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Questions and Answers
How much less ATP is yielded by unsaturated compounds compared to their saturated analogs?
What is the first step in the conversion of propionyl CoA to succinyl CoA?
What is the primary form of energy storage in humans?
Which fatty acid type yields propionyl CoA during β-oxidation?
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What happens to triacylglycerols when energy is needed?
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What is the primary function of acyl in biochemistry?
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Which metal is central in the structure of coenzyme B12?
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How is D-methylmalonyl CoA converted in the second step of propionyl CoA metabolism?
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Which enzymes are responsible for breaking down triacylglycerols in the intestine?
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What is required for the activation of fatty acids?
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Which intermediate is formed when a fatty acid is activated?
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What type of reactions do vitamin B12 dependent enzymes generally catalyze?
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What are the end products of triacylglycerol digestion in the intestine?
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What substance is produced when the TCA cycle is insufficient for acetyl CoA utilization?
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What drives the conversion of the fatty acyl adenylate to acylCoA?
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Which metabolic pathway is used to convert fatty acids to acetylCoA?
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What is the result of converting the cis-Δ4 double bond to its trans-Δ2 form?
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How are excess fatty acids and glycerol combined to form triacylglycerols?
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How does fatty acylCoA enter the mitochondrial matrix?
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Where do dietary lipids undergo emulsification?
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What catalyzes the transfer of fatty acyl group from acylCoA to carnitine?
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What component is integrated into glycolysis or gluconeogenesis from lipid digestion?
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What is the nature of the reaction that transfers the fatty acyl group to carnitine?
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What occurs to the acyl group after it has been transferred to carnitine?
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What happens to the fifth carbon atom during the first reaction of fatty acid biosynthesis?
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Which molecule serves as the reductant in the second reaction of fatty acid biosynthesis?
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What type of reaction occurs in the second step of fatty acid biosynthesis?
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What configuration do the hydroxyl groups produced in the second reaction assume?
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What is released from the thioester bond during the first reaction, contributing to the reaction's favorable thermodynamics?
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What factor makes the first reaction of fatty acid biosynthesis thermodynamically favorable?
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In what process is the acetylCoA carboxylated to form malonylCoA?
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Which statement is true regarding the β-oxidation cycle compared to fatty acid biosynthesis?
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What enzyme catalyzes the removal of elements of water to create a trans carbon-carbon double bond?
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Which product is formed from the final step of the fatty acid biosynthesis cycle?
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What is the key advantage of the glyoxylate cycle?
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What is a unique reaction catalyzed by isocitrate lyase in the glyoxylate cycle?
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Why can humans not synthesize glucose from acetyl-CoA?
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What do succeeding rounds of fatty acid biosynthesis achieve?
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What happens after the formation of butyryl-ACP in fatty acid biosynthesis?
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What are the two four-carbon products produced by the glyoxylate cycle?
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What component of triacylglycerols is integrated into glycolysis or gluconeogenesis after digestion?
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What is the initial step in fatty acid biosynthesis?
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In the process of lipid biosynthesis, what happens when there is excess energy?
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Which best describes the role of bile acids in the lipid digestion process?
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What is a major outcome of the glyoxylate cycle?
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Which of the following statements about the β-oxidation cycle is accurate compared to fatty acid biosynthesis?
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What is the function of lipases in lipid metabolism?
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Which pathway is primarily associated with the respiration of fatty acids for energy?
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What is released during the first reaction of fatty acid biosynthesis that contributes to the thermodynamic favorability of the reaction?
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Which of the following best describes the role of NADPH in the second reaction of fatty acid biosynthesis?
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What type of configuration is produced in the hydroxyl group during the second reaction of fatty acid biosynthesis?
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What is the primary thermodynamic contributor when breaking the thioester bond in the first reaction of fatty acid biosynthesis?
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Which reaction in the fatty acid biosynthesis cycle is similar to dehydration processes in other metabolic pathways?
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In the complete cycle of fatty acid biosynthesis, how does the structure of the product compare to that of the reactants during the initial step?
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How does the fatty acid biosynthesis cycle primarily differ from the β-oxidation cycle?
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What thermodynamic property does the process of carboxylation of acetylCoA to malonylCoA primarily enhance in the biosynthetic pathway?
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What type of fatty acid will yield propionyl CoA during β-oxidation?
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What is the final product of the metabolic conversion of propionyl CoA?
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Which enzyme is responsible for the carboxylation of propionyl CoA to D-methylmalonyl CoA?
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What is the intermediate formed during the isomerization of L-methylmalonyl CoA to succinyl CoA?
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What structural feature does coenzyme B12 possess?
