Phospholipids are formed from four components: fatty acids, a negatively charged phosphate group, an alcohol and a backbone. Phospholipids with a glycerol backbone are known as glycerophospholipids or phosphoglycerides.

Only one type of phospholipid possesses a sphingosine backbone – sphingomyelin. Sphingomyelin is present in all eukaryotic cell membranes, but is mainly present in cells of the nervous system. Phospholipids, along with glycolipids and cholesterol, are a major component of all biological membranes. (click to enlarge image)

Phospholipases are enzymes that hydrolyze specific ester bonds in phosphoglycerides or glycerophosphatidates, converting the phospholipids into fatty acids and other lipophilic substances. Phospholipases are involved in signaling cascades. Phospholipase A1 hydrolyzes the acyl group attached to the 1-position, while phospholipase A2 hydrolyzes the acyl group attached to the 2-position to form fatty acid and lysophospholipid products. Phospholipase A2 is responsible for the release of arachidonic acid from membranes (flow diagram PLA2 pathway). Arachidonic acid is a signalling molecule and is the precursor for eicosanoid signaling molecules, which include leukotrienes and prostaglandins. Some eicosanoids are synthesized from diacylglycerol, and are released from the lipid bilayer by phospholipase C.



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