Membranes

1. Which of the following are increased by low ATP, high ADP, and high AMP?

• Fatty acid oxidation is increased.

• Fatty acid synthesis is decreased.

• Low ATP and high AMP activate AMP-activated protein kinase (AMPK), which phosphorylates and inhibits acetyl CoA carboxylase (ACC), reducing fatty acid synthesis and increasing fatty acid oxidation (Slide 27).

2. What is the function of the Carnitine Shuttle?

• The Carnitine Shuttle transfers long-chain fatty acyl-CoA from the cytosol into the mitochondrial matrix for β-oxidation.

• CPTI (carnitine palmitoyltransferase I) on the outer mitochondrial membrane catalyzes the rate-limiting step of fatty acid oxidation by converting fatty acyl-CoA into fatty acyl-carnitine.

• Once inside the mitochondrial matrix, CPTII converts fatty acyl-carnitine back into fatty acyl-CoA for β-oxidation (Slides 15-16).

3. What is the function of the Citrate Shuttle?

• The Citrate Shuttle transfers acetyl-CoA from the mitochondrial matrix to the cytosol for fatty acid synthesis.

• Acetyl-CoA is converted to citrate, which can cross the mitochondrial inner membrane into the cytosol.

• In the cytosol, citrate is converted back to acetyl-CoA, which is then used for fatty acid synthesis (Slides 22-23).

4. What is the rate-limiting enzyme in fatty acid oxidation?

• Carnitine Palmitoyltransferase I (CPTI) is the rate-limiting enzyme in fatty acid oxidation (Slide 15).

5. What is the rate-limiting enzyme in fatty acid synthesis?

• Acetyl-CoA Carboxylase (ACC) is the rate-limiting enzyme in fatty acid synthesis. It catalyzes the conversion of acetyl-CoA to malonyl-CoA (Slide 25).

6. How is ACC allosterically regulated?

• Citrate activates ACC, increasing fatty acid synthesis.

• Long-chain fatty acids inhibit ACC, decreasing fatty acid synthesis (Slide 26).

7. Is phosphorylated ACC active or inactive?

• Phosphorylated ACC is inactive.

• AMP-activated protein kinase (AMPK) phosphorylates ACC, inhibiting fatty acid synthesis (Slide 27).

8. What is the key building block for even-chain fatty acid synthesis?

• Acetyl-CoA is the key building block for even-chain fatty acid synthesis (Slide 28).

9. What is the key building block for odd-chain fatty acid synthesis?

• Propionyl-CoA is the key building block for odd-chain fatty acid synthesis (Slide 28).

10. What is the rate-limiting enzyme in ketogenesis?

• HMG-CoA Synthase (mitochondrial isoform) is the rate-limiting enzyme in ketogenesis.

• It catalyzes the formation of HMG-CoA from acetoacetyl-CoA and acetyl-CoA (Slide 20).

11. What high-energy molecules are produced by fatty acid oxidation for ATP production?

• NADH, FADH₂, and Acetyl-CoA are produced during β-oxidation of fatty acids.

• Acetyl-CoA enters the TCA cycle, while NADH and FADH₂ drive ATP synthesis via oxidative phosphorylation(Slide 17).

12. What molecule inhibits fatty acid oxidation and why does this make sense?

• Malonyl-CoA inhibits CPTI, the rate-limiting enzyme in fatty acid oxidation.

• This makes sense because malonyl-CoA is a key precursor in fatty acid synthesis, and its presence signals an anabolic (energy-storing) state, preventing simultaneous fatty acid oxidation and synthesis (Slide 30).

13. Do increased glucose and insulin increase fatty acid synthesis and fatty acid oxidation?

• Increased glucose and insulin promote fatty acid synthesis but inhibit fatty acid oxidation.

• Insulin activates protein phosphatase (PP2A), which dephosphorylates ACC, activating it and increasing fatty acid synthesis.

• Insulin also increases the availability of citrate, which activates ACC (Slide 27).

14. Is phosphorylated ACC active or inactive?

• Phosphorylated ACC is inactive.

