15. Topic : Beta oxidation and regulation of lipid / FA

Question 1

Part 1 : Biosynthesis, storage and mobilisation of fat

Describe how the liver contributes to lipid metabolism.

Answer 1

1. Liver : biosynthesis of FA, β-oxidation of FA, ketone body generation

Question 2

Part 1 : Biosynthesis, storage and mobilisation of fat

Describe how the muscle / kidney contributes to lipid metabolism.

Answer 2

β-oxidation of FA occurs to generate energy (when blood glucose is low)

Question 3

Part 1 : Biosynthesis, storage and mobilisation of fat

Describe how the adipose tissues contributes to lipid metabolism.

Answer 3

They store fat in the form of TAG

Question 4

Part 1 : Biosynthesis, storage and mobilisation of fat

Describe how the red blood cells / brain contributes to lipid metabolism.

Answer 4

They do not use FA as fuel.
- But for the brain, when blood glucose is low, it utilises ketones for energy generation (FA cannot cross the blood brain barrier)

Question 5

Part 1 : Biosynthesis, storage and mobilisation of fat

Fat storage and release mainly occurs in ___ tissues. How is this fat storage and release regulated?

Answer 5

Adipose.
Hormonal regulation.

Question 6

Part 1 : Biosynthesis, storage and mobilisation of fat

Explain how glucagon regulates fat mobilisation / storage in adipose tissues.

Answer 6

• When blood glucose is low, insulinglucagon bind to surface receptors of adipose tissues.
• This triggers an increase in [cAMP], which activates PKA (protein kinase A).
• PKA phosphorylates endogenous adipocyte lipase, increasing digestion of TAG -> FA and thus releasing FA to be utilised for energy

Question 7

Part 1 : Biosynthesis, storage and mobilisation of fat

Explain how insulin regulates fat mobilisation / storage in adipose tissues.

Answer 7

When blood glucose is high,
- insulin activates lipoprotein lipase in blood vessels, causing TAG in VLDL to be broken down into FA
- At the same time, cAMP is inhibited by insulin, imhibiting fat breakdown in the adipose tissues.
- FA diffuses from bloodstream into adipose tissues, and is eventually stored as fat

Question 8

Part 1 : Biosynthesis, storage and mobilisation of fat

Explain what is ectopic fat storage and what metabolic disease it is associated with.

Answer 8

Ectopic fat storage is a phenomenon whereby excess dietary fat is consumed and exceeds storage capacity of adipose tissues & rate of b-oxidation, leading to fat being stored in tissues that usually dont store fat (peripheral tissue).

It is associated with insulin resistance in muscle and liver cells as excessive fat impairs insulin signalling.

Question 9

Part II : β-oxidation of fatty acids

Where does β-oxidation of fatty acids
occur? (organ, organelle)

Answer 9

Liver (and muscle, kidney to a smaller extent), mitochondria

Question 10

Part II : β-oxidation of fatty acids

Why is oxidation of fatty acids called β-oxidation ?

Answer 10

Oxidation of FA occurs on the second C atom from carboxyl (CoA) group

Generates acetyl CoA

Question 11

Part II : β-oxidation of fatty acids

Before β-oxidation of fatty acids, they have to be …? [2]

Answer 11

Activated and transported out of cytosol into mitochondria of liver cells

Question 12

Part II : β-oxidation of fatty acids

How are FAs activated in the cytosol of adipose tissues before being transported out?
State the reaction equation and the enzyme that catalyses this reaction.

Answer 12

FA + CoA-SH + ATP → Acyl-CoA + AMP + PP_i

• Catalysed by Acyl-CoA synthethase

Need energy to form chemical bond between FA and CoA-SH

Question 13

Part II : β-oxidation of fatty acids

After FA is activated into acyl-CoA in the cytosol, can it be transported into mitochondria through the inner mitochondrial membrane?

If not, state the reaction equation

Answer 13

No, acyl CoA cannot cross the inner mitochondria membrane.
Acyl CoA + carnithine → acyl-carnithine + H-SCoA

Question 14

Part II : β-oxidation of fatty acids

How are FAs transported out of the adipocyte tissue → bloodstream → other tissues / cells ?

