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How do defects in lipoprotein proteins, receptors, or enzymes cause lipid metabolism disorders?
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Lipid
Structure and properties of two representative lipids. Both stearic acid (a fatty acid) and phosphatidylcholine (a phospholipid) are composed of chemical groups that form polar “heads” and nonpolar “tails.” The polar heads are hydrophilic, or soluble in water, whereas the nonpolar tails are hydrophobic, or insoluble in water. Lipid molecules of this composition spontaneously form aggregate structures such as micelles and lipid bilayers, with their hydrophilic ends oriented toward the watery medium and their hydrophobic ends shielded from the water.
Lipid metabolism disorders often arise from genetic defects in the structural proteins of lipoproteins, cell surface receptors, or metabolic enzymes. These defects disrupt the normal processing, transport, and cellular uptake of lipids, leading to their pathological accumulation in tissues or blood vessels.1
Mechanisms of Disorder
- Receptor Defects: In familial hypercholesterolemia, a deficiency or dysfunction of the LDL receptor prevents cells from internalizing cholesterol. This not only causes cholesterol to build up in the blood but also disrupts feedback inhibition, causing cells to synthesize even more cholesterol, which accelerates atherosclerosis.12
- Structural Protein Defects: Abnormalities in apoproteins, such as apoprotein E, can impair the proper clearance of lipoproteins, leading to elevated blood cholesterol and triglycerides, as seen in familial dysbetalipoproteinemia.1
- Enzyme Deficiencies: Defects in enzymes can prevent the breakdown of lipids. For example, lipid storage diseases like Gaucher’s disease or Tay-Sachs disease occur when specific enzymes fail to metabolize lipids within the cell, causing toxic accumulation that interferes with organ and nervous system function.13