Lipid Metabolism Overview
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Questions and Answers

What is produced when a fatty acid undergoes oxidation and loses two carbon atoms?

  • Acetyl CoA and acyl CoA (correct)
  • Acetyl CoA and oxaloacetate
  • Acetyl CoA and succinyl CoA
  • Acetyl CoA and propionyl CoA

What happens to the propionyl CoA produced from the oxidation of fatty acids with an odd number of carbon atoms?

  • It enters the respiratory chain directly.
  • It is converted to acetyl CoA.
  • It is converted to succinyl CoA. (correct)
  • It is stored as fat.

Why must unsaturated fatty acids undergo additional steps before oxidation?

  • They must first be reduced. (correct)
  • They require coenzymes for oxidation.
  • They produce fewer energy molecules.
  • They are converted to saturated fatty acids.

What is necessary for acetyl CoA to effectively enter the Krebs cycle?

<p>Presence of oxaloacetate. (B)</p> Signup and view all the answers

How is oxaloacetate generated from pyruvate?

<p>By pyruvate carboxylase. (A)</p> Signup and view all the answers

What percentage of triacylglycerols (TAGs) are typically hydrolyzed in the stomach during lipid digestion?

<p>10% (A)</p> Signup and view all the answers

Which substance is primarily responsible for emulsifying lipids in the small intestine?

<p>Bile salts (C)</p> Signup and view all the answers

What is formed when monoacylglycerols and free fatty acids combine in the intestinal cells?

<p>Triacylglycerols (TAGs) (B)</p> Signup and view all the answers

How do chylomicrons transport TAGs from intestinal cells?

<p>Through the lymphatic system (B)</p> Signup and view all the answers

Which two components do chylomicrons combine with to form a complete structure?

<p>Phospholipids and cholesterol (A)</p> Signup and view all the answers

What happens to some fats in the liver after being transported there?

<p>They are changed to phospholipids (D)</p> Signup and view all the answers

Which term refers to the small droplets formed by fatty acids, monoacylglycerols, and bile salts?

<p>Micelles (C)</p> Signup and view all the answers

What is the role of phospholipids like sphingomyelin and lecithin produced in the liver?

<p>To form nerve and brain tissues (B)</p> Signup and view all the answers

What is the primary function of Acyl Carrier Protein (ACP) in fatty acid synthesis?

<p>To link intermediates in fatty acid synthesis (C)</p> Signup and view all the answers

Which reaction occurs first in the chain elongation process of fatty acid biosynthesis?

<p>Condensation (A)</p> Signup and view all the answers

What is the final product after the first complete cycle of fatty acid biosynthesis starting from acetyl ACP?

<p>Butyryl ACP (B)</p> Signup and view all the answers

Which fatty acid represents the end product of the chain elongation process in fatty acid biosynthesis?

<p>Palmitic acid (D)</p> Signup and view all the answers

In the biosynthesis of unsaturated fatty acids, what compound is essential for introducing double bonds into the acyl group?

<p>Molecular oxygen (O2) (D)</p> Signup and view all the answers

Which of the following unsaturated fatty acids must be obtained from the diet in humans?

<p>Linoleic acid (D)</p> Signup and view all the answers

Where in the body do the enzymes that introduce double bonds into fatty acids primarily operate?

<p>Different cellular locations (A)</p> Signup and view all the answers

What is the role of NADPH in the chain elongation process of fatty acid synthesis?

<p>It reduces the keto group to an alcohol (A)</p> Signup and view all the answers

What is the primary role of adipocytes in the body?

<p>To store excess fat and provide insulation (D)</p> Signup and view all the answers

What is the first step in the oxidation of fatty acids?

<p>Activation by binding to Coenzyme-A (B)</p> Signup and view all the answers

What happens to fatty acids after they are hydrolyzed in the bloodstream?

<p>They are further broken down for energy or stored as lipids (D)</p> Signup and view all the answers

What is the primary purpose of fatty acid activation?

