Quiz 9 (L30-L32 Beginning)

Journey of the Burger: Fatty acid transport from gut through lymph and capillaries then into cytosol of tissue cells:

• Know journey of TAG/FA from gut into cytosol of cells anywhere in body (intestine, absorptive cells, lacteal/lymph, capillaries, cells close to capillaries).

1) In the small intestine: Bile salts emulsify dietary fat into small droplets. Pancreatic lipase breaks TAG down mainly into free fatty acids and 2-monoacylglycerol. These products join bile salts in mixed micelles, which carry them to the surface of intestinal absorptive cells.

2) Inside intestinal absorptive cells (enterocytes): The fatty acids and monoacylglycerol enter the cells, where they are reassembled into TAG. TAG is packaged with other lipids and proteins into transport particles called chylomicrons.

3) Into lymph, then blood: Enterocytes release chylomicrons into lacteals, the lymphatic capillaries inside intestinal villi. Chylomicrons travel through lymphatic vessels and enter the bloodstream through the thoracic duct.

4) At tissue capillaries: Lipoprotein lipase (LPL), located on the blood-facing surface of capillary endothelial cells, breaks down chylomicron TAG and releases fatty acids.

5) Into nearby tissue cells: The released fatty acids cross the capillary lining and enter nearby cells, such as muscle cells or adipocytes, through membrane transport processes. Inside the cell, fatty acid–binding proteins help carry them through the cytosol. They can then be used for energy or reassembled into TAG for storage.

Journey of the Burger: Fatty acid transport from gut through lymph and capillaries then into cytosol of tissue cells:

• Know the role of bile salts, lipases.

• Bile salts help emulsify the fats from our food in small intestine(breaking big droplets into small ones)

• Lipase is an enzyme that breaks down TAG to release fatty acids

Journey of the Burger: Fatty acid transport from gut through lymph and capillaries then into cytosol of tissue cells:

• Know where disassembly and rebuilding of TAG happens (hint: it happens multiple times!).

TAG is disassembled twice and rebuilt at two locations along this journey:

1. Small intestine: Lipases break dietary TAG down, releasing fatty acids for absorption.

2. Intestinal absorptive cells: Fatty acids are rebuilt into TAG, which is packaged into chylomicrons.

3. Tissue capillary walls: Lipoprotein lipase breaks down chylomicron TAG, releasing fatty acids and glycerol.

4. Inside nearby tissue cells: Fatty acids can be rebuilt into TAG for storage in muscle cells or adipocytes. Alternatively, they can be oxidized for energy.

Journey of the Burger: Fatty acid transport from gut through lymph and capillaries then into cytosol of tissue cells:

• Describe the different particle structures involved and where they occur (emulsion particles, micelles, chylomicrons).

Particle	Structure	Where it occurs
Emulsion particles	Dietary fat droplets, mainly TAG, with bile salts stabilizing their surface. About 1–5 μm in diameter.	Small intestine lumen, where lipases act on their surface to digest TAG.
Micelles	Small assemblies containing fatty acids and bile salts, with water-compatible surfaces and hydrophobic portions inward. About 0.004 μm in diameter.	Small intestine lumen, where they deliver fatty acids to absorptive cells. The fatty acids enter the cells.
Chylomicrons	A TAG-rich core, surrounded by a phospholipid monolayer with cholesterol and proteins. About 0.1–1 μm in diameter.	Assembled inside intestinal absorptive cells, then released into lacteals → lymph → blood, delivering TAG to tissue capillaries.

REGULATED transport of fatty acid from the cytosol into mitochondrial matrix:

Describe how FA gets from the cytosol of tissue cells into the mitochondrial matrix.

• Describe the enzymes involved and and what they do (fatty acyl-CoA synthetase, carnitine acyltransferase I, carnitine acyltransferase II).

• Fatty acyl-CoA synthetase activates the fatty acid.

  • Located in the outer mitochondrial membrane, accessible from the cytosol.

  • Uses ATP to attach CoA to the fatty acid, forming fatty acyl-CoA.

