Catabolism of Lipids (Triacylglycerol)

B.Sc. (Hons.) Sport and Exercise Science

Craig Boyd

Learning Outcomes1. Outline the role of lipid in the replenishment of ATP.

2. Describe the entry points of glycerol and fatty acids in previously covered energy systems (glycolysis and TCA).

3. Compare and contrast the replenishment of ATP from glycogen and lipids (TAG).

4. Describe the role of exercise intensity in the energy continuum.

Lipid Catabolism - Overview

• Can only replenish ATP aerobically.

• Stored lipids are the body’s most plentiful source of potential energy.

• A relatively ‘unlimited’ source of energy.

• Can supply up to 80% of the energy required for exercise - intensity dependent.

Triacylglycerol (TG or TAG)• Also known as triglyceride, consist of 3 fatty acids +

glycerol.

• The fatty acids can be combined (esterified) or free (non-esterified) FFA [NEFA].

• Major source of exogenous lipid ~100-150g consumed∙day-1.

• Stored in the body as adipose tissue or within muscle (intramuscular TAG stores).

Catabolism of Lipidhormone sensitive lipase(lipolysis)

Catabolism of Glycerol

• Glycerol enters glycolytic pathway as 3-phosphoglyceraldehyde pyruvate (slide 7).

• Pyruvate oxidised in the TCA (Krebs) cycle (slide 8).

• Free energy for 19 ATP to be replenished:• 4 from glycolysis, 15 from TCA (slide 9).

glycerol

single molarity

2 ATP!

1 ATP

1 ATP

go back

3 ATP

3 ATP

3 ATP

2 ATP

1 GTP

Remember: singlemolarity not double

like CHO!

go back

3 ATP

Transport of Free Fatty Acids

• FFA diffuse from adipocytes (fat cells) into bloodstream and combine with albumin (a soluble protein).

• Fatty acids are then delivered to active tissues.

• Uptake of FFA into tissue is via diffusion (gradient dependent) and is proportional to blood flow.

Catabolism of Fatty Acids• Occurs within mitochondria of the cell.

• Process called beta oxidation ( ß-oxidation).

• Involves the progressive removal of two-carbon (2C) acetyl units from the FFA chain.

• These are cyclically degraded to acetyl-CoA which enters the TCA cycle and is oxidised.

35 ATP total

12 ATP per cycle

3 ATP per cycle

ATP Replenishment from FFA

• For each 18-carbon fatty acid molecule - 147 (146) ATP are formed (ß-oxidation + TCA).

• Each triacylglycerol has 3 fatty acids 3 x 147 (146) 441 (438) ATP

• Total yield = 19 (glycerol) + 441 (438) ATP

= 460 (457) ATP per 18-C TAG

Energy Continuum• Relative contribution of energy systems shifts

• CHO, lipid, and protein interact metabolically to replenish ATP.

• Exercise intensity is the main ‘effector’.

• Understanding this can help to identify :• predominant substrates during exercise.• sport & exercise specific energy needs.