Metabolism of Lipids

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Questions and Answers

Where do ketone bodies diffuse from?

  • Cytosol
  • Liver mitochondria (correct)
  • Endoplasmic reticulum
  • Peripheral tissues

What is the fate of acetone in the body?

  • It is converted back to acetyl-CoA
  • It is excreted in the urine (correct)
  • It is stored in the liver
  • It is converted to NADH

What is the precursor molecule required for cholesterol synthesis?

  • HMG-CoA
  • Mevalonic acid
  • Squalene
  • Acetyl-CoA (correct)

Which enzyme catalyzes the conversion of HMG-CoA to mevalonic acid?

<p>HMG-CoA reductase (D)</p> Signup and view all the answers

Where is the subcellular location of cholesterol synthesis?

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

What is the function of cholesterol synthesis?

<p>De novo synthesis of cholesterol (B)</p> Signup and view all the answers

Which organs make the largest contributions to the body's cholesterol pool?

<p>Liver, intestine, adrenal cortex, ovaries, testes, and placenta (B)</p> Signup and view all the answers

What is the role of cholesterol in the body?

<p>Constituent of cellular membranes (A)</p> Signup and view all the answers

Where is the reaction of converting acetyl-CoA to HMG-CoA located?

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

What is the product of the reaction catalyzed by HMG-CoA reductase?

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

Flashcards

Lipid Digestion

The process of breaking down dietary fats into smaller molecules for absorption.

Bile Salts

Substances produced from cholesterol, crucial for emulsifying fats and aiding digestion.

Pancreatic Lipase

Enzyme that breaks down triglycerides into fatty acids and monoglycerides.

Lipolysis

Breakdown of triglycerides into glycerol and free fatty acids.

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Fatty Acid Activation

Conversion of fatty acids to fatty acyl-CoA for entry into mitochondria.

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Beta-Oxidation

Metabolic process that breaks down fatty acids into acetyl-CoA.

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Ketone Bodies

Alternative energy sources produced from fatty acids, crucial during starvation.

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Cholesterol Synthesis

The creation of cholesterol from acetyl-CoA, a vital process.

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Chylomicrons

Packages formed to facilitate the transport of digested fats in the bloodstream.

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HMG-CoA Reductase

Key enzyme in cholesterol biosynthesis, regulating the rate of production.

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Study Notes

Metabolism of Lipids

  • Covers key aspects such as digestion, degradation, biosynthesis, eicosanoids, ketone bodies, and cholesterol metabolism.

Digestion of Dietary Lipids

  • Lipids aggregate in large droplets in watery environments, requiring emulsification to form smaller droplets for digestion.
  • Mechanical breakdown occurs in the mouth and stomach primarily through chewing and churning.
  • Lingual and gastric lipases contribute limited digestion of triacylglycerol before reaching the small intestine.
  • Most lipid digestion occurs in the small intestine by pancreatic lipase, stimulated by the presence of fat.

Role of Bile in Digestion

  • Bile acids, synthesized from cholesterol, play a crucial role in emulsifying fats.
  • Bile salts, made from bile acids in the liver and stored in the gallbladder, are secreted in response to cholecystokinin, aiding in digestion.

Digestion and Absorption Processes

  • Triacylglycerols (TG) are broken down into two free fatty acids and monoacylglycerol (2MG) by pancreatic lipases.
  • Phospholipids are hydrolyzed to lysophospholipids by phospholipases A2.
  • Cholesterol esters are converted to free cholesterol by cholesterol esterase.
  • Absorbed lipid digestion products are re-esterified and packaged into chylomicrons for safe circulation.

Lipolysis: Breakdown of Triglycerides

  • Lipolysis is the process that breaks down TG into glycerol and free fatty acids, occurring in the cytosol of adipose tissue.
  • Hormone-sensitive lipase facilitates lipolysis and is stimulated by glucagon and epinephrine, while inhibited by insulin.

Fates of Lipolysis Products

  • Glycerol enters the liver for glycolysis and gluconeogenesis due to lack of glycerol kinase in adipocytes.
  • Free fatty acids can be re-esterified into TG or released into the bloodstream for transport and oxidation by tissues.

Fatty Acid Oxidation (β-Oxidation)

  • Fatty acids must be activated to fatty acyl-CoA before entering the mitochondrial matrix for β-oxidation.
  • This process repeats until the fatty acid is fully converted into acetyl-CoA, producing NADH and FADH2.

Energy Yield from β-Oxidation

  • Each β-oxidation cycle generates acetyl-CoA, which is further utilized in the TCA cycle to produce ATP, CO2, and H2O.
  • Odd-numbered fatty acids produce propionyl-CoA, which is converted into succinyl-CoA.

Ketone Body Metabolism

  • Ketone bodies, produced in the liver, are transported to peripheral tissues and converted back into acetyl-CoA, generating NADH.
  • Acetone is unable to revert to acetyl-CoA and is excreted in urine.

Cholesterol Biosynthesis

  • Cholesterol synthesis initiates with acetyl-CoA, converted to HMG-CoA, and then to mevalonic acid through HMG-CoA reductase, the key regulatory step.
  • The synthesis occurs in the cytosol and endoplasmic reticulum of tissues, primarily the liver, intestine, adrenal cortex, and reproductive organs.

Importance of Cholesterol

  • Cholesterol is essential for cellular membrane structure and is found in all animal tissues.
  • It also serves as a precursor for bile salts and steroid hormones, playing a vital role in physiological processes.

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