Lipolysis & Lipid Synthesis

Question 0

Why are some fatty acids essential?

Answer 0

Humans cannot introduce a double bond beyond C9, so some polyunsaturated acids need to be consumed in the diet.

Question 1

What is the structure of lipids compared to carbohydrates?

Answer 1

Insoluble (hydrophobic)

More reduced than carbohydrates, therefore releases more energy when oxidised than carbohydrates.

Question 2

Name the fat soluble vitamins.

Answer 2

ADEK

Question 3

What are triacylglycerols composed of? What are TAGs used for? How are TAGs stored?

Answer 3

Glycerol bound to fatty acid side chains by ester bonds (-O-C=O)

Used in prolonged exercise, pregnancy, and starvation.

Hydrophobic, therefore stored in anhydrous form.

Question 4

How are lipid stores transported to tissues for oxidation?

Answer 4

Fatty acids are bound to albumin by hormone-sensitive lipase and are transported to consumer tissues, e.g. muscles.

Hormone-sensitive lipase is activated by glucagon and adrenaline, and inhibited by insulin.

Question 5

Which cells cannot utilise fatty acids as an energy source?

Answer 5

Cells without mitochondria, e.g. RBCs

CNS (as TAGs cannot pass the blood-brain barrier)

Question 6

Where are lipids from the diet digested, and by what enzymes?

Answer 6

Extracellular digestion: small intestine (pancreatic lipases)

Question 7

How are fatty acids transported into mitochondria for beta-oxidation?

Answer 7

Carnitine Shuttle Transporter (inhibited by malonyl-CoA)

Fatty-acyl CoA bound to carnitine (FA-acyl carnitine) using a carnitine acyltransfease enzyme and shuttled across inner mitochondrial membrane.

Fatty-acyl CoA removed by another carnitine acyltransferase enzyme and enters the TCA cycle using FAD2H & NADH. Carnitine shuttles back across.

Question 8

Describe the mechanism of fatty acid oxidation.

Answer 8

Beta-oxidation.

Two carbons removed from fatty acid each cycle

Water and FAD/NAD+ used each cycle.

Occurs in mitochondrial matrix.

Question 9

What does a defect in the carnitine shuttle cause?

Answer 9

Exercise intolerance.

Lipid droplets accumulate in muscle during exercise

Question 10

How is glycerol metabolised?

Answer 10

Glycerol kinase catalyses phosphorylation of glycerol, which can either be converted back to TAGs, or can be converted to DHAP for glycolysis in the liver.

Question 11

How are lipids synthesised?

Answer 11

Fatty acid (n) + Malonyl-CoA -> Fatty acid (n+2)-Acyl carrier protein
                                   (fatty acid synthase)