B. Lipoprotein lipase on capillary endothelial surfaces in muscle and adipose tissue degrades the triacylglycerols (TAG) in chylomicrons into fatty acids (FA) and glycerol. This degradation provides a source of FA to these tissues for degradation or storage and provides glycerol for hepatic metabolism. In the duodenum, TAG is degraded into one 2-monoacylglycerol + two free FA, which are then absorbed. Medium and short-chain FA enter directly into the blood, not lymph, and they do not require micelles or get packaged into chylomicrons. A defect in fat absorption would lead to a decrease in chylomicron production.
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I. OVERVIEW
Fatty acids can exist as free (nonesterified) in the body or as fatty acyl esters in more complex molecules like triacylglycerols (TAG). Although all tissues have low levels of free fatty acids (FFA), substantial amounts can be found in the plasma, especially during fasting. Plasma FFA, transported on serum albumin, travel from their point of origin (TAG in adipose tissue or circulating lipoproteins) to their destination (most tissues) for consumption. FFA can be oxidized by various tissues, particularly the liver and muscle, to provide energy and serve as a substrate for ketone body synthesis in the liver. Fatty acids also serve as structural components in membrane lipids, such as phospholipids and glycolipids. Fatty acids attached to specific proteins enhance their ability to associate with membranes. Additionally, fatty acids act as precursors for prostaglandins, hormone-like molecules. White adipose tissue (WAT) stores esterified fatty acids in the form of TAG, which serves as the body's primary energy reserve. Obesity and diabetes are associated with alterations in fatty acid metabolism. Figure 16.1 illustrates the metabolic pathways of fatty acid synthesis and degradation and their relationship to carbohydrate metabolism.
II. FATTY ACID STRUCTURE
A fatty acid consists of a hydrophobic hydrocarbon chain with a carboxyl group at the end, which has a pKa of approximately 4.8. At physiological pH, the terminal carboxyl group (–COOH) becomes ionized, forming –COO−. The anionic group has an affinity for water, giving the fatty acid its amphipathic nature, with both a hydrophilic and a hydrophobic region. However, for long-chain fatty acids (LCFA), the hydrophobic portion predominates. LCFA molecules are highly insoluble in water and require transport in the circulation in association with proteins. More than 90% of the fatty acids in plasma are in the form of fatty acid esters, primarily TAG, cholesteryl esters, and phospholipids, which are contained in circulating lipoprotein particles. FFA are transported in the circulation in association with albumin, the most abundant protein in serum.
A. Fatty acid saturation
Correct answer = B. The triacylglycerols (TAG) in chylomicrons are degraded to fatty acids (FA) and glycerol by lipoprotein lipase on capillary endothelial surfaces in muscle and adipose tissue, thus providing a source of FA to these tissues for degradation or storage and providing glycerol for hepatic metabolism. In the duodenum, TAG are degraded to one 2-monoacylglycerol + two free FA that get absorbed. Medium-and short-chain FA enter directly into blood (not lymph), and they neither require micelles nor get packaged into chylomicrons. Because chylomicrons contain dietary lipids that were digested and absorbed, a defect in fat absorption would result in decreased production of chylomicrons.
Fatty Acid, Triacylglycerol, and Ketone Body Metabolism 16
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I. OVERVIEW
Fatty acids exist free in the body (that is, they are nonesterified) and as fatty acyl esters in more complex molecules such as triacylglycerols (TAG). Low levels of free fatty acids (FFA) occur in all tissues, but substantial amounts can sometimes be found in the plasma, particularly during fasting. Plasma FFA (transported on serum albumin) are in route from their point of origin (TAG of adipose tissue or circulating lipoproteins) to their site of consumption (most tissues). FFA can be oxidized by many tissues, particularly liver and muscle, to provide energy and, in the liver, to provide the substrate for ketone body synthesis. Fatty acids are also structural components of membrane lipids, such as phospholipids and glycolipids (see p. 201). Fatty acids attached to certain proteins enhance the ability of those proteins to associate with membranes (see p. 206). Fatty acids are also precursors of the hormone-like prostaglandins (see p. 213). Esterified fatty acids, in the form of TAG stored in white adipose tissue (WAT), serve as the major energy reserve of the body. Alterations in fatty acid metabolism are associated with obesity and diabetes. Figure 16.1 illustrates the metabolic pathways of fatty acid synthesis and degradation and their relationship to carbohydrate metabolism.
II. FATTY ACID STRUCTURE
A fatty acid consists of a hydrophobic hydrocarbon chain with a terminal carboxyl group that has a pKa (see p. 6) of ~4.8 (Fig. 16.2). At physiologic pH, the terminal carboxyl group (–COOH) ionizes, becoming –COO−. [Note: When the pH is above the pK, the deprotonated form predominates (see p. 7).] This anionic group has an affinity for water, giving the fatty acid its amphipathic nature (having both a hydrophilic and a hydrophobic region). However, for longchain-length fatty acids (LCFA), the hydrophobic portion is predominant. These molecules are highly water insoluble and must be transported in the circulation in association with protein. More than 90% of the fatty acids found in plasma are in the form of fatty acid esters (primarily TAG, cholesteryl esters, and phospholipids) contained in circulating lipoprotein particles (see p. 227). FFA are transported in the circulation in association with albumin, the most abundant protein in serum.
A. Fatty acid saturation