Hint : FA are hydrophobic

Answer 14

Since FAs are hydrophobic, but still need to circulate in bloodstream before reaching other tissues, several FAs are binded in albumin to form a complex, increasing their solubility in water

Question 15

Part II : β-oxidation of fatty acids

What are the 4 main steps of β-oxidation of fatty acids?
State if any electron carriers are produced and class of enzymes involved

Answer 15

1) Dehydrogenation (elimination rxn) of C_α – C_β bond to form C=C (2H atoms removed transferred to FADH, forming 1 FADH₂)
- acyl-CoA dehydrogenase

2) Hydration reaction, addition of H₂O across the C=C bond (CH=CH → CH(OH)-CH₂)
- enzyme class : hydratase

3) Oxidation (& dehydrogenation) to form C=O (oxidation of alcohol group) → 2H atoms removed are transferred to 1 NADH
- enzyme class : dehydrogenase

4) Cleavage at beta carbon by thiolase : β-ketoacyl CoA + CoA-SH → fatty acyl CoA + acetyl CoA

Question 16

Part II : β-oxidation of fatty acids

For each round of β-oxidation, what are the products formed?

Answer 16

1 acetyl-CoA
1 NADH
1 FADH₂

Question 17

Part II : β-oxidation of fatty acids

For a complete oxidation of a 16 carbon fatty acid, how many ATP is generated?

Answer 17

16 C = (16/2)/2 = 7 rounds of oxidation
8 acetyl CoA generated
- each acetyl CoA undergoing TCA : 3 NADH, 1FADH₂, 1 GTP
- 8 acetyl CoA : 24 NADH, 8 FADH₂, 8 GTP

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1 x 7 = 7 NADH formed
1 x 7 = 7 FADH₂
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Total = 31 NADH, 15 FADH₂, 8 GTP
= 31 (3) + 15 (2) + 8 = 131 ATP

Question 18

Part II : β-oxidation of fatty acids

One problem with β-oxidation of FAs is the presence of a cis β,γ bond (C3 and C4) instead of a cis α,β bond (C2 and C3). Explain how this is overcome.

Answer 18

Explain how this is overcome.The cis β,γ bond is converted into a cis α,β bond with an isomerase.

Question 19

Part II : β-oxidation of fatty acids

Other than the presence of a cis β,γ bond instead of a cis α,β bond, what are 2 other challenges faced during β oxidation of unsaturated FAs, and how is it overcome?

Answer 19

1) The presence of C=C bond between C5 and 6 causes the isomerisation of cis-α,β bond into cis β,γ (C3,4) bond to form conjugated double bonds (alternating) → isomerase is used to shift both C=C (which leads to another problem stated below)

2) Presence of 2 double bonds, Δ² (between C2, 3) and Δ⁴ (C4,5) double bonds inhibits the action of enoyl hydratase, water cannot be added across the Δ2 double bond.
- reduce Δ⁴ bond, but the Δ² bond will become a Δ³ bond (between C3 and 4 instead of between C2 and 3)
- Use an isomerase to shift back bond to Δ²

Question 20

Part II : β-oxidation of fatty acids

What is the difference between β-oxidation of odd chain FA and even chain FA?

Answer 20

The only difference is the last step of oxidation, where 5C acyl intermediate is split into acetyl CoA (2C0) and propionyl CoA (3C)

Question 21

Part II : β-oxidation of fatty acids

For β-oxidation of odd chain FA:
After 5c acyl intermediate is converted into acetyl CoA (2C) + propionyl CoA (3C), what happens to propionyl CoA afterward?

Answer 21

It undergoes a 3 step reaction to eventually get converted into succinyl-CoA, whhich enters the TCA cycle

Question 22

Part II : β-oxidation of fatty acids

For β-oxidation of odd chain FA:
During 3 step conversion of propionyl CoA into succinyl CoA, what is the role of methylmalonyl-CoA mutase enzyme?

Answer 22

Propionyl CoA gets converted into methylmalonyl-CoA after carboxylation.
The enzyme is involved in the reaction : methylmalonyl CoA → Succinyl CoA, where -C=O-SCOA group is swapped from C2 to C3 through a radical mechanism

Question 23

Part II : β-oxidation of fatty acids

β-oxidation of very long chain fatty acids (>22C) occurs in what organelle?

Answer 23

Peroxisome

Question 24

Part II : β-oxidation of fatty acids

β-oxidation of very long chain fatty acids (>22C)
In the first step, elimination to form C=C, what is the main difference between β-oxidation of very long chain FA and β-oxidation of FA?