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What occurs when there is an inadequate rate of the TCA cycle during fatty acid oxidation?
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What is the process that converts cis-Δ4 double bond in linoleic acid into a trans-Δ2 bond?
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Which reaction step follows the carboxylation of propionyl CoA to produce L-methylmalonyl CoA?
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What is the product formed after the reduction of the trans carbon-carbon double bond during fatty acid biosynthesis?
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Which of the following statements about the glyoxylate cycle is correct?
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Which reaction catalyzed by malate synthase is unique to the glyoxylate cycle?
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What is the role of 3-hydroxyacyl-ACP dehydratase in fatty acid biosynthesis?
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How many carbon atoms does each round of fatty acid biosynthesis elongate the acyl-ACP by?
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Which of the following accurately describes the products of the glyoxylate cycle?
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What distinguishes the acetyl-CoA utilization in the glyoxylate cycle from that in mammals?
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Which enzyme is responsible for cleaving isocitrate into glyoxylate and succinate during the glyoxylate cycle?
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What contributes to the thermodynamic favorability of the first reaction in fatty acid biosynthesis?
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Which configuration does the hydroxyl group produced in the second reaction of fatty acid biosynthesis assume?
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What role does NADPH play in the second reaction of fatty acid biosynthesis?
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How is malonylCoA formed from acetylCoA, and what is its primary purpose in fatty acid biosynthesis?
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Which statement accurately describes a difference between fatty acid biosynthesis and the β-oxidation cycle?
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What is the primary function of the antiporter protein in the carnitine cycle?
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During β-oxidation, what configuration is introduced between the α and β carbons?
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What is the primary product of the first reaction catalyzed by the condensing enzyme in fatty acid biosynthesis?
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What happens to carbon dioxide during the first reaction of fatty acid biosynthesis?
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What enzyme catalyzes the conversion of a hydroxyl group to a ketone in the β-oxidation cycle?
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Which enzyme is responsible for the reduction of β-ketone to an alcohol in fatty acid biosynthesis?
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What is released as a product at the end of one complete cycle of β-oxidation?
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What is the role of FAD in the first reaction of the β-oxidation cycle?
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What reaction primarily characterizes the second step of β-oxidation?
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Which intermediate is formed as a result of the actions of β-ketothiolase in the final step of β-oxidation?
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How does the oxidative process of β-oxidation compare to the reductive steps in fatty acid biosynthesis?
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Study Notes
Lipid Metabolism Overview
- Triacylglycerols are the most important energy store in humans.
- Stored fats provide enough energy for basal metabolism for one to two months.
Lipid Metabolism Summary
- Lipid catabolism occurs when energy is needed.
- Triacylglycerols are broken down into fatty acids and glycerol by lipases.
- Fatty acids are converted to acetylCoA by the β-oxidation pathway.
- AcetylCoA can enter the TCA cycle or be used for biosynthesis.
- Glycerol is integrated into glycolysis or gluconeogenesis.
- Lipid biosynthesis occurs when there is excess energy.
- Fatty acids can be synthesized from acetylCoA, and glycerol from glycolysis intermediates.
- These components are combined to form triacylglycerols.
Lipid Digestion
- Dietary lipids are emulsified in the intestine by bile acids.
- Pancreatic lipases hydrolyze triacylglycerols into free fatty acids and 2-monoglycerides in the intestinal lumen.
Fatty Acid Activation
- Fatty acids are carried in the blood by serum albumin protein.
- They are activated in the cytosol by conversion to acylCoA derivatives.
- AcylCoA formation is thermodynamically unfavorable as acylCoA is a thioester.
- ATP is used to activate fatty acids with the help of inorganic pyrophosphate and a fatty acyl adenylate.
- Pyrophosphate cleavage releases energy, contributing to reaction favorability.
- A high-energy acyl adenylate intermediate is swapped for CoA in a thermodynamically neutral reaction.
Fatty Acyl Transport
- Fatty acylCoA derivatives are synthesized in the cytosol but oxidized in the mitochondrial matrix.
- Fatty acylCoA is transferred to carnitine in the cytosol.
- The fatty acyl carnitine derivative is transported across the mitochondrial inner membrane.
- In the matrix, the fatty acyl group is transferred back to CoA.
Fatty Acyl Transfer from CoA to Carnitine
- Transfer from fatty acylCoA to carnitine is catalyzed by an acyl transferase enzyme.
- The reaction is energetically neutral and reversible.
β-Oxidation
- β-oxidation occurs in the mitochondrial matrix.
- It is a four-step cycle that repeatedly removes two-carbon units from fatty acid chains as acetylCoA.