  • ATP becomes AMP + pyrophosphate (PPᵢ). Hydrolysis of PPᵢ helps drive activation forward.

• Carnitine acyltransferase I transfers the fatty acyl group from CoA to carnitine.

  • Forms fatty acyl-carnitine, leaving the original CoA outside the matrix.

  • This is the regulated step that controls fatty acid entry for breakdown.

• A transporter carries fatty acyl-carnitine across the inner mitochondrial membrane.

  • Fatty acyl-CoA cannot cross this membrane directly.

• Carnitine acyltransferase II transfers the fatty acyl group from carnitine to a CoA inside the matrix.

  • Re-forms fatty acyl-CoA, which can undergo β-oxidation.

  • Carnitine returns outside the matrix for reuse.

REGULATED transport of fatty acid from the cytosol into mitochondrial matrix:

Describe how FA gets from the cytosol of tissue cells into the mitochondrial matrix.

• Describe what happens in two steps in fatty acyl-CoA synthetase.

• Fatty acyl-CoA synthetase: Activates a cytosolic fatty acid to fatty acyl-CoA, at the outer mitochondrial membrane.

  1. Fatty acid attacks ATP’s α-phosphate → fatty acyl-AMP + PPᵢ.

  2. CoA’s sulfur attacks the fatty acyl carbonyl carbon → fatty acyl-CoA + AMP.

  PPᵢ is separately hydrolyzed to 2 Pᵢ, helping drive the reaction forward. Activation costs 2 ATP equivalents, although only one ATP molecule is consumed.

• Carnitine acyltransferase I: Transfers the fatty acyl group from CoA to carnitine, forming fatty acyl-carnitine, which a transporter carries across the inner mitochondrial membrane. This is the one regulated step in FA breakdown emphasized in your lecture.

• Carnitine acyltransferase II: Transfers the fatty acyl group from carnitine to matrix CoA, regenerating fatty acyl-CoA inside the matrix. Carnitine is released and transported back out for reuse.

REGULATED transport of fatty acid from the cytosol into mitochondrial matrix:

Describe how FA gets from the cytosol of tissue cells into the mitochondrial matrix.

• Explain how carnitine acyltransferase I is required to make FA-carnitine that has a transporter to cross inner mitochondrial membrane.
  
  

Fatty acyl-CoA cannot cross the inner mitochondrial membrane directly. Carnitine acyltransferase I transfers its fatty acyl group from CoA to carnitine, forming fatty acyl-carnitine (FA-carnitine).

A specific transporter then carries fatty acyl-carnitine across the inner membrane into the matrix. There, carnitine acyltransferase II transfers the fatty acyl group to a matrix CoA, regenerating fatty acyl-CoA for β-oxidation.

CoA stays outside; carnitine carries the fatty acyl group inside.

REGULATED transport of fatty acid from the cytosol into mitochondrial matrix:

Describe how FA gets from the cytosol of tissue cells into the mitochondrial matrix.

• Identify carnitine acyltransferase I as the ONE regulated enzyme in FA catabolism.

Carnitine acyltransferase I is the ONE regulated enzyme in fatty acid catabolism as It transfers the fatty acyl group from CoA to carnitine, forming fatty acyl-carnitine, which can be transported across the inner mitochondrial membrane.

By controlling this step, the cell controls how much fatty acid enters the matrix for β-oxidation.

REGULATED transport of fatty acid from the cytosol into mitochondrial matrix:

• Explain why there are separate pools of CoA in the cytosol and in the mitochondrial matrix. [This is why ketone bodies have no CoA, they are made in the mitochondrial matrix then exported to other tissues!]

Separate CoA pools allow the cell to control metabolic reactions independently in the cytosol and mitochondrial matrix. Because many reactions require CoA, controlling its availability helps prevent those reactions from proceeding excessively.

Ketone bodies also preserve the matrix CoA pool. The liver converts acetyl-CoA into ketone bodies that contain no CoA, allowing fuel to leave for other tissues while keeping CoA in the liver’s mitochondrial matrix for continued metabolism.

