34 Questions
Which best describes the type of fatty acid found on carbon 2 of a TAG?
Unsaturated
What effect does the presence of unsaturated fatty acids have on the Tm of lipids?
Decreases it
What stimulates the mobilization of fat from adipocytes?
Epinephrine and glucagon
Where is Carnitine primarily synthesized in the body?
Liver and kidneys
How can Carnitine be obtained besides its synthesis in the body?
From animal products
What can Carnitine deficiency lead to in tissues?
Decreased LCFA utilization for energy
Why are lipids considered more efficient for storage compared to carbohydrates and proteins?
Higher energy yield per gram
What type of lipids are stored as liquid droplets in adipocytes?
Triacylglycerol
What is the fate of mitochondrial Acetyl-CoA in de novo fatty acid synthesis?
It is transported to the cytosol as citrate.
Which enzyme catalyzes the conversion of Acetyl-CoA to Malonyl CoA in de novo fatty acid synthesis?
Acetyl-CoA carboxylase (ACC)
What stimulates the synthesis of Acetyl-CoA carboxylase (ACC) in de novo fatty acid synthesis?
High carbohydrate diets
What is biotin (Vit H or B7) required for?
Fatty acid de novo synthesis
What triggers a high energy signal to be produced in the de novo fatty acid synthesis pathway?
High mitochondrial citrate levels
Where does the de novo synthesis of fatty acids primarily occur?
Adipocytes
What is required for de novo synthesis of fatty acids?
ATP and NADPH
Which organelle is essential for providing Acetyl Coenzyme A (ACoA) for de novo synthesis of fatty acids?
Mitochondria
What is the primary end product of de novo synthesis of fatty acids?
Palmitic Acid
Which type of fatty acid can the brain produce for the synthesis of brain phospholipids?
VLCFA
What happens when physiological conditions promote fatty acid synthesis?
Oxidation is inhibited
Which tissues can synthesize fatty acids in small quantities?
Kidney, brain, lungs
What is the role of smooth ER in fatty acid metabolism?
Desaturation and elongation of LCFA
Which molecule is required to transport fatty acyl CoA into the mitochondria?
Carnitine
What inhibits the carnitine shuttle (CPT1) from transferring newly synthesized fatty acids to the mitochondria?
Malonyl CoA
In beta oxidation, what is the rate-limiting step for transporting fatty acyl CoA into the mitochondria?
Carnitine shuttle
Which of the following is NOT produced during each cycle of beta oxidation?
Malonyl CoA
What is the main catabolism pathway for fatty acids?
Fatty Acid Beta Oxidation
What is the end product of oxidizing 1 palmitoyl CoA?
8 acetyl-CoA, 7 NADH, 7 FADH2
Which four reactions are involved in the first cycle of beta oxidation that shortens a fatty acid by 2 carbons?
$1$: Oxidation, $2$: Hydration, $3$: Oxidation, $4$: Thiolysis
What stimulates the production of ketone bodies in the liver?
Increased acetyl-CoA from beta oxidation of FFA
Why can't mammalian RBC and liver use ketones as an energy source?
Due to the lack of mitochondria in RBC and lack of thiophorase enzyme in hepatocytes
What is the main function of ketone bodies during episodes of fasting?
To save glucose and provide energy to peripheral tissues
During fasting, what triggers the increase in ketone body production in the liver?
High levels of NADH exceeding oxidative capacity in the liver
What is one reason why ketones cannot be used as an energy source by hepatocytes?
Absence of thiophorase enzyme in hepatocytes
What is the first step of ketolysis?
3-hydroxybutyrate is oxidized to acetoacetate
Study Notes
Lipid Storage
- Lipids are stored as triacylglycerol (TAG) molecules, consisting of three fatty acids.
- Fatty acids in TAG are generally not identical.
- Carbon 1 is often saturated, carbon 2 is unsaturated, and carbon 3 can be either saturated or unsaturated.
- Unsaturated fatty acids lower the melting point (Tm) of lipids.
- Lipids can be stored as mono-, di-, or triacylglycerol.
- Lipid caloric mass is over twice that of carbohydrates and proteins, making them more efficient for energy storage.
Lipid Mobilization
- Lipids are stored as liquid droplets in adipocytes.
- A small amount of lipids are stored in the liver and released into the blood as VLDL.
- Mobilization of fat from adipocytes requires release from TAG form via lipolysis, with the help of hormone-sensitive lipase (HSL).
- Lipolysis is stimulated by epinephrine and glucagon.
Carnitine and Fatty Acid Beta Oxidation
- Carnitine is a compound synthesized by methionine and lysine amino acids in the liver and kidneys.
- Carnitine deficiency can cause decreased ability of tissues to use long-chain fatty acids (LCFA) for energy.
- Carnitine deficiency can be due to cellular defects, genetic or medical conditions, or liver or kidney pathologies.
De Novo Synthesis of Fatty Acids
- Step 1: Cytosolic acetyl-CoA production involves mitochondrial ACoA production, citrate formation, and ATP-citrate lyase.
- Step 2: ACoA carboxylation to malonyl-CoA is catalyzed by acetyl-CoA carboxylase (ACC), which is allosterically activated by citrate and inactivated by palmitoyl-CoA.
- Step 3: Synthesis of palmitate (16:0) involves the addition of 2 carbons from malonyl-CoA to the carboxyl end of acyl acceptors.
Fatty Acid Metabolism
- Fatty acid synthesis normally occurs in the cytosol of adipocytes, hepatocytes, and mammary glands.
- Short fatty acids are specifically produced in the mammary glands.
- The kidney, brain, and lungs can synthesize FA in small quantities.
- Animals can synthesize all fatty acids they need, except for essential fatty acids.
- De novo synthesis of fatty acids requires ATP and NADPH.
Fatty Acid Beta Oxidation
- Beta oxidation is the major catabolism pathway for fatty acids and takes place in the mitochondria.
- LCFA must form an active intermediate (fatty acyl-CoA) before being oxidized in the mitochondria.
- The carnitine shuttle is required to transport fatty acyl-CoA into the mitochondria.
- Each cycle of beta oxidation produces 1 acetyl-CoA, 1 NADH, and 1 FADH2.
Ketones
- Ketones are produced during high-fat diet, fasting, or prolonged exercise.
- Ketones are used proportionally to their concentration in the blood by extrahepatic tissue.
- Ketones can be used by the cardiac and skeletal muscle, intestinal mucosa cells, renal cortex, brain, and even by a fetus.
- Ketones' major function is to save glucose during episodes of fasting and prolonged exercise.
Test your knowledge of the synthesis and oxidation of fatty acids as part of lipid metabolism. Learn about the physiological conditions that regulate fatty acid synthesis and oxidation, and how this process prevents futile recycling in cells.
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