Lipid Catabolism Study Notes

Chapter 17: Lipid Catabolism

  • Overview: Lipid catabolism refers to the metabolic processes that break down lipids, particularly fatty acids, for energy. This process is critical for fuel utilization in fasting and exercise.

Dietary Lipids

  • Transport and Digestion:
    • Dietary lipids, primarily triglycerides (TAGs), are digested in the lumen of the small intestine.
    • The process involves:
    • Hydrolysis of TAGs into fatty acids (FAs) and monoacylglycerol
    • Transport into enterocytes (intestinal cells)
    • Re-esterification of FAs into TAGs
    • Packaging of TAGs into chylomicrons

Chylomicrons

  • Description:

    • Chylomicrons are the largest class of lipoproteins composed primarily of lipids, where 98% of their weight is lipid, with 85% being TAGs.

  • Function:

    • Transport absorbed lipids from the intestines through lymph ducts into the bloodstream.
    • The predominant protein component is ApoB48.
    • During circulation, TAGs and some cholesterol are extracted, resulting in a chylomicron remnant.

Lipolysis

  • Definition: Lipolysis is the metabolic process that breaks down stored lipids (triglycerides) into free fatty acids and glycerol when energy is needed.

  • Triggers:

    • Occurs during fasting and exercise periods.

  • Mechanism:

    • Lipolysis is regulated by the phosphorylation of hormone-sensitive lipase and perilipin.
    • Perilipin: A protein that coats lipid droplets and ensures they are accessible only during lipolysis.
    • CGI-58: Released due to perilipin phosphorylation; activates TAG lipase for fat breakdown.

  • FA Transport in Bloodstream:

    • Fatty acids are transported bound to albumin in the bloodstream, with the heart and muscles being the primary consumers of these fatty acids.
    • Glycerol is transported to the liver where it can enter glycolysis or gluconeogenesis.

Fatty Acid Activation

  • Process: Before fatty acids can enter the mitochondria for oxidation, they are activated to form acyl-CoA on the outer mitochondrial membrane.
    • This activation uses two equivalents of ATP.

Carnitine Shuttle

  • Rationale: Acyl-CoA cannot cross the mitochondrial membranes.

    • Process:
    1. Fatty acyl-CoA is converted to acylcarnitine by Carnitine acyltransferase I (CAT I).
    2. Acylcarnitine is then transported across the mitochondrial membrane by a carrier protein.
    3. Inside the mitochondria, acylcarnitine is converted back to acyl-CoA by Carnitine acyltransferase II (CAT II).

  • Weight Loss: Contrary to some beliefs, carnitine does not assist with weight loss.

  • Design Argument: Discussion on the lack of a direct transporter for fatty acyl-CoA is suggested as an argument against intelligent design.

Part 2: Fatty Acid Oxidation

  • Stages of Fatty Acid Oxidation:

    1. Stage 1: β-Oxidation: Breakdown of fatty acids into acetyl-CoA units.
    2. Stage 2: Citric Acid Cycle (CAC): Acetyl-CoA enters CAC for energy production.
    3. Stage 3: Electron Transport Chain: Further ATP generation through oxidative phosphorylation.

  • Overall Yield: Complete oxidation of fatty acids via β-oxidation results in multiple acetyl-CoA units which are further processed in the CAC and electron transport chain.

β-Oxidation Cycle

  • Processes: Each cycle produces:

    • 1 NADH
    • 1 FADH2
    • 1 Acetyl-CoA
    • The last cycle generates an additional Acetyl-CoA.

  • Example with Palmitic Acid (C16):

    • Undergoes 7 cycles, resulting in:
    • 7 NADH
    • 7 FADH2
    • 8 Acetyl-CoA

  • Equation for Palmitic Acid Breakdown:
    extPalmitoyl−CoA+7extCoASH+7extFAD+7extNAD++7extH<em>2extOightarrow8extAcCoA+7extFADH</em>2+7extNADH+7extH+ext{Palmitoyl-CoA} + 7 ext{CoASH} + 7 ext{FAD} + 7 ext{NAD}^+ + 7 ext{H}<em>2 ext{O} ightarrow 8 ext{AcCoA} + 7 ext{FADH}</em>2 + 7 ext{NADH} + 7 ext{H}^+

  • Enzyme Complex: The β-oxidation enzyme complex consists of hydratase, NAD+-linked dehydrogenase, and thiolase, contributing to the overall reaction efficiency.

Monounsaturated and Polyunsaturated FAs

  • Monounsaturated FAs:

    • Lose one FADH2 as the first step of β-oxidation is skipped (example: Oleic acid C18:1 yields 8 NADH, 7 FADH2, and 9 Acetyl-CoA).

  • Polyunsaturated FAs:

    • Each additional C=C bond requires the use of NADPH.

    • Example with Linoleic Acid (C18:2):

    • Yields slightly lower ATP than oleic acid due to the conversion of NADH to FADH2.

Oxidation of FAs

  • Typical Yield Calculation:
    • For Stearic Acid (C18), the expected ATP yield is broken down into:
    • β-oxidation: 8 NADH, 8 FADH2, -2 ATP (activation) ; Total: 35 ATP from oxidation.
    • The cumulative ATP yield accounts for both β-oxidation and CAC, totaling 120.

Water Formation During Fatty Acid Oxidation

  • Process: Water formation occurs with every cycle of oxidation:
    • β-Oxidation of Palmitic Acid requires 7 H2O cycles.
    • CAC of 8 Acetyl-CoA requires 16 H2O.
    • Total water formation from electron transport: 46 H2O produced from 31 NADH and 15 FADH2.

Glucose Production from Odd-Chain Fats

  • Conversion Process:
    • Odd-chain fatty acids yield 1 propionyl-CoA which can be converted to glucose, showing the potential use of fats for maintaining glucose levels in the body.

Regulation of Fatty Acid Oxidation

  • Factors Influencing Regulation:
    • Linked to fatty acid synthesis.
    • Availability of O2 and fatty acids, which is regulated by glucagon levels.

Part 3: Ketone Bodies

  • Function:

    • Ketone bodies serve as alternative fuel during low glucose availability, easily transported to heart and muscle tissues, allowing continued β-oxidation and gluconeogenesis.

  • Overproduction:

    • High-fat, low-carbohydrate diets and diabetes can lead to ketoacidosis due to excess β-hydroxybutyrate, lowering blood pH.