• ACC is inhibited by AMP-activated protein kinase (AMPK) when energy levels are low (Slide 27).

15. What is the key building block for even-chain fatty acid synthesis?

• Acetyl-CoA (Slide 28).

16. What is the key building block for odd-chain fatty acid synthesis?

• Propionyl-CoA (Slide 28).

1. When cholesterol levels are low, where is SCAP-SREBP?

• SCAP-SREBP is in the Golgi.

• When ER sterol levels are low, SCAP-SREBP complex moves from the ER to the Golgi, where proteolytic cleavage of SREBP occurs, allowing its mature form to translocate to the nucleus and activate cholesterol synthesis genes (Slides 7-12).

2. What is the transcription factor that activates the expression of cholesterol synthesis genes?

• Sterol Regulatory Element-Binding Protein (SREBP) (Slide 7).

3. What is the mechanism of action of statins?

• Statins competitively inhibit HMG-CoA reductase, the rate-limiting enzyme of cholesterol synthesis.

• Statins mimic the transient intermediate mevaldyl-CoA, blocking cholesterol production (Slide 18).

4. What is the mechanism of action of Zetia?

• Zetia (ezetimibe) inhibits cholesterol absorption at the small intestine brush border.

• It does not block the absorption of fat-soluble vitamins or triglycerides, making it a selective cholesterol absorption inhibitor (Slide 19).

5. What is the mechanism of action of Vytorin?

• Vytorin (ezetimibe + simvastatin) blocks both cholesterol absorption and cholesterol synthesis.

• Ezetimibe inhibits cholesterol absorption at the intestine, while simvastatin inhibits HMG-CoA reductase in the liver.

• This lowers cholesterol levels more effectively than statins alone (Slide 19).

6. What organ stores bile acids and bile salts?

• The gallbladder stores bile acids and bile salts (Slide 26).

7. When is HMG-CoA reductase likely to be degraded?

• HMG-CoA reductase is degraded when intracellular cholesterol levels are high.

• High cholesterol leads to ubiquitin-mediated degradation of HMG-CoA reductase, reducing cholesterol synthesis (Slide 14).

8. When intracellular cholesterol levels are low, is HMG-CoA reductase phosphorylated or unphosphorylated?

• HMG-CoA reductase is unphosphorylated when cholesterol is low.

• AMP-activated protein kinase (AMPK) phosphorylates HMG-CoA reductase, inhibiting its activity when ATP is low.

• When cholesterol is low and ATP is available, HMG-CoA reductase remains unphosphorylated and active(Slide 16).

9. Which lipoprotein particle has the highest percentage of triglycerides & cholesterol esters?

• Chylomicrons have the highest percentage of triglycerides.

• LDL has the highest percentage of cholesterol esters (Slide 24).

10. Where are chylomicrons, VLDL, LDL, and HDL particles formed?

• Chylomicrons – Assembled in the small intestine and released into the lymphatic system (Slide 29).

• VLDL – Synthesized in the liver and secreted into the bloodstream (Slide 35).

• LDL – Formed from the degradation of VLDL and IDL in circulation (Slide 35).

• HDL – Synthesized in the liver and intestines (Slide 37).

11. Which apolipoproteins are required for the assembly and secretion of chylomicrons, LDLs, and HDLs?

• Chylomicrons – Require ApoB-48 for assembly and secretion (Slide 29).

• LDL – Requires ApoB-100 for assembly and binding to LDL receptors (Slide 35).

• HDL – Requires ApoA-I for formation and cholesterol transport (Slide 22).

12. Increased levels of which lipoprotein particles increase the risk of cardiovascular disease and stroke, and why?

• LDL and Lp(a) increase cardiovascular disease (CVD) and stroke risk.

• LDL contributes to atherosclerosis, forming plaques that narrow arteries.

• Lp(a) is a derivative of LDL with Apo(a), which increases CVD and stroke risk significantly (Slides 24, 27).

13. What is reverse cholesterol transport?

• Reverse cholesterol transport is the process by which HDL removes excess cholesterol from peripheral tissues and returns it to the liver.