<p>To produce high energy acyl CoA (B)</p> Signup and view all the answers

How does glycerol enter metabolic pathways?

<p>By converting into dihydroxyacetone phosphate (D)</p> Signup and view all the answers

What role does carnitine play in fatty acid metabolism?

<p>It transports acyl CoA into the mitochondrial matrix (C)</p> Signup and view all the answers

Which statement accurately describes the beta-oxidation process?

<p>It removes two carbon atoms at a time from the fatty acid chain (B)</p> Signup and view all the answers

Which of the following best describes triacylglycerol energy reserves?

<p>They are the primary source of stored energy in the body (D)</p> Signup and view all the answers

What is produced during the first step of the beta-oxidation pathway?

<p>FADH2 (D)</p> Signup and view all the answers

What is the product of the breakdown of acyl Co-A during fatty acid oxidation?

<p>Acetyl Co-A, FADH2, and NADH (C)</p> Signup and view all the answers

Where are adipocytes primarily located in the body?

<p>Near the vital organs and under the skin (B)</p> Signup and view all the answers

What happens during the hydration step of beta-oxidation?

<p>Water is added to form a secondary alcohol (B)</p> Signup and view all the answers

What distinguishes acyl Co-A from acetyl Co-A?

<p>Acyl Co-A consists of a mixed-size fatty acid chain (A)</p> Signup and view all the answers

How does the fatty acid chain change during each cycle of beta-oxidation?

<p>It decreases by two carbon atoms (B)</p> Signup and view all the answers

Which enzyme is NOT directly involved in the beta-oxidation pathway?

<p>ATP synthase (D)</p> Signup and view all the answers

Which component is NOT produced in the beta-oxidation pathway?

<p>Glucose (B)</p> Signup and view all the answers

What condition arises when oxaloacetate supplies are insufficient for processing acetyl CoA in the TCA cycle?

<p>Ketogenesis (C)</p> Signup and view all the answers

What are the primary sites for the synthesis of ketone bodies?

<p>Liver mitochondria (A)</p> Signup and view all the answers

Which of the following conditions is NOT associated with the occurrence of ketosis?

<p>High carbohydrate diet (B)</p> Signup and view all the answers

What is a possible consequence of ketone body accumulation in the blood?

<p>Decreased blood pH (B)</p> Signup and view all the answers

During which condition is acetone likely to be detected on a patient's breath?

<p>Ketosis (D)</p> Signup and view all the answers

What may lead to dehydration in diabetics experiencing ketosis?

<p>Polyuria due to glucose (A), Reduced water intake (D)</p> Signup and view all the answers

In which of the following scenarios will the body predominantly burn fats instead of carbohydrates?

<p>Prolonged fasting (C)</p> Signup and view all the answers

What is the main issue that arises from inadequate glucose utilization in diabetic conditions?

<p>Enhanced fat oxidation and ketogenesis (A)</p> Signup and view all the answers

Which factor is crucial for the activity of acetyl CoA carboxylase in converting acetyl CoA to malonyl CoA?

<p>Biotin and Mn2+ (D)</p> Signup and view all the answers

What is the primary location of lipogenesis within the cell?

<p>Cytosol (C)</p> Signup and view all the answers

What is the role of the citrate-malate shuttle system in lipogenesis?

<p>It helps transport acetyl CoA to the cytosol. (B)</p> Signup and view all the answers

Which of the following statements accurately describes the differences between lipogenesis and fatty acid degradation?

<p>Fatty acid degradation involves independent enzymatic steps, while lipogenesis uses a multi-enzyme complex. (B)</p> Signup and view all the answers

What complication arises due to severe liver damage regarding carbohydrate metabolism?

<p>Reduced glycogen storage leading to ketosis (B)</p> Signup and view all the answers

What is the primary source of the body's major energy reserves?

<p>Fat reserves (A)</p> Signup and view all the answers

What is the first substance produced when fatty acids are activated for oxidation?

