14. Digestion, absorption and transport of dietary fat + De novo biosynthesis of fatty acids

Question 1

Part I : Digestion, absorption and transport of dietary fats

Give examples of different types of lipids [5]

Answer 1

1. Triacylglyceride / fatty acids (constitute 90%)
2. Glycerophospholipids / sphingolipids – structural lipids part of cell membrane
3. Steroids : cholesterol, bile acids, steroid hormones
4. Eicosanoids : lipid-based signalling molecules
5. Fat-solbule vitamins (lipid derivatives)

Question 2

Part I : Digestion, absorption and transport of dietary fats

Describe the role of bile salts in lipid digestion.

Answer 2

Bile salts emulsify big fat globules into tiny fat droplets (physical digestion), forming micelles and increasing the surface area for lipase to digest TAGs

Question 3

Part I : Digestion, absorption and transport of dietary fats

Name the 3 parts of the (pancreatic) lipase enzyme.

Answer 3

1. Co-lipase (coenzyme)
2. Active site of lipase
3. Lid

Question 4

Part I : Digestion, absorption and transport of dietary fats

Explain the mechanism on how pancreatic lipase is activated to digest triacylglycerides (TAG)

Answer 4

As lipase comes to the lipid-water interface, co-lipase binds to the lid that covers the active site of lipase. Thus, with the active site now exposed, lipase can digest TAGs within the emulsified fat globules (micelles).

Question 5

Part I : Digestion, absorption and transport of dietary fats

Describe the composition of mixed micelles in lipid digestion. [5]

Answer 5

Mixed micelles contain **products of lipid digestion **
- Monoacylglycerol
- Diacylglycerol
- Fatty acids
- Cholesterol
- Bile acids

Question 6

Part I : Digestion, absorption and transport of dietary fats

Lipases are active in aqueous medium. True or False?

Answer 6

False. Lipases are inactive in aq medium as the lid binds to the active site, preventing reactions from occuring.
- lipases are have hydrophobic portions and thus only work well at the oil-water interface

Question 7

Part I : Digestion, absorption and transport of dietary fats

Explain how lipids are absorbed into epithelial cells of small intestine.

Answer 7

When micelles reach the brush border membrane of cells, the products of digestion (fatty acid, monoacylglycerol, diacylglycerol, cholesterol → hydrophobic) are diffuse through the epithelial cell walls as mixed micelles

Question 