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Initiating Fat Metabolism
- Epinephrine/glucagon initiate fat breakdown
- Can be used for ATP through: catabolism, mobilization, and oxidation
- Steps in Fat mobilization (β adrenergic signalling)
1. hormone binds GPCRs on adipose cells
2. Signal transduction: Gα-GTP, adenylate cyclase, cAMP production, PKA activation
3. hormone sensitive lipase is activated by PKA
4. TAGs --> glycerol and 3 fatty acids
Fatty Acid Activation
- In the cytoplasm, fatty acids are activated through the addition of a CoA (traps them in the cell)
- Performed by Acyl-CoA synthetase
- Acyl-CoA moves into the mitochondrial matrix where beta-oxidation occurs
- Passes though the outer membrane through porins, but through the inner membrane via carnitine shuttle
beta oxidation occurs in 4 steps:
1. Acyl CoA dehydrogenase: double bond formation in the fatty acid chain (FADH2 produced)
2. Hydratase: adds water across the bond, introducing a hydroxyl group
3. Dehydrogenase: converts hydroxyl group to ketone group (NADH produced)
4. Thiolase: cleaves acetyl-CoA from the fatty acid chain and leaves the fatty acyl-CoA with two less carbons
One 16:0 CoA yeilds:
- 8 acetyl CoA + 7 NADH + 7 H+ + 7 FADH2
- This is would yield significantly more ATP than glucose, but the Citric Acid Cycle is limited by oxaloacetate (derived from pyruvate)
- "Hitting the wall" during exercise is when no more glycogen is available
- ATP production from fat is limited by oxaloacetate
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Which statement is FALSE about fatty acid catabolism?
a. Fat catabolism is a response to glucagon signalling
b. Triacylglycerol lipase is activated by PKA to degrade TAGs
c. Beta oxidation of acyl-CoAs has the potential to provide equivalent quantities of ATP as oxidative phosphorylation
d. The carnitine shuttle transports acyl-CoA from the mitochondrial matrix to the cytosol
e. None of the above
f. More than one of the above
f. Beta-oxidation has the potential to provide more ATP than oxidative phosphorylation, and the carnitine shuttle moves acyl-CoA from the cytosol to the mitochondrial matrix.
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How many oxidoreductase reactions are there during one round of beta-oxidation?
a. 1
b. 2
c. 3
d. 4
b. 2. The first step and the third step are oxidoreductase reactions. They yield one FADH2 and one NADH.