<p>Acyl CoA (C)</p> Signup and view all the answers

Which metabolic pathway is glycerol converted to dihydroxyacetone phosphate a part of?

<p>Glycolysis (D)</p> Signup and view all the answers

What are the three main processes involved in the oxidation of fatty acids?

<p>Activation, transport, and repeated oxidation (D)</p> Signup and view all the answers

What role do adipocytes primarily serve in the body?

<p>Energy storage (D)</p> Signup and view all the answers

Where in the body is adipose tissue predominantly located?

<p>In the abdomen and beneath the skin (B)</p> Signup and view all the answers

What product is specifically generated from glycerol metabolism in the liver?

<p>Dihydroxyacetone phosphate (A)</p> Signup and view all the answers

What is the main function of the fatty acid spiral during oxidation?

<p>To oxidize fatty acids and produce energy-rich molecules (B)</p> Signup and view all the answers

What happens to fatty acids containing an odd number of carbon atoms during oxidation?

<p>They produce acetyl CoA and propionyl CoA. (D)</p> Signup and view all the answers

Which requirement is crucial for acetyl CoA to effectively participate in the Krebs cycle?

<p>Sufficient levels of oxaloacetate. (B)</p> Signup and view all the answers

What is the fate of the majority of acetyl CoA produced from the fatty acid spiral?

<p>It is processed through the Krebs cycle. (B)</p> Signup and view all the answers

What must occur before unsaturated fatty acids can participate in the fatty acid oxidation process?

<p>They must be reduced by dehydrogenases. (D)</p> Signup and view all the answers

What is a consequence of low oxaloacetate availability during the metabolism of acetyl CoA?

<p>Inhibition of the Krebs cycle. (D)</p> Signup and view all the answers

What is the primary role of bile salts in lipid digestion?

<p>To form micelles (C)</p> Signup and view all the answers

Which component is formed when monoacylglycerols and free fatty acids are repackaged in intestinal cells?

<p>Chylomicrons (D)</p> Signup and view all the answers

What primarily determines the length of time high-fat foods remain in the stomach?

<p>The lipid content of the meal (A)</p> Signup and view all the answers

What are chylomicrons responsible for transporting in the body?

<p>TAGs (B)</p> Signup and view all the answers

How does the liver processing of fats influence blood composition post-digestion?

<p>It converts some fats to phospholipids (B)</p> Signup and view all the answers

What percentage of triacylglycerols is typically hydrolyzed by pancreatic lipase in the small intestine?

<p>66% (B)</p> Signup and view all the answers

What is the initial step in lipid digestion that occurs in the stomach?

<p>Hydrolysis by gastric lipase (C)</p> Signup and view all the answers

What two components combine with TAGs to form chylomicrons in intestinal cells?

<p>Phospholipids and lipoproteins (D)</p> Signup and view all the answers

What is the primary role of the molecule carnitine in fatty acid metabolism?

<p>To shuttle acyl CoA into the mitochondrial matrix (A)</p> Signup and view all the answers

During beta-oxidation, what occurs in the second step?

<p>Hydration of a double bond at the beta-carbon (B)</p> Signup and view all the answers

What molecule is produced during the first oxidation step of the beta-oxidation pathway?

<p>FADH2 (D)</p> Signup and view all the answers

What characterizes the process of beta-oxidation as it pertains to the fatty acid chain length?

<p>It removes two carbon atoms with each cycle (D)</p> Signup and view all the answers

Which enzyme is responsible for the cleavage of the fatty acid chain in beta-oxidation?

<p>The fatty acyl-CoA thioesterase (B)</p> Signup and view all the answers

What occurs to an acyl CoA molecule during the chain cleavage step of beta-oxidation?

<p>It releases acetyl CoA and produces a shorter acyl CoA (D)</p> Signup and view all the answers

Which step in the beta-oxidation pathway involves the production of NADH?

