What is Lipid Metabolism and Beta-Oxidation?
Lipid metabolism is how the body stores and breaks down fats for energy. Beta-oxidation is the central pathway: it chops fatty acid chains into 2-carbon acetyl-CoA units inside the mitochondria, feeding the Krebs cycle and generating large amounts of ATP.
Beta-oxidation is the mitochondrial pathway that repeatedly removes 2-carbon acetyl-CoA units from a fatty acid chain, producing FADH2 and NADH each cycle until the whole fatty acid is converted into acetyl-CoA for the Krebs cycle.
- 1↓ActivationA fatty acid is attached to coenzyme A in the cytoplasm, forming fatty acyl-CoA (costs 2 ATP-equivalents).
- 2↓Carnitine ShuttleFatty acyl-CoA is carried across the mitochondrial membrane bound to carnitine.
- 3↓Beta-Oxidation CyclesEach cycle removes one 2-carbon acetyl-CoA unit and produces 1 FADH2 and 1 NADH, repeating until the chain is fully broken down.
- 4Krebs Cycle & ETCAcetyl-CoA units enter the Krebs cycle; FADH2/NADH feed the electron transport chain to generate ATP.
Step-by-step worked examples
Palmitic acid has 16 carbons. How many beta-oxidation cycles and how many acetyl-CoA molecules does it yield?
Cycles = (n/2) − 1 = (16/2) − 1 = 7 cycles Acetyl-CoA produced = n/2 = 16/2 = 8 Result: 7 cycles, producing 8 acetyl-CoA, 7 FADH2, and 7 NADH
Stearic acid has 18 carbons. How many acetyl-CoA molecules and beta-oxidation cycles does it produce?
Acetyl-CoA = n/2 = 18/2 = 9 Cycles = (n/2) − 1 = 9 − 1 = 8 Result: 8 cycles, producing 9 acetyl-CoA
Using P/O ratios (NADH = 2.5 ATP, FADH2 = 1.5 ATP, and 10 ATP per acetyl-CoA through the Krebs cycle/ETC), estimate the net ATP yield of complete oxidation of one palmitic acid (8 acetyl-CoA, 7 FADH2, 7 NADH from beta-oxidation).
Acetyl-CoA via Krebs cycle: 8 × 10 = 80 ATP Beta-oxidation FADH2: 7 × 1.5 = 10.5 ATP Beta-oxidation NADH: 7 × 2.5 = 17.5 ATP Total = 80 + 10.5 + 17.5 = 108 ATP Minus activation cost (2 ATP-equivalents) = 106 ATP Result: about 106 net ATP per palmitic acid
Flashcards
Quick quiz
Q1.What molecule is released each cycle of beta-oxidation?
Q2.How many acetyl-CoA molecules does a 16-carbon fatty acid (palmitic acid) yield?
Q3.What carries fatty acyl-CoA across the mitochondrial membrane?
Q4.Which two electron carriers are produced during each beta-oxidation cycle?
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Common mistakes
Beta-oxidation occurs in the cytoplasm. — Correct: Beta-oxidation takes place in the mitochondrial matrix; fatty acid synthesis (the reverse-like process) occurs in the cytoplasm.
Fatty acids can freely enter mitochondria without help. — Correct: Long-chain fatty acids need the carnitine shuttle to cross the mitochondrial membrane.
Beta-oxidation produces glucose directly. — Correct: It produces acetyl-CoA, which enters the Krebs cycle for ATP — it cannot be converted back into glucose in animals (acetyl-CoA can't feed gluconeogenesis).
All fatty acids yield the same number of acetyl-CoA regardless of length. — Correct: The number of acetyl-CoA units equals half the carbon count, so longer chains yield more acetyl-CoA and more ATP.
FAQ
What is beta-oxidation?
It's the mitochondrial pathway that breaks fatty acids down into 2-carbon acetyl-CoA units, generating NADH and FADH2 along the way.
What is the formula for beta-oxidation cycle number?
Number of cycles = (n/2) − 1, and acetyl-CoA produced = n/2, where n is the number of carbons in the fatty acid.
What are examples of beta-oxidation in the body?
During fasting or exercise, stored triglycerides release fatty acids that undergo beta-oxidation in liver and muscle mitochondria to generate ATP.
How do you calculate ATP yield from a fatty acid?
Sum ATP from each acetyl-CoA through the Krebs cycle/ETC (~10 ATP each) plus ATP from the FADH2 and NADH made directly in beta-oxidation, then subtract the 2 ATP-equivalent activation cost.