Lipid metabolism

Question 1

What are the 3 sources of lipids?

Answer 1

1. Diet
2. De novo biosynthesis
3. Storage depots in adipose tissue

Question 2

Function of bile

Answer 2

Lipid digestion is aided using bile salts produced by the liver and stalled in the gall bladder. They are used to emulsify fats in the small intestine increasing the rate of digestion using lipase allowing for easier absorption of fats as well as fat soluble vitamins.

Question 3

What are chylomicrons?

Answer 3

Initially the dietary products are absorbed by enterocytes in the lining of the small intestine and are then integrated into chylomicrons which transport the dietary product from the lacteals of the small intestine to the thoracic duct and to the left subclavian vein where they enter the bloodstream. The chylomicron can be broken down using lipoprotein lipase releasing the dietary products. The fatty acids undergo beta oxidation whilst the glycerol is returned to the liver for gluconeogenesis.

Question 4

What are HDLs?

Answer 4

HDLs transfer apoproteins to nascent chylomicrons when they enter the bloodstream.

They also carry cholesterol from peripheral tissues to the liver.

Question 5

What are LDLs?

Answer 5

LDLs carry cholesterol synthesised in the liver to the peripheral tissues. Elevated levels of LDLs are associated with atherosclerosis.

Question 6

How is acyc coA generated for beta oxidation?

Answer 6

Fatty acids react with CoA using ATP to form acyl CoA, AMP and PPi using acyl CoA synthetase. The acyl CoA is produced outside the mitochondrial matrix therefore the carnitine shuttle is used to transport the acyl CoA into the matrix by forming acyl carnitine which can be moved into the matrix by a translocase enzyme.

Question 7

Outline beta oxidation

Answer 7

1. First the long fatty acyl CoA is oxidised using acyl CoA dehydrogenase to trans 2 enoyl CoA reducing FAD to FADH2 in the process.
2. Trans 2 enoyl CoA is hydrated then dehydrated.
3. Thiolysis occurs using thiolase enzyme to form acetyl coA and a new fatty acyl CoA with 2 less carbons.

This process repeats until all the carbons in the fatty acyl CoA chain are converted into acetyl CoA.

Question 8

Why are ketone bodies formed during periods of strenuous exercise or fasting?

Answer 8

Acetyl CoA generated by β-oxidation enters the TCA cycle only if β-oxidation and carbohydrate metabolism are balanced, since oxaloacetate is needed for entry into the TCA cycle.

Acetyl CoA is therefore used to synthesise ketone bodies instead by switching off acetyl coA carboxylase and switching on malonyl coa decarboxylase instead which intiaties ketogenesis.

Question 9

Outline lipogenesis

Answer 9

Firstly, acetyl CoA and malonyl CoA undergo group transfer reactions using malonyl ACP to activate acetyl CoA and acetyl ACP to activate malonyl CoA. Then, decarboxylative condensation reactions occur between acetyl ACP and malonyl ACP, elongating the fatty acyl ACP chain. After each round of elongation, the fatty acyl ACP chain undergoes reduction and dehydration by the sequential action of ketoreductase, dehydratase and enol reductase.