<p>Step 3: Oxidation (B)</p> Signup and view all the answers

What is the main outcome of the repeated cycles of beta-oxidation?

<p>Production of acetyl CoA, FADH2, and NADH (A)</p> Signup and view all the answers

What is the primary role of Acyl Carrier Protein (ACP) in fatty acid synthesis?

<p>To carry acyl groups during fatty acid elongation. (A)</p> Signup and view all the answers

Which of the following indicates a step in the chain elongation process of fatty acid biosynthesis?

<p>Conversion of acetyl-ACP to acetoacetyl-ACP. (D)</p> Signup and view all the answers

Where in humans is the enzymatic introduction of double bonds into fatty acids primarily confined?

<p>Only the liver. (A)</p> Signup and view all the answers

What type of fatty acids must be obtained from the diet in humans?

<p>Polyunsaturated fatty acids like linoleic and linolenic acids. (A)</p> Signup and view all the answers

Which molecule is essential for the biosynthesis of unsaturated fatty acids?

<p>Molecular oxygen (O2). (B)</p> Signup and view all the answers

What happens to the acyl group chain during fatty acid biosynthesis after the first cycle?

<p>It lengthens by two carbons. (C)</p> Signup and view all the answers

Which reaction occurs following the hydrogenation step in the chain elongation process during fatty acid biosynthesis?

<p>Dehydration to form an alkene. (A)</p> Signup and view all the answers

What compound is formed at the end of the fatty acid biosynthetic pathway involving chain elongation?

<p>Palmitic acid (C16). (B)</p> Signup and view all the answers

Flashcards

Lipid Digestion

The process of breaking down lipids, primarily triacylglycerols, into smaller molecules that can be absorbed by the body. It begins in the stomach and continues in the small intestine.

Gastric Lipase

An enzyme found in the stomach that initiates the breakdown of triacylglycerols by hydrolyzing ester bonds, yielding two free fatty acids and one monoacylglycerol. However, it only breaks down about 10% of triacylglycerols.

Chyme

The mixture of partially digested food that leaves the stomach and enters the small intestine. It is mixed with bile salts in the small intestine.

Pancreatic Lipase

A type of lipase produced by the pancreas. It plays the main role in lipid digestion by hydrolyzing ester bonds in triacylglycerols, yielding free fatty acids and glycerol.

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Micelles

Tiny droplets formed in the small intestine during lipid digestion. They consist of monoacylglycerols, free fatty acids, and bile salts. Their hydrophobic chains are in the interior, allowing them to move through the watery environment of the intestine.

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Lipid Absorption

The process of absorbing digested lipids, primarily fatty acids and monoacylglycerols, from the intestinal lumen into the intestinal cells.

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Chylomicrons

A type of lipoprotein formed within the intestinal cells. They package newly formed triacylglycerols, along with phospholipids and cholesterol, and transport them from the intestinal cells to the bloodstream via the lymphatic system.

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Bile Salts

A substance produced by the liver that helps to emulsify fats, allowing them to be more easily digested by lipase enzymes. It also helps to transport lipids through the lymphatic system.

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Lipase

A type of enzyme that hydrolyzes (breaks down) fats into fatty acids and glycerol. These enzymes are found in the digestive system.

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Fat Reserves

The primary energy reserve in the human body, stored in specialized cells called adipocytes.

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Adipocytes

Specialized cells that store fat, large in size and found primarily beneath the skin and around vital organs.

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Glycerol Metabolism

The process of converting glycerol (from the breakdown of fats) into DHAP (dihydroxyacetone phosphate).

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DHAP (Dihydroxyacetone Phosphate)

A compound involved in the glycolysis pathway, which can be converted into lactic acid, glycogen, or pyruvic acid.

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Fatty Acid Oxidation

The process of breaking down fatty acids to generate energy. It involves three steps: activation, транспорт, and repeated oxidation.

