BCH2022 Chap.6 Lipid Mobilisation and Beta-Oxidation
Lipid Mobilisation and Beta-Oxidation
Lipid catabolism begins before beta-oxidation. Stored triacylglycerol is mobilised, fatty acids travel to tissues, enter cells, are activated and cross the mitochondrial inner membrane through regulated transport. The beta-oxidation spiral then repeats oxidation, hydration, oxidation and thiolysis, releasing acetyl-CoA and reduced carriers. Carbon counting gives the number of acetyl units and cycles for a saturated even chain.
The chapter separates circulating fatty acid concentration from tissue oxidation, explains how ketone production depends on hepatic routing, and makes substrate structure and activation assumptions explicit in every quantitative result. Lipid use begins before the beta-oxidation spiral.
Stored triacylglycerol must be mobilised, fatty acids travel in an appropriate bound form, cellular uptake and activation occur, and long chains require controlled mitochondrial entry. Only then does repeated beta-oxidation shorten the acyl chain and produce acetyl-CoA, NADH and a reduced FAD carrier. This ordering prevents a common mistake in which every change in circulating fatty acid is attributed directly to the spiral.
Hormonal state can alter mobilisation while malonyl-CoA-related control alters mitochondrial entry, so substrate availability and mitochondrial access must be assessed separately. For a saturated even chain, carbon counting gives the acetyl-CoA units first and the cycle count second; the last cleavage releases two acetyl units, making cycles one fewer than units.
Keep activation cost and any ATP-yield convention separate unless the question specifies how to count them. When oxidation is impaired, predict the first blocked transfer and then distinguish accumulation before the lesion from reduced products after it. Oxygen consumption, ketone production and fatty-acid abundance are related but not interchangeable readouts.
Ketone formation is a liver routing response to acetyl-CoA and whole-body fuel conditions, not a universal fate in every tissue. A strong experiment controls fatty-acid delivery, mitochondrial amount, time and cell viability, while measuring both disappearance of labelled substrate and appearance of oxidation products. An endpoint pool alone can rise because of increased mobilisation, reduced uptake or reduced oxidation.
In a disease-style answer, trace the effect from adipose release through liver or muscle handling, identify the affected compartment, and state which result would distinguish transport failure from enzyme failure within the spiral.
What this chapter covers
- 01
Triacylglycerol mobilisation
- 02
Fatty acid transport
- 03
Acyl-CoA activation
- 04
Mitochondrial entry
- 05
Beta-oxidation spiral
- 06
Ketone-body routing
Worked example · free
Count a fourteen-carbon chain
- +1Confirm the substrate is a saturated even-chain fatty acid, so the basic counting rule applies.
- +1Divide fourteen by two to obtain seven acetyl-CoA units.
- +1Subtract one from the acetyl-unit count because the final cleavage yields two units, giving six cycles.
- +1Keep activation cost and any ATP-equivalent convention separate until the question defines them.
Key terms
- Lipolysis
- Lipolysis releases fatty acids and glycerol from stored triacylglycerol.
- Fatty acyl-CoA
- Fatty acyl-CoA is the activated form of a fatty acid used in oxidation or lipid synthesis.
- Carnitine shuttle
- The carnitine shuttle transfers long-chain fatty acyl groups across the mitochondrial inner membrane.
- Beta-oxidation
- Beta-oxidation repeatedly shortens fatty acyl chains and releases acetyl-CoA and reduced carriers.
- Thiolysis
- Thiolysis cleaves a beta-ketoacyl intermediate using coenzyme A and releases acetyl-CoA.
- Ketone body
- A ketone body is a liver-derived fuel produced when acetyl-CoA supply exceeds local cycle routing.
Lipid Mobilisation and Beta-Oxidation FAQ
Why are cycles one fewer than acetyl units?
The final four-carbon intermediate is split into two acetyl-CoA molecules during the last cycle. Therefore an even chain with several acetyl units needs one fewer cycle than the number of units.
Does high plasma fatty acid prove oxidation?
No. Concentration reflects release minus uptake and removal. Increased mobilisation, reduced uptake or altered tissue use can all raise the pool, so tracer or product evidence is needed for oxidation.
Why is mitochondrial entry regulated?
Long-chain fatty acids require controlled transfer after activation. The entry gate coordinates oxidation with the synthetic state and helps prevent simultaneous high fatty acid synthesis and breakdown.
When do ketone bodies increase?
Hepatic ketone production rises when fatty acid-derived acetyl-CoA supply is high relative to citric acid cycle routing, commonly during longer low-carbohydrate states. Hormones and oxaloacetate availability shape the response.
Exam move
Write four verbs across the page: mobilise, transport, import and oxidise. Place the responsible tissue or compartment under each one. Learn the beta-oxidation spiral by its chemical logic rather than enzyme names alone, then count acetyl units and cycles for several even chains. Add separate cases for odd and unsaturated chains so the shortcut is not overgeneralised.
For experiments, distinguish fatty acid disappearance from actual oxidation by using labelled products and controlling chain length, binding, cell number and oxygen. Connect hepatic acetyl-CoA routing to oxaloacetate and ketone production, but avoid claiming that extra acetyl-CoA alone determines the result. State activation and energy conventions beside every final calculation.
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