Metabolism of Macromolecules: Lipid Digestion and Absorption

Digestion, Absorption, and Transportation of Lipids

• Triacylglycerols (triglycerides) are the major form of energy, making up 90% of dietary lipids
• They provide 6 times more energy per unit weight than glycogen
• Triacylglycerols are water-insoluble and require emulsification by bile salts and bile acids in the small intestine for digestion
• Bile acids have a detergent character, helping to solubilize and absorb lipids in the gut
• Lipid absorption occurs in enterocytes as micelles with bile salts and lecithin or as lipid-protein complexes
• Lipids are transported as lipoproteins, which are lipid-protein complexes that make lipids soluble

Lipid Metabolism

• Lipid metabolism involves the breakdown of fats (lipids) for energy and storage
• The process involves multiple steps and enzymes that are tightly regulated
• Biosynthesis of triacylglycerols (lipogenesis) involves the incorporation of fatty acids into triacylglycerols for storage or into phospholipid components of membranes
• Triacylglycerols and glycerophospholipids share two precursors: fatty acyl-CoA and L-glycerol 3-phosphate
• The majority of glycerol 3-phosphate is derived from dihydroxyacetone phosphate (DHAP) by the action of cytosolic NAD-linked glycerol 3-phosphate dehydrogenase
• Fatty acyl-CoAs are formed from fatty acids by acyl-CoA synthetases, the same enzymes responsible for the activation of fatty acids for β-oxidation

Biosynthesis of Triacylglycerols

• The first stage in biosynthesis is the acylation of the two free hydroxyl groups of L-glycerol 3-phosphate by two molecules of fatty acyl-CoA to yield diacylglycerol 3-phosphate (phosphatidic acid)
• Phosphatidic acid is a central intermediate in lipid biosynthesis and can be converted to a triacylglycerol or a glycerophospholipid
• In the pathway to triacylglycerols, phosphatidic acid is hydrolyzed by phosphatidic acid phosphatase (lipin) to form a 1,2-diacylglycerol
• Diacylglycerols are then converted to triacylglycerols by transesterification with a third fatty acyl-CoA

Lipolysis

• Lipolysis is the process by which stored fat in adipose tissue is broken down into free fatty acids and glycerol
• This process is regulated by hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL)
• Fatty acids are mobilized from triacylglycerols and oxidized to meet the energy needs of the cell or organism

Fatty Acid Oxidation

• Fatty acid oxidation involves the breakdown of fatty acids into acetyl-CoA, which is then processed in the citric acid cycle
• The process of degradation converts an aliphatic compound into a set of activated acetyl units (acetyl-CoA) that can be processed by the citric acid cycle
• The repetitive four-step process by which fatty acids are converted into acetyl-CoA is called β-oxidation
• Triacylglycerols are highly concentrated stores of metabolic energy because they are reduced and anhydrous

Ketone Bodies

• Ketogenesis occurs when excess fatty acids are converted into ketone bodies, such as acetoacetate, β-hydroxybutyrate, and acetone
• This process occurs in the liver and is regulated by enzymes such as β-ketoacyl-CoA transferase and β-hydroxybutyrate dehydrogenase
• Ketone bodies are produced when the supply of carbohydrates is low and the body needs to use fat as its primary fuel source
• Acetoacetate and D-β-hydroxybutyrate are transported by the blood to tissues other than the liver, where they are converted to acetyl-CoA and oxidized in the citric acid cycle

Digestion, Absorption, and Transportation of Lipids

• Triacylglycerols (triglycerides) are the major form of energy, making up 90% of dietary lipids
• They provide 6 times more energy per unit weight than glycogen
• Triacylglycerols are water-insoluble and require emulsification by bile salts and bile acids in the small intestine for digestion
• Bile acids have a detergent character, helping to solubilize and absorb lipids in the gut
• Lipid absorption occurs in enterocytes as micelles with bile salts and lecithin or as lipid-protein complexes
• Lipids are transported as lipoproteins, which are lipid-protein complexes that make lipids soluble

Lipid Metabolism

• Lipid metabolism involves the breakdown of fats (lipids) for energy and storage
• The process involves multiple steps and enzymes that are tightly regulated
• Biosynthesis of triacylglycerols (lipogenesis) involves the incorporation of fatty acids into triacylglycerols for storage or into phospholipid components of membranes
• Triacylglycerols and glycerophospholipids share two precursors: fatty acyl-CoA and L-glycerol 3-phosphate
• The majority of glycerol 3-phosphate is derived from dihydroxyacetone phosphate (DHAP) by the action of cytosolic NAD-linked glycerol 3-phosphate dehydrogenase
• Fatty acyl-CoAs are formed from fatty acids by acyl-CoA synthetases, the same enzymes responsible for the activation of fatty acids for β-oxidation

Biosynthesis of Triacylglycerols

• The first stage in biosynthesis is the acylation of the two free hydroxyl groups of L-glycerol 3-phosphate by two molecules of fatty acyl-CoA to yield diacylglycerol 3-phosphate (phosphatidic acid)
• Phosphatidic acid is a central intermediate in lipid biosynthesis and can be converted to a triacylglycerol or a glycerophospholipid
• In the pathway to triacylglycerols, phosphatidic acid is hydrolyzed by phosphatidic acid phosphatase (lipin) to form a 1,2-diacylglycerol
• Diacylglycerols are then converted to triacylglycerols by transesterification with a third fatty acyl-CoA

Lipolysis

• Lipolysis is the process by which stored fat in adipose tissue is broken down into free fatty acids and glycerol
• This process is regulated by hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL)
• Fatty acids are mobilized from triacylglycerols and oxidized to meet the energy needs of the cell or organism

Fatty Acid Oxidation

• Fatty acid oxidation involves the breakdown of fatty acids into acetyl-CoA, which is then processed in the citric acid cycle
• The process of degradation converts an aliphatic compound into a set of activated acetyl units (acetyl-CoA) that can be processed by the citric acid cycle
• The repetitive four-step process by which fatty acids are converted into acetyl-CoA is called β-oxidation
• Triacylglycerols are highly concentrated stores of metabolic energy because they are reduced and anhydrous

Ketone Bodies

• Ketogenesis occurs when excess fatty acids are converted into ketone bodies, such as acetoacetate, β-hydroxybutyrate, and acetone
• This process occurs in the liver and is regulated by enzymes such as β-ketoacyl-CoA transferase and β-hydroxybutyrate dehydrogenase
• Ketone bodies are produced when the supply of carbohydrates is low and the body needs to use fat as its primary fuel source
• Acetoacetate and D-β-hydroxybutyrate are transported by the blood to tissues other than the liver, where they are converted to acetyl-CoA and oxidized in the citric acid cycle

This quiz covers the metabolism of macromolecules, specifically the digestion, absorption, and transportation of lipids, including their energy content and solubility.