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Acyl CoA

A molecule formed when a fatty acid is activated by attaching to Coenzyme A. This molecule is transported to the mitochondria for further breakdown.

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Beta-Oxidation Pathway

A series of four reactions that break down fatty acids, generating energy in the form of ATP. This cycle repeats until the fatty acid is completely broken down.

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First Step of Beta-Oxidation: Dehydrogenation

This stage involves the removal of hydrogen atoms from the beta-carbon of the fatty acid, producing a double bond. FAD is used as the oxidizing agent in this step.

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Second Step of Beta-Oxidation: Hydration

Water is added across the double bond formed in the first step, creating a hydroxyl group on the beta-carbon. This step transforms the double bond into a single bond with a hydroxyl group.

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Third Step of Beta-Oxidation: Dehydrogenation

The hydroxyl group on the beta-carbon is oxidized to a keto group. This step uses NAD+ as the oxidizing agent.

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Fourth Step of Beta-Oxidation: Chain Cleavage

The fatty acid chain is cleaved between the alpha and beta carbons to release acetyl-CoA. A new CoA molecule is attached to the remaining fatty acid chain, setting up the cycle for another round of reactions.

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Fatty Acid Activation

Fatty acids must be activated before they can be broken down. This process involves attaching Coenzyme A (CoA) to the fatty acid, requiring two ATP molecules.

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Carnitine Shuttle

Carnitine is a molecule that acts as a shuttle, transporting activated fatty acids across the inner mitochondrial membrane to the matrix where the beta-oxidation machinery is located.

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Fatty Acid Oxidation Cycle

Each round of fatty acid oxidation produces an acetyl CoA molecule and a shortened acyl CoA molecule, which then goes through the cycle again. This process continues until the entire fatty acid is broken down into acetyl CoA.

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Acetyl CoA and Krebs Cycle

Most acetyl CoA produced from fatty acid oxidation enters the Krebs cycle for further processing. This process requires a balance between carbohydrate and fat metabolism.

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Oxaloacetate Importance in Krebs Cycle

The first step in the Krebs cycle involves the reaction of oxaloacetate with acetyl CoA. Sufficient oxaloacetate is crucial for this reaction to occur.

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Pyruvate to Oxaloacetate Conversion

Pyruvate, produced from glycolysis, can be converted into oxaloacetate by the enzyme pyruvate carboxylase. This conversion is important for maintaining the necessary oxaloacetate levels for the Krebs cycle.

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Ketone Bodies Production

When there is an imbalance in carbohydrate and lipid metabolism, acetyl CoA cannot be processed through the Krebs cycle effectively, leading to the production of ketone bodies. Ketone bodies are alternative energy sources produced by the liver during conditions like prolonged fasting or low carbohydrate intake.

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Ketogenesis

The process of converting excess acetyl CoA into ketone bodies, primarily occurring in the liver mitochondria.

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Ketone Bodies

Chemical compounds produced during ketogenesis, used as an alternative energy source when glucose is limited.

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Ketosis

A condition where ketone bodies accumulate in the blood and urine, often associated with diabetes, starvation, or high-fat diets.

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Acidosis

A state of abnormally high acidity in the blood, often caused by the accumulation of ketone bodies.

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Polyuria

Excessive urination, often seen in diabetic individuals due to high blood glucose levels.

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Dehydration

A condition of severe dehydration, often occurring in diabetic individuals due to polyuria and increased water loss.

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Fat Burning

A metabolic state where the body primarily burns fat for energy instead of carbohydrates.

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High-Fat, Low-Carb Diet

A diet high in fats and low in carbohydrates, potentially leading to ketosis.

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ACP (Acyl Carrier Protein)

A carrier protein in fatty acid biosynthesis, acting as a 'giant CoA-SH molecule', to which all intermediates are linked during the process.

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Fatty Acid Chain Elongation

The process of elongating a fatty acid chain by adding two-carbon units from malonyl-ACP. It involves four key reactions: condensation, hydrogenation (twice), and dehydration.