- The steps involve oxidation, hydration, oxidation, and thiolysis.
- Each cycle produces one acetylCoA, one FADH2, and one NADH.
Oxidation of Unsaturated Fatty Acids
- Unsaturated fatty acids require additional enzymes to isomerize double bonds for β-oxidation.
- Cis double bonds are isomerized to trans double bonds.
- The unsaturated fatty acids produce less ATP because of the extra steps.
Oxidation of Odd-Chain Fatty Acids
- Odd-chain fatty acids produce propionyl CoA in the last cycle of β-oxidation.
Propionyl CoA Metabolism
- Propionyl CoA is converted to succinyl CoA, an intermediate of the TCA cycle.
- This requires three steps:
- Carboxylation to D-methylmalonyl CoA.
- Conversion to L-methylmalonyl CoA.
- Isomerization to succinyl CoA by a vitamin B12–dependent enzyme.
Ketone Body Formation
- Ketone bodies are produced when energy demand is high, and the TCA cycle cannot utilize all the acetylCoA.
Fatty Acid Biosynthesis
- Occurs in the cytosol of the cell.
- Uses acetylCoA as a starting material.
- Requires NADPH as a reducing agent.
- Involves a four-step cycle that adds two-carbon units to the growing fatty acid chain.
- Steps include condensation, reduction, dehydration, and reduction.
Glyoxylate Cycle
- A metabolic pathway in plants and some microorganisms.
- Allows for the synthesis of carbohydrates from acetylCoA.
- Absent in animals, including humans.
- Uses many of the reactions of the TCA cycle, but adds two unique reactions:
- Isocitrate lyase cleaves isocitrate into glyoxylate and succinate.
- Malate synthase condenses glyoxylate and acetylCoA to produce malate.
- This cycle provides a mechanism for converting two-carbon acetylCoA units into four-carbon products.
Lipid Catabolism Overview
- Triacylglycerol catabolism is a key energy source for living cells
- Triacylglycerols are the body's most important energy store
- The body can store enough energy in triacylglycerols to fuel basal metabolism for one or two months
- Carbohydrate stores (glycogen) provide enough energy for less than one day
Lipid Metabolism Summary
- Lipid catabolism occurs when energy is needed
- Triacylglycerols are hydrolyzed by lipases into fatty acids and glycerol
- Fatty acids are converted to Acetyl-CoA through the β-oxidation pathway
- Glycerol is integrated into glycolysis or gluconeogenesis
- Acetyl-CoA can enter the TCA cycle or be used in many biosynthetic pathways
- Lipid biosynthesis occurs when there is excess energy
- Fatty acids are synthesized from Acetyl-CoA
- Glycerol is produced from glycolysis intermediates
- Triacylglycerols are formed from fatty acids and glycerol in ATP-dependent reactions
Lipid Digestion in the Intestine
- Most dietary lipids are ingested as triacylglycerols
- Bile acids emulsify dietary lipids in the intestine
- Pancreatic lipases hydrolyze triacylglycerols into fatty acids and 2-monoglycerides in the intestinal lumen
Digestion and Absorption of Triacylglycerols
- Dietary triacylglycerols are hydrolyzed into free fatty acids and 2-monoglycerides
- Free fatty acids and 2-monoglycerides are packaged into micelles
- Micelles carry lipids to the intestinal epithelium
- Lipids are absorbed into enterocytes
- Lipids are reassembled into triacylglycerols in enterocytes
- Triacylglycerols are packaged into chylomicrons
- Chylomicrons are secreted into the lymph system
β-Oxidation Pathway
- The β-oxidation pathway occurs in the mitochondrial matrix
- Fatty acids are activated by attaching to CoA
- Fatty acids are transported into the mitochondrial matrix by carnitine palmitoyltransferase I and II
- Fatty acids are broken down into Acetyl-CoA units in a four step cycle
- Each cycle produces one Acetyl-CoA, one NADH, and one FADH2
Unsaturated Fatty Acid Oxidation
- β-oxidation cannot occur directly on a cis-double bond
- cis-double bonds are isomerized to trans-double bonds by enoyl CoA isomerase
- Isomerization requires energy derived from NADPH
- The trans-double bonds can then be processed by β-oxidation
Linoleic Acid Oxidation
- Linoleic acid is a C-18 fatty acid with two double bonds
- β-oxidation of linoleic acid is more complex due to the isomerization of cis-Δ4-enoyl-CoA to trans-Δ2-enoyl-CoA
- Isomerization requires three reactions
Odd-Chain Fatty Acid Oxidation
- Odd-chain fatty acids produce Acetyl-CoA and propionyl-CoA during the β-oxidation pathway
- Propionyl-CoA is a three carbon compound that is metabolized in a distinct pathway
Propionyl CoA Metabolism
- Propionyl CoA is converted to succinyl CoA (a four-carbon compound), an intermediate in the TCA cycle.
