glycosphingolipids?
Cerebrosides and gangliosides; cerebosides have a glucose or galactose attached to the ceramide; gangliosides contain oligosaccharides
When this lipid is hydrolyzed by phospholipase C, it produces two signaling molecules, diacylglycerol and inositol triphosphate; diacylglycerol activates protein kinase C; inositol triphosphate increases intracellular calcium levels
Sphingomyelinase acts on sphingomyelin in the membrane and releases ceramine into the cell; ceramide is an important 2° messenger
In what forms are membrane carbohydrates present?
Glycoproteins and glycolipids; only found on the exoplasmic surface of the cell
How do integral proteins diffuse in the phospholipid bilayer?
Diffuse parallel to the plane of the bilayer
What is the role of flippases?
Maintain the asymmetry of lipids in membranes by catalyzing unidirectional movement of specific lipids from one leaflet to another
What do platelets, aged or sickled erythrocytes, erythrocytes infected with malarial parasites, and apoptotic cells have in common?
In these cells the asymmetry of phospholipid distribution has broken down, and anionic phospholipids are exposed on the exoplasmic surface; this allows recognition by scavenger receptors on circulating mononuclear phagocytes and initiates phagocytosis
Binds annexin V; fluorescein-labeled annexin V will bind to apoptotic cells but not to normal healthy cells
How are integral membrane proteins embedded in the nonpolar hydrocarbon regions of the phospholipid bilayer?
Exist in an α-helical formation, with hydrophobic AAs (Leu, Ile, Val, Trp, Phe, Tyr, Met) as the predominate AA; porins exist in β-barrel composed of antiparallel β sheets with hydrophobic AAs facing outside the barrel and hydrophilic AAs are facing inward forming the aqueous pore
How are peripheral membrane proteins attached to the membrane?
There are other ways, but two main ways are: electrostatic binding between the negatively charged head groups of the membrane lipids and the positively charged regions of proteins; hydrophobic region containing nonpolar AAs at one end of the protein, which acts as an anchor in the membrane lipid
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