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Condensation (Fatty Acid Synthesis)

The first step in fatty acid chain elongation where acetyl-ACP and malonyl-ACP combine to form acetoacetyl-ACP, a four-carbon compound.

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Hydrogenation (Fatty Acid Synthesis)

The reduction of a keto group to an alcohol group using NADPH during fatty acid synthesis.

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Dehydration (Fatty Acid Synthesis)

The removal of water from an alcohol to form an alkene, a double bond, during fatty acid synthesis.

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Unsaturated Fatty Acid Biosynthesis

The process of introducing double bonds into a fatty acid chain, requiring molecular oxygen and specialized enzymes.

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Essential Fatty Acids

Essential fatty acids, like linoleic and linolenic acid, cannot be synthesized by humans and must be obtained from the diet.

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Palmitic Acid

The final product of fatty acid synthesis in humans, a 16-carbon saturated fatty acid.

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What is Gastric Lipase?

The enzyme that initiates lipid digestion in the stomach, breaking down triacylglycerols into two free fatty acids and one monoacylglycerol. However, it only breaks down about 10% of triacylglycerols.

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What are Micelles?

Tiny droplets formed in the small intestine during lipid digestion. They consist of monoacylglycerols, free fatty acids, and bile salts. Their hydrophobic chains are in the interior, allowing them to move through the watery environment of the intestine.

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Which enzyme completes the breakdown of fats in the small intestine?

The main enzyme responsible for lipid breakdown in the small intestine. It hydrolyzes ester bonds in triacylglycerols, yielding free fatty acids and glycerol.

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What are Chylomicrons?

A type of lipoprotein formed within the intestinal cells. They package newly synthesized triacylglycerols, along with phospholipids and cholesterol, and transport them from the intestinal cells to the bloodstream via the lymphatic system.

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What is Glycerol Metabolism?

The process of converting glycerol (from the breakdown of fats) into DHAP (dihydroxyacetone phosphate).

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What is Fatty Acid Oxidation?

The process of breaking down fatty acids to generate energy. It involves three steps: activation, transport, and repeated oxidation.

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What is the Beta-Oxidation Pathway?

A series of four reactions that break down fatty acids, generating energy in the form of ATP. This cycle repeats until the fatty acid is completely broken down.

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What is DHAP?

A compound involved in the glycolysis pathway, which can be converted into lactic acid, glycogen, or pyruvic acid.

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Beta-oxidation

Repeated oxidation of a fatty acid, breaking it down two carbons at a time, to produce acetyl CoA, FADH2, and NADH.

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Carnitine

A carrier molecule used to transport activated fatty acids across the mitochondrial membrane into the matrix for beta-oxidation.

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Dehydrogenation (First Step)

The first step in beta-oxidation, where FAD removes hydrogen atoms from the beta-carbon of the fatty acid, creating a double bond.

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Hydration (Second Step)

The second step in beta-oxidation, where water is added across the double bond formed during dehydrogenation, creating a hydroxyl group on the beta-carbon.

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Dehydrogenation (Third Step)

The third step in beta-oxidation, where NAD+ oxidizes the hydroxyl group on the beta-carbon to a keto group.

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Chain Cleavage (Fourth Step)

The fourth and final step in beta-oxidation, where the fatty acid chain is cleaved between the alpha and beta carbons, releasing acetyl-CoA.

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Transport of Acyl CoA

The process of moving activated fatty acids (acyl CoA) from the cytoplasm into the mitochondria, where the beta-oxidation pathway takes place.

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Oxaloacetate

The first step in the Krebs cycle requires the reaction between oxaloacetate and acetyl CoA. This molecule is crucial for the efficiency of the cycle.

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Lipogenesis

The process of synthesizing fatty acids from simpler precursors like acetyl CoA. It involves the use of NADPH as a reducing agent and is catalyzed by a multi-enzyme complex called fatty acid synthase.

