Lipid Metabolism

Question 1

What are the three dietary lipids?

Answer 1

Triglycerides = 95%
Cholesterol - don’t get any ATP from it - need to make membranes
Phospholipids - to make membranes

Question 2

Types of fatty acids in triglycerides?

Answer 2

Saturated FA - no C=C
Mono-unsaturated - one C=C
Polyunsaturated - multiple C=C

Question 3

Briefly describe lipid digestion

Answer 3

Occurs in the small intestine.
Bile salts emulsify fats to smaller fat droplets - to increase surface area for enzymes to work on
Bile salts and pancreatic lipase and collapse break down triglycerides into 2 fatty acids and monoglyceride

Question 4

What happens when fatty acids enter the intestinal cell??

Answer 4

They are reformed as triglycerides and packaged with cholesterol, lipoproteins and other lipids to form chylomicrons

Question 5

Why do fatty acids have to be packaged up?

Answer 5

Because they start to break down cell walls - quite dangerous

Question 6

How do chylomicrons exit intestinal cell?

Answer 6

They are released into lymphatic system by exocytosis

Question 7

What does lipoprotein lipase do?

Answer 7

Rips off the lipid component from the chylomicron and these then go to adipose tissues for:

• storage (TG)
• structural (phospholipids)
• oxidation (ATP)

Question 8

What happens to the rest of the chylomicron after LPL action?

Answer 8

It is transported to the liver

Question 9

What do lipoproteins do (concerning lipid transport)?

Answer 9

They stop dangerous fatty acids from being free in circulation
Largely in VLDLs then LDL then HDL
Lipoprotein lipase takes the triglycerides away and leaves the cholesterol to be taken to cells

Question 10

What is beta-oxidation?

Answer 10

The generation of energy (ATP) from fatty acids.
Acetyl CoA is formed (and FADH2 and NADH) which is further oxidised in the TCA cycle and oxidative phosphorylation to get more ATP

Question 11

Where does beta oxidation occur?

Answer 11

Within the mitochondria which has a double membrane

Fatty acids need a carrier protein to carry them

Question 12

What carrier protein helps fatty acids over the mitochondrial membrane for beta-oxidation?

Answer 12

Carnitine - an amino acid

• derived from lysine and methionine
• lots in muscle

Question 13

What enzyme is needed for fatty acid entry to the mitochondrion for beta oxidation?

Answer 13

Carnitine Palmitoyl Transferase

CPT1 and CPT2

Question 14

How do fatty acids enter the mitochondrion for beta oxidation?

Answer 14

CPT1 (in first membrane) splits carnitine and fatty acyl Co-A into CoA and fatty acyl carnitine
FA carnitine crosses the second membrane
CPT2 then transfers them back to carnitine and fatty acyl CoA which is used for beta oxidation

Question 15

Regulating factor in beta oxidation?

Answer 15

Activity of the CPT enzymes - especially CPT1

Question 16

What happens in beta oxidation?

Answer 16

Fatty acyl-CoA is degraded by oxidation at the beta carbon
Fatty acyl chain is reduced by 2 carbons each time
Produces 1 FADH2, NADH and Acetyl CoA each time
Re-enters cycle and chops off 2 carbons each time

Question 17

What happens to the acetyl CoA?

Answer 17

Further oxidised to get ATP, through the TCA cycle and oxidative phosphorylation

Question 18

Why are fatty acids such a good source of ATP?

Answer 18

Lots of ATP per molecule vs glucose

Can store TG and lipid droplets together very densely in adipose tissues

Question 19

How are triglycerides synthesised?

Answer 19

Synthesised from the esterification of three fatty acids and glycerol

Fatty acids:
LPL hydrolyses the FAs in chylomicrons/VLDL - takes away the FA
FAs transported in/out of cell with proteins

Glycerol is obtained from glycolysis
DHAP - G3P stage

Diacylglycerol acyl transferase (DGAT) re-esterifies to triglycerides

Question 20

What happens in triglyceride breakdown? (lipolysis)

Answer 20

TG is broken down into glycerol and three fatty acids

Enzyme involved = hormone sensitive lipase

Question 21

Hormone sensitive lipase

Answer 21

Found in adipose tissue (liver also has)

Activated by cAMP-dependent phosphorylation in response to adrenaline in FASTED state

Inhibited by insulin in FED state

Question 22

Is adipose tissue innervated??

Answer 22

Yes, very highly innervated

Has lots of adrenergic nerves (the autonomic nervous system)

Question 23

What does lipolysis result in?

Answer 23

Fatty acids transported mainly to muscle (largest tissue) but also other tissues

Glycerol transported to liver for gluconeogenesis to make blood glucose (as stimulated in fasted state remember!!)