Fatty acid release from storage (lipid droplets in cells):

• Describe the hormones that regulate release (activate = glucagon and epinephrine vs inhibit = insulin) and think about the signaling pathway

• Glucagon and epinephrine (activators):

  • (1) Binds to its receptor on cell surface in liver, increasing metabolic activity →

  • (2) activation of adenylate cyclase →

  • (3) Make cAMP →

  • (4) Activate Protein Kinase A →

  • (5) phosphorylated perilipin (target of Arg-Arg-Ser-Ser) on lipid droplet surface to allow enzymatic activity + phosphorylate triacylglycerol lipase (hormone-sensitive lipase) makes fatty acids and glycerol for energy

    
    

• Insulin (inhibitor):

  • (1) Drives degradation of cyclic AMP with TAG formation favored over fatty acid formation →

  • (2) Decrease in hormone-sensitive lipase activity (makes fatty acid and glycerol that body uses for energy(Phosphorylated lipase hydrolyzes TAG, releasing free fatty acids which allow fatty acid traveling in blood bound to serum albumin (non-specific binding protein) and enter cell near capillary.)

Fatty acid release from storage (lipid droplets in cells):

• Describe the enzymes and proteins involved in FA release and transport to other tissues (familiar signaling pathway proteins like PKA, perilipin, hormone-sensitive lipase, albumin).

1) Protein kinase A (PKA)Phosphorylates perilipin and hormone-sensitive lipase, promoting stored fat breakdown.

2) Perilipin Coats lipid droplets and restricts lipase access to TAG, but phosphorylation allows lipases to reach TAG.

3) Hormone-sensitive lipaseWhen phosphorylated by PKA, becomes more active and helps break down stored TAG, ultimately releasing fatty acids and glycerol.

4) Serum albumin binds released fatty acids in blood, allowing these poorly water-soluble molecules to travel to other tissues for fuel.

b-oxidation of FA:

• Describe what happens at each step.

Beta Oxidaton Occurs in the Mitochondrial Matrix

Step and enzyme	Substrate → Product	What happens
1. Oxidation: acyl-CoA dehydrogenase	Fatty acyl-CoA → trans-Δ²-enoyl-CoA	Forms a trans double bond between α and β carbons. FAD → FADH₂.
2. Hydration: enoyl-CoA hydratase	trans-Δ²-Enoyl-CoA → β-hydroxyacyl-CoA	Adds water across the double bond, placing OH on the β carbon.
3. Oxidation: β-hydroxyacyl-CoA dehydrogenase	β-Hydroxyacyl-CoA → β-ketoacyl-CoA	Oxidizes the β-carbon’s OH to a ketone. NAD⁺ → NADH + H⁺.
4. Thiolysis: thiolase	β-Ketoacyl-CoA + CoA-SH → acetyl-CoA + shortened fatty acyl-CoA	CoA’s sulfur performs nucleophilic attacks the β-carbonyl carbon, breaking the bond between α and β carbons.

b-oxidation of FA:

• Write from memory the overall reaction equation for a given fatty acyl-coA molecule.

Palmitoyl-CoA + 7CoA + 7FAD + 7NAD⁺ + 7H₂O → 8 Acetyl-CoA + 7FADH₂ + 7NADH + 7H⁺ (dependent on the number of carbons in fatty acid chain)

• Acetyl-CoA is a 2-carbon molecule so # of Acetyl-CoA molecules (half of the # of carbons in fatty acid chain)

• CoA, FAD, NAD⁺, H₂O, and H⁺ is (½ the # of carbons in fatty acid chain - 1)

Ketone bodies:

• List their names (acetoacetate, b-hydroxybutyrate, acetone)

Ketone bodies:

• Describe where they are made (liver mitochondrial matrix) and why (high acetyl-CoA, for example from b-oxidation of FA, needs to be transported to other tissues without   CoA that must stay in MM).

Ketone bodies:

• Describe what happens to ketone bodies after they are made in the MM (transported to other tissues where they are broken down into acetyl-CoA that can enter the CAC).

Ketone bodies:

• Explain why blood acidosis is possible if insulin signaling does not work properly.