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Fatty Acid Synthase

A multi-enzyme complex responsible for catalyzing the reactions in lipogenesis. It operates in the cell cytosol and uses acyl carrier protein (ACP) to bind intermediates.

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Citrate-Malate Shuttle

A system that transports acetyl CoA from mitochondria to the cytosol for lipogenesis. It involves citrate and malate as intermediates.

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Acetyl CoA Carboxylation

The conversion of acetyl CoA to malonyl CoA, a key step in lipogenesis. It requires CO2 and ATP, and is catalyzed by acetyl CoA carboxylase.

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Ketosis in Liver Damage

A condition where liver damage prevents proper glycogen storage, leading to the breakdown of fats for energy and the production of ketone bodies. This can result in ketoacidosis.

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What is ACP (Acyl Carrier Protein)?

A carrier protein that acts as a 'giant CoA-SH molecule' in fatty acid biosynthesis, and to which all intermediates are linked during the process.

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What is Fatty Acid Chain Elongation?

The process of elongating a fatty acid chain by adding two-carbon units from malonyl-ACP, which is crucial in building longer fatty acids.

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What is Condensation (Fatty Acid Synthesis)?

The first step in fatty acid chain elongation where acetyl-ACP and malonyl-ACP combine, forming a four-carbon compound called acetoacetyl-ACP, that kickstarts the process of building fatty acids.

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What is Hydrogenation (Fatty Acid Synthesis)?

The reduction of a keto group to an alcohol group using NADPH during fatty acid synthesis. A key step that changes the structure of the growing fatty acid.

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What is Dehydration (Fatty Acid Synthesis)?

The removal of water from an alcohol to form an alkene (a double bond) within the growing fatty acid chain. A vital step that introduces unsaturation.

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What is Unsaturated Fatty Acid Biosynthesis?

The process of introducing double bonds into a fatty acid chain, often requiring specialized enzymes and molecular oxygen.

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What are Essential Fatty Acids?

Essential fatty acids, like linoleic and linolenic acid, that cannot be synthesized by humans and must be obtained from the diet. These are crucial for various bodily functions.

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What is Palmitic Acid?

The final product of fatty acid synthesis in humans, a 16-carbon saturated fatty acid.

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Study Notes

Lipid Metabolism

  • Lipid metabolism encompasses the digestion, absorption, storage, and utilization of lipids in the body.
  • Digestion begins in the stomach with gastric lipase, hydrolysing ester bonds in triglycerides (TAGs), releasing fatty acids and monoacylglycerol. This process is limited (about 10% of total TAGs).
  • High fat meals cause a feeling of fullness and slow stomach emptying.
  • In the small intestine, bile salts emulsify lipids, pancreatic lipase further hydrolyzes TAGs into fatty acids and glycerol.
  • Micelles form, carrying fatty acids and monoacylglycerols for absorption across the intestinal cells.
  • In intestinal cells, monoacylglycerols and free fatty acids are repackaged into TAGs, which assemble with cholesterol and phospholipids forming chylomicrons.
  • Chylomicrons transport lipids from the intestinal cells to the lymphatic system, then into the bloodstream.
  • In the liver, some TAGs are converted to phospholipids.
  • Blood leaving the liver contains both fats and phospholipids.
  • Phospholipids are essential for nerve and brain tissue.
  • Lipids are transported through the bloodstream to other parts of the body.
  • Excess fat is stored in adipocytes located primarily beneath the skin, especially in abdominal regions and vital organs.

Glycerol Metabolism

  • Glycerol is absorbed into the liver or kidney and converted into dihydroxyacetone phosphate (DHAP), a glycolysis intermediate.
  • Glycerol is converted to glycerol 3-phosphate by the enzyme glycerol kinase consuming ATP and producing ADP.
  • DHAP is further metabolized through the glycolysis pathway, entering the TCA cycle.

