MCAT Biochemistry
Lipid transport is essential for the distribution of lipids throughout the body. Most lipids hitch a ride inside water-soluble lipoproteins that include cholesterol, specialized proteins called apoproteins, and a single layer of phospholipids. However, fatty acids are the exception as they are transported in the blood bound to the serum protein, albumin. There are various lipoproteins responsible for lipid transport, which can be distinguished by their density, lipids they carry, origin, and final destination.
Chylomicrons have the lowest density and primarily transport triglycerides from the intestinal mucosal cells to peripheral tissues. Very Low-Density Lipoproteins (VLDL) are assembled in the liver and transport triglycerides to peripheral tissues. Low-Density Lipoproteins (LDL) are often referred to as “bad” cholesterol and deliver cholesterol to tissues for biosynthesis. High-Density Lipoproteins (HDL), also known as “good” cholesterol, are synthesized in the liver and intestines. They pick up excess cholesterol in the bloodstream and carry it to the liver for removal from the body. Some HDL also delivers cholesterol to tissues that synthesize steroid hormones, including sex steroids and corticosteroids. Lastly, Intermediate Density Lipoproteins (IDL) are formed when triglycerides are removed from VLDL. In the liver, they can either be further processed to become LDL or reabsorbed directly.
Lesson Outline
<ul> <li>Lipid transporters: lipoproteins</li> <li>Fatty acids: transported by serum albumin (a plasma protein)</li> <li>Lipoproteins</li> <ul> <li>Composition: phospholipids, cholesterol, apoproteins</li> <li>Carry lipids internally (associated with fatty acids)</li> </ul> <li>Differ by density, lipids carried, origin, and destination</li> <ul> <li>Chylomicrons: least dense</li> <li>Very low density lipoproteins (VLDL)</li> <li>Intermediate density lipoproteins (IDL)</li> <li>Low density lipoproteins (LDL): "bad" cholesterol</li> <li>High density lipoproteins (HDL): "good" cholesterol</li> </ul> <li>Chylomicrons</li> <ul> <li>Carry triglycerides and cholesterol</li> <li>Transport from mucosal cells to bloodstream</li> <li>Deliver triglycerides to peripheral tissues</li> </ul> <li>Very low density lipoproteins (VLDL)</li> <ul> <li>Produced in the liver</li> <li>Transport triglycerides and fatty acids to peripheral tissues</li> </ul> <li>Low density lipoproteins (LDL)</li> <ul> <li>Deliver cholesterol to tissues for biosynthesis</li> <li>Contribute to obstructive plaques in blood vessels if in excess</li> </ul> <li>High density lipoproteins (HDL)</li> <ul> <li>Synthesized and released in the liver and intestines</li> <li>Pick up excess cholesterol from bloodstream and deliver to liver for removal</li> <li>Also deliver cholesterol to tissues that synthesize steroid hormones</li> </ul> <li>Intermediate density lipoproteins (IDL)</li> <ul> <li>Transition particles between triglyceride transport and cholesterol transport</li> <li>Created from VLDL when triglycerides are removed</li> <li>Can be processed into LDL or reabsorbed by the liver</li> </ul> </ul>
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FAQs
Lipoproteins have a vital role in lipid transport as they help the transportation of water-insoluble lipids in the blood and lymph. They consist of a hydrophilic shell and hydrophobic core and carry lipids such as cholesterol, phospholipids, and triglycerides throughout the body. There are different types of lipoproteins, including chylomicrons, very low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL), each with specific functions and characteristics.
Chylomicrons are the largest lipoproteins synthesized in the intestines after ingestion of dietary lipids. Their primary function in lipid transport is to transport dietary lipids (mainly triglycerides) from the intestines to other tissues such as adipose (fat) and muscle for energy storage and consumption. Once the triglycerides are hydrolyzed by lipoprotein lipase, the chylomicron remnants are cleared by the liver, where their lipid components can be repurposed or metabolized.
High-density lipoproteins (HDL) contribute to lipid transport by primarily facilitating the removal of cholesterol from peripheral tissues to the liver for breakdown and excretion. This process is known as reverse cholesterol transport. As HDL removes excess cholesterol from the bloodstream and prevents its deposition in arteries, it helps maintain arterial health, reducing the risk of atherosclerosis and cardiovascular disease. Therefore, HDL is often referred to as "good" cholesterol.
Apoproteins, also known as apolipoproteins, are protein components of lipoproteins that play essential roles in lipoprotein function and lipid transport. They serve as structural components, co-factors for enzymes, and ligands for receptors necessary for lipoprotein metabolism. Some apoproteins stabilize lipoproteins, while others have a regulatory function, assisting enzymes or binding to receptors to facilitate lipid uptake and clearance. These interactions ultimately contribute to maintaining lipid homeostasis in the body.
Fatty acids, also known as free fatty acids, are transported in the blood bound to serum albumin, a plasma protein. Serum albumin has multiple binding sites for fatty acids and acts as a carrier, facilitating their transportation to cells that require them for energy production or other metabolic processes. This interaction allows the water-insoluble fatty acids to remain soluble in the blood and effectively reach their target sites in the body.
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