- Conversion of propionyl CoA to succinyl CoA requires three steps:
- Propionyl CoA is carboxylated by a biotin-dependent enzyme to form D-methylmalonyl CoA
- D-methylmalonyl CoA is converted to L-methylmalonyl CoA
- L-methylmalonyl CoA is isomerized to succinyl CoA by a vitamin B12-dependent enzyme
Vitamin B12
- Vitamin B12 is a coenzyme used by enzymes that catalyze isomerization reactions
- B12 contains a cobalt ion and a porphyrin ring system
Ketone Body Formation
- Ketone bodies are produced when there is high energy demand or when the TCA cycle cannot process all of the Acetyl-CoA produced by fatty acid oxidation
Fatty Acid Biosynthesis Overview
- Fatty acid biosynthesis is a four-step process that mirrors the reversal of the β-oxidation pathway
Fatty Acid Biosynthesis Steps
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Step 1: Acetyl-CoA is carboxylated to malonyl-CoA using a biotin-dependent enzyme
- Carbon dioxide release drives this reaction forward
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Step 2: Malonyl-CoA reacts with acetyl-CoA to form acetoactyl-CoA
- This reaction is catalyzed by a condensing enzyme
- ACP is released, providing additional energy
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Step 3: Acetoactyl-CoA is reduced to β-hydroxybutyryl-CoA
- The reaction uses NADPH as a reductant
- The hydroxyl group is in the D-configuration, unlike the L-configuration produced in β-oxidation
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Step 4: Dehydration occurs, followed by a final reduction using NADPH.
- The resulting fatty acid is elongated by two carbons
Fatty Acid Biosynthesis
- Water molecules are removed from the fatty acid chain, creating a double bond between carbon atoms.
- This reaction is catalyzed by 3-hydroxyacyl-ACP dehydratase.
- In the last step of the cycle, enoyl-ACP reductase uses NADPH to reduce the double bond back to a single bond.
- The product of this process is a four-carbon butyryl-ACP.
- This four-carbon acyl-ACP can then condense with a malonylACP, initiating a second round of biosynthesis
- Each subsequent round of biosynthesis elongates the acyl-ACP by two carbon atoms.
- Most naturally occurring fatty acids have an even number of carbon atoms due to the two-carbon addition in each biosynthesis round.
Glyoxylate Cycle
- An anabolic metabolic pathway occurring in plants and several microorganisms.
- Enables organisms to use acetylCoA for the net synthesis of carbohydrates.
- Absent in animals, including humans.
- This absence explains why humans cannot synthesize glucose from acetylCoA.
Glyoxylate Cycle Reactions
- Utilizes many reactions of the TCA cycle but adds two unique reactions.
- Isocitrate lyase: Cleaves isocitrate into glyoxylate and succinate.
- Malate synthase: Glyoxylate condenses with acetylCoA to produce malate.
- These two reactions combined with other TCA cycle reactions produce four-carbon products – succinate and malate
- These products can then be utilized to produce other four, five, or six carbon intermediates in the TCA cycle.
Fatty Acid Biosynthesis Cycle
- Four steps in the cycle, mimicking the reversal of the β-oxidation cycle.
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Step 1: Condensing enzyme catalyzes the reaction of malonylACP with acetylACP, producing a four-carbon β-ketoacylACP.
- A two-carbon compound and a three-carbon compound condense to form a four-carbon compound.
- Carbon dioxide is released from the malonylACP, providing energy for the reaction.
- The release of carbon dioxide is thermodynamically favorable, contributing to the reaction's progression.
- An ACP molecule is released, breaking a thioester bond, contributing to the negative ΔG of the reaction.
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Step 2: β-ketoacyl-ACP reductase uses NADPH to reduce the β-ketone group to an alcohol.
- This is typical of anabolic oxidation-reduction reactions.
- The resulting hydroxyl group is in the D-configuration unlike the L-configuration in the β-oxidation cycle.
- Step 3: Dehydration reaction occurs, removing water.
- Step 4: Enoyl-ACP reductase uses NADPH to reduce the double bond to a single bond.
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Description
This quiz covers the essential concepts of lipid metabolism, detailing processes such as lipid catabolism, biosynthesis, and digestion. Learn about the role of triacylglycerols, fatty acids, and the metabolic pathways involved in energy storage and utilization in humans.