• HDL also removes oxidized cholesterol from atherosclerotic plaques, reducing the risk of cardiovascular disease (Slide 37).

14. Non-functional LDL receptors result in:

• Higher plasma levels of cholesterol because LDL is not taken up by cells.

• Lower intracellular levels of cholesterol, leading to increased cholesterol synthesis and further worsening hypercholesterolemia (Slide 38).

15. What are the effects of PCSK9 on the levels of LDL receptors at the plasma membrane?

• PCSK9 binds the LDL receptor and promotes its degradation in lysosomes, reducing the number of LDL receptors at the plasma membrane.

• This leads to higher plasma LDL levels, increasing the risk of cardiovascular disease (Slide 43).

16. Do anti-PCSK9 antibodies increase or decrease the levels of LDL receptors at the plasma membrane? Why?

• Anti-PCSK9 antibodies increase LDL receptor levels at the plasma membrane.

They prevent PCSK9 from degrading LDL receptors, allowing for more LDL uptake from the blood, thereby reducing cholesterol levels (Slide 44).

Primary Fuel Sources for ATP Production

• Main fuel sources: Carbohydrates and triglycerides/fatty acids.

• Ketone bodies serve as an energy source when carbohydrate availability is low (e.g., fasting, prolonged exercise, diabetes).

• Heart and skeletal muscle predominantly use fatty acids as their energy source.

• Brain, pancreatic β-cells, and red blood cells primarily rely on glucose oxidation or glycolysis.

Structure and Storage of Triglycerides

• Triglycerides are stored in adipose tissue and adipocytes within cytoplasmic lipid droplets.

• Other cell types can store triglycerides, but the amount varies by cell type.

• Function: Serve as an energy reserve for ATP production.

Fatty Acid Types

Saturated Fatty Acids (No double bonds)

• Examples: Palmitic acid (found in cheese, butter, milk, and meat).

• Characteristics:

  • Strong intermolecular interactions.

  • Less readily oxidized for ATP production.

  • Increases cardiovascular disease risk.

Unsaturated Fatty Acids (One or more double bonds)

• Example: Oleic acid (found in olive oil).

• Characteristics:

  • Weaker intermolecular interactions.

  • More readily oxidized for ATP production.

Two Essential Fatty Acids (Must Be Obtained from Diet)

1. α-Linolenic Acid (Omega-3) – Found in kiwi seeds, flax, soybean, broad green leaves, and fish oils.

2. Linoleic Acid (Omega-6) – Found in safflower, grape seed, sunflower, corn, wheat germ, cottonseed, soybean, walnuts.

Triglyceride Uptake from Diet

• Chylomicrons transport triglycerides to cells throughout the body.

• Pancreatic lipase breaks down triglycerides in the small intestine into free fatty acids and monoglycerides, which are then absorbed.

• Bile salts emulsify lipids, aiding digestion and absorption.

Hormones Stimulate Breakdown of Triglycerides into Fatty Acids

• Glucagon, cortisol, and epinephrine activate hormone-sensitive lipase (HSL) via protein kinase A (PKA).

• Triglycerides in adipocytes are broken down, and fatty acids are released into circulation.

• Albumin transports medium- and long-chain fatty acids to target cells.

Metabolism of Fatty Acids

β-Oxidation (Occurs in the Mitochondria)

• Produces FADH₂, NADH, and Acetyl-CoA.

• More ATP is generated from palmitic acid oxidation than glucose oxidation (~3x more ATP).

• Rate-limiting step: Carnitine Palmitoyltransferase I (CPTI).

• Carnitine shuttle's main function: To transport long-chain fatty acids into the mitochondrial matrix.

Fatty Acid Synthesis (Occurs in the Cytosol)

• Requires Acetyl-CoA, which is transported via the citrate shuttle.

• Acetyl-CoA is converted to malonyl-CoA by Acetyl-CoA Carboxylase (ACC) (rate-limiting step).

• Fatty acid synthesis is activated by insulin and inhibited by glucagon.