Oxidation of Fatty Acids

  • Fatty acid oxidation breaks down fatty acids to produce energy (Acetyl CoA).

  • The process involves three main phases: activation, transport, and repeated oxidation.

    • Activation happens by binding to coenzyme A forming acyl CoA. Acyl CoA is transported to the mitochondrial matrix.
    • Transport to the mitochondrial matrix involves acyl carnitine.

  • Repeated oxidation, or fatty acid spiral, breaks down the fatty acid molecule, generating acetyl CoA, NADH, and FADH₂.

Oxidation of Fatty Acids: Transport

  • Fatty acid activation occurs in the outer mitochondrial membrane.
  • Acyl-CoA is converted to acyl-carnitine before it can cross into the mitochondrial membrane.
  • A shuttle mechanism involving the molecule carnitine facilitates the transport of acyl CoA into the mitochondrial matrix.

Reactions of the Beta-Oxidation Pathway

  • Fatty acid oxidation follows the β-oxidation pathway.
  • It involves a repeating sequence of four reactions. -Repeated oxidation cycles remove two carbons at a time from the fatty acid chain.
  • The cycle produces acetyl CoA, FADH2, and NADH.
  • The process continues until the entire fatty acid molecule is broken down into acetyl CoA molecules.

Oxidation of Fatty Acids: Steps

  • Step 1: Oxidation (dehydrogenation): FAD is the oxidizing agent producing FADH2.
  • Step 2: Hydration: Water is added.
  • Step 3: Oxidation (dehydrogenation): NAD+ is the oxidizing agent, producing NADH.
  • Step 4: Chain Cleavage: Coenzyme A breaks the fatty acid chain, forming acetyl CoA and a shortened acyl CoA molecule, repeating the cycle.

Fate of Fatty-Acid Generated Acetyl CoA

  • Acetyl CoA is further processed through several routes depending on the body's needs.
    • Oxidation in the citric acid cycle: Used for energy production
    • Ketone body formation: This is important when carbohydrate availability is limited, leading to a buildup of excess acetyl CoA and the production of ketone bodies.
    • Fatty acid biosynthesis: Excess acetyl CoA can be used to synthesize fatty acids.
    • Cholesterol biosynthesis: Acetyl CoA is a precursor for cholesterol synthesis.

Cholesterol

  • Cholesterol is an essential component of cell membranes and a precursor for vital molecules (e.g., bile salts, steroid hormones).
  • Cholesterol synthesis occurs in the liver from acetyl CoA, involving many enzymatic steps.
  • Daily body synthesis of cholesterol is 1.5-2.0 g; dietary intake is about 0.3 g.

Lipogenesis

  • Lipogenesis is the synthesis of fatty acids from acetyl CoA.
  • It occurs in the cytosol.
  • A multi-enzyme complex (fatty acid synthase) catalyzes lipogenesis.
  • NADPH is the reducing agent.
  • Acetyl CoA is transported from the mitochondria to the cytosol using the citrate-malate shuttle system, helping to generate acetyl CoA within the cytosol for lipogenesis.

B Vitamins and Lipid Metabolism

  • B Vitamins serve as coenzymes in lipid metabolism.
  • Various B vitamins (e.g., niacin, riboflavin, pantothenic acid, biotin) are involved in different steps of lipid metabolic pathways.
  • These vitamins are essential for the body to use lipids for energy.

Relationships Between Lipid and Carbohydrate Metabolism

  • Acetyl CoA is a central metabolite.
  • Acetyl CoA is important in both lipid and carbohydrate metabolism.
  • Acetyl CoA is a key component in both the pathways of lipid and carbohydrate synthesis.
  • Metabolic pathways of both carbohydrates and lipids often share common intermediates or reactions.

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This quiz covers the essential processes of lipid metabolism, including digestion, absorption, and utilization of lipids in the body. Explore how fats are broken down, transported, and stored, and the physiological effects of high-fat meals on digestion. Test your understanding of this vital metabolic pathway.

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