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Phosphoglycerides are lipids that contain glycerol with a phosphate ester on one end and two fatty acid ester side chains. Phosphoglycerides such as phosphatidylethanolamine and phosphatidylserine are the major component of lipid bilayer membranes in which their detergent-like properties help to provide membranes the quality of self-assembly. Phosphatidylglycerol is a phospholipid in which the phosphate ester on glycerol is joined to another glycerol. Phosphatidylglycerol is a precursor in the synthesis of both cardiolipin and phosphatidylinositol. In cardiolipin, two phosphatidylglycerol units are joined together. Cardiolipin was first identified in the heart, thus its name, and is abundant in the heart. Antibodies against cardiolipin have been observed in the blood of individuals with autoimmune disorders such as lupus. In mammals, cardiolipin is localized in mitochondria and may play a role in oxidative phosphorylation. Changes in cardiolipin levels with age might alter mitochondrial function and contribute to the aging process. Another derivative of phosphatidylglycerol is phosphatidylinositol in which the sugar inositol forms the phosphate ester. Inositol is an important intermediary in intracellular signaling pathways. Esterification of inositol with phosphates at various positions alters its interaction with different signaling proteins. Phosphatidylinositol can be phosphorylated at two positions to create PIP2. Hydrolysis of PIP2 by phospholipase C in response to GPCR activation releases IP3, which acts as a cytoplasmic second messenger to stimulate calcium release. (This definition may be outdated - see the DesignNote.)
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