Ketone Body Synthesis (Ketogenesis)

• Takes place in the liver when glucose availability is low (e.g., fasting, prolonged exercise, carbohydrate restriction).

• Ketone bodies produced:

  • Acetoacetate.

  • β-hydroxybutyrate.

  • Acetone (not significantly used for energy).

• Rate-limiting enzyme: HMG-CoA synthase (mitochondrial isoform).

• Increased ketone bodies can lead to ketoacidosis (decreased blood pH).

• Ketone bodies are utilized (ketolysis) in the brain, heart, and muscle.

Acetyl-CoA Carboxylase (ACC) – Regulation of Fatty Acid Synthesis

Allosteric Regulation

• Citrate activates ACC (increases fatty acid synthesis).

• Long-chain fatty acids inhibit ACC (decreases fatty acid synthesis).

Phosphorylation Regulation

• AMP-activated protein kinase (AMPK) inhibits ACC via phosphorylation (when ATP is low).

• Protein phosphatase (PP2A) dephosphorylates ACC, activating it (when ATP is high).

Hormonal Regulation

• Insulin activates ACC by stimulating PP2A, increasing fatty acid synthesis.

• Glucagon inhibits ACC by activating AMPK, decreasing fatty acid synthesis.

Ethanol Metabolism and Its Impact

• Increases the NADH:NAD+ ratio, leading to:

  • Inhibition of the TCA cycle.

  • Increased fatty acid synthesis and triglyceride accumulation (fatty liver disease).

  • Decreased fatty acid oxidation.

  • Increased ketone body synthesis (ketoacidosis).

  • Increased reactive oxygen species (ROS), causing liver damage.

Clinical Implications

• Excessive alcohol consumption disrupts multiple metabolic pathways, leading to fatty liver disease, acidosis, and oxidative stress.

Cholesterol Synthesis and Transport

Cholesterol Concentration Increases Toward the Periphery

• Cholesterol moves from the ER to the plasma membrane (PM).

Steroid Hormones, Vitamin D, and Bile Acids Are Derived from Cholesterol

• Steroid hormones: Estrogen, glucocorticoids.

• Bile acids: Cholic acid.

Structure of Cholesterol

• 27 carbons.

• C-3 hydroxyl group.

• C-17 hydrocarbon tail.

Cholesterol Synthesis

• HMG-CoA reductase is the rate-limiting enzyme.

• Cholesterol synthesis is ATP-intensive (requires ~36 ATP per molecule).

• Synthesis occurs only when ATP is plentiful.

SREBP-SCAP Pathway (When Cholesterol Levels Are Low)

1. SCAP-SREBP complex moves from the ER to the Golgi.

2. SREBP undergoes proteolytic cleavage in the Golgi.

3. Processed SREBP translocates to the nucleus.

4. Activates cholesterol synthesis genes.

Regulation of Cholesterol Synthesis

• HMG-CoA reductase is regulated by:

  • Phosphorylation (AMPK inhibits it).

  • Hormones (Insulin activates, Glucagon inhibits).

  • Protein degradation (HMG-CoA reductase is degraded when cholesterol is high).

Drugs That Lower High Cholesterol

• Statins: Inhibit HMG-CoA reductase by mimicking mevaldyl-CoA.

• Zetia (Ezetimibe): Inhibits cholesterol absorption in the small intestine.

• Vytorin (Ezetimibe + Simvastatin): Blocks both cholesterol absorption and synthesis.

Major Classes of Lipoproteins

Lipoprotein Metabolism

• Chylomicrons are assembled in the intestine, released into the lymph, and acquire ApoC-II and ApoE from HDL.

• VLDL is assembled in the liver, acquiring ApoC-II and ApoE from HDL, and eventually converting into LDL.

• LDL is taken up by receptor-mediated endocytosis.

PCSK9 Function and Inhibition

• PCSK9 degrades LDL receptors, increasing LDL levels.

• Anti-PCSK9 antibodies prevent LDL receptor degradation, increasing LDL uptake and reducing blood cholesterol.