Fatty Acid Oxidation, Ketone Bodies and Choosing a Fuel
Fatty acids are shortened two carbons at a time, feeding acetyl groups into the cycle and reduced carriers into the chain, which is why fat yields more ATP per carbon than sugar. When carbohydrate runs low the liver converts some of that acetyl into ketone bodies the brain can use.
What a learner can do afterwards
- Calculates the ATP yield of a stated fatty acid from the rounds it takes to break down.
- Explains why fat cannot be converted to glucose in humans.
- Links ketone body production to a shortage of cycle intermediates rather than to fat excess.
1 · Read
Fatty acids are shortened two carbons at a time by beta oxidation. Each round produces one acetyl CoA plus one NADH and one FADH2, with the chain two carbons shorter. Those carriers feed the electron transport chain and the acetyl groups feed the cycle, which is why fat yields more ATP per carbon than sugar.
Count the rounds from the chain length: rounds equal carbons divided by two, minus one, while acetyl units equal carbons divided by two. An 8-carbon acid therefore takes 3 rounds and gives 4 acetyl CoA. Each NADH is worth 2.5 ATP, each FADH2 is worth 1.5 ATP, and each acetyl CoA is worth 10 ATP through the cycle, so add the carriers plus the cycle output per acetyl to price the gross yield. Activating the fatty acid costs 2 ATP equivalents, so take 2 off the gross total to get the net yield. For the 8-carbon acid, the gross yield is 52 ATP and the net yield is 50 ATP.
Acetyl CoA from fat cannot become net glucose in humans, because the pyruvate step is irreversible and the cycle releases two carbons for every two that enter. Fat is superb fuel but no sugar source, so the fasting brain runs on ketone bodies. Ketones appear when oxaloacetate runs short and stranded acetyl piles up, which signals scarce cycle capacity, not simply abundant fat.
Read ketones as an oxaloacetate report: fasting diverts it to glucose making, so acetyl piles up and ketones rise. Most tissues including the fasting brain burn ketones, but red blood cells cannot, since they lack mitochondria.
Fat is chopped two carbons per round for a rich carrier harvest, yet its acetyl cannot become glucose, so fasting livers park stranded acetyl as ketones.
More to read
4.5 Connections to Other Metabolic Pathways
Fats enter respiration as fatty acids that are chopped into acetyl CoA units by beta oxidation. Each round shortens the chain by two carbons and produces one NADH and one FADH2 alongside the acetyl CoA. Those carriers and the acetyl CoA then feed the electron transport chain and the citric acid cycle. Counting rounds for a given chain length predicts the total ATP yield.
7.6 Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways
Acetyl CoA from fat cannot become net glucose in humans, because the pyruvate dehydrogenase step is irreversible and the cycle releases two carbons for every two that enter. Fat is therefore a superb fuel but a poor sugar source. The brain, which prefers glucose, must be supplied by gluconeogenesis or, during starvation, by ketone bodies. This one way chemistry shapes every fuel choice the body makes.
8.2 Catabolism of Carbohydrates
Ketone body production begins when acetyl CoA accumulates faster than the cycle can accept it. That happens when oxaloacetate runs short, typically because it is diverted to gluconeogenesis during fasting. The liver then converts excess acetyl CoA into ketone bodies for other tissues to burn. Ketones therefore signal scarce cycle capacity, not simply abundant fat.
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