Answer 24

The β-oxidation of very long chain FA does not generate FADH₂ as electrons are directly passed to oxygen

Question 25

# **Part II : β-oxidation of fatty acids** β-oxidation of very long chain fatty acids (>22C) The last step (cleavage) is catalysed by what enzyme? Does oxidation of the whole 22C long FA occur in the peroxisome?

Answer 25

Peroxisomal thiolase No, the enzyme cannot cleave FA with C8 or less, thus acyl CoAs are converted into carnitine esters and passively diffuse out of peroxisome to mitochondria for complete oxidation

Question 26

# **Part II : β-oxidation of fatty acids** What are the differences between 1. Location 2. Carrier group 3. Electron donor / acceptors in β-oxidation and FA synthesis?

Answer 26

1. Location : β-oxidation occurs into the mitochondria, while FA synthesis occurs in the cytosol 2. Carrier group : main carrier group in β-oxidation is CoA (acyl-CoA), while main carrier group in FA synthesis is ACP (acyl-ACP) 3. Electron donor and acceptor : electron **acceptor** in β-**oxidation** is FAD, NAD⁺ ;; while electron donor in FA synthesis is NADPH

Question 27

# **Part III : Regulation of FA metabolism** What are the 3 main regulation strategies in FA metabolism?

Answer 27

1. Hormonal control 2. Allosteric regulation by metabolic intermediates 3. Compartmentalisation of FA synthesis (cytosol) and β-oxidation (mitochondria)

Question 28

# **Part III : Regulation of FA metabolism** In FA synthesis, what is the main point of regulation?

Answer 28

Main point of regulation is the first committed step : acetyl-CoA + HCO₃^- + ATP → malonyl CoA + ADP + P_i, catalysed by acetyl CoA carboxylase (ACC). Thus, enzymatic activity of ACC is tightly regulated.

Question 29

# **Part III : Regulation of FA metabolism** How is the activity of acetyl CoA carboxylase (ACC) regulated? State the 2 regulatory strategies and all relevant details

Answer 29

1) Allosteric regulation - feedforward activation by citrate (which then breaksdown into acetyl CoA + oxaloacetate) - product inhibition by palmitate (C16 FA) 2) Hormonal regulation a. insulin activates phosphatases, dephosphorylating ACC and thus increasing its activity - FA synthesis increase - TAG packaged into VLDL to be transported to adipocyte - In blood vessel, lipoprotein lipase digests TAG → FFA - FFA diffuses into adipose tissue, and FA → TAG to be stored as fat in adipocytes b. glucagon causes increase in [cAMP] -> increase in protein kinase A (PKA) activity, leading to phosphorylation and deactivation of ACC ## Footnote **ACC : for FA synthesis -> when blood glucose is high and enough energy in body**

Question 30

# **Part III : Regulation of FA metabolism** Which metabolic intermediate regulates both the FA synthesis and β-oxidation of FA? Explain how it does so.

Answer 30

Malonyl CoA. - When [malonyl CoA] is high, it inhibits acetyl CoA carboxylase (ACC), thus inhibiting FA synthesis (high conc of intermediate -> FA not synthesised at such a high rate and lead to accumulation, suggesting that the body has sufficient energy) - When body is low in energy, malonyl CoA allosterically inhibits carnitine acyltransferase I , so that FA in cytosol cannot be transported into the mitochondria for β-oxidation. Thus this metabolic intermediate is diverted to FA synthesis instead.

Question 31

# **Part III : Regulation of FA metabolism** Explain how FA synthesis is activated by hormones when energy state of the cell is low. Also describe the process of mobilisation of fat stores

Answer 31

Glucagon is secreted. - Glucagon increases concentration of cAMP, which increases activity of protein kinase A (PKA) - PKA phosphorylates hormone sensitive lipases within the adipocyte tissues, causing local fat stores in adipocytes to be converted in free fatty acids (TAG → FA by lipase) - Several FA binds to albumin and gets transported out of the adipocyte tissue as albumin complex, which then goes through blood stream and into cytosol of liver, muscle tissue. - FA is activated into fatty acyl CoA and combines with carnitine to form carnitine ester. It diffuses from cytosol of tissue (liver,muscle) into the mitochondria - Fatty acyl CoA regenerated in mitochondria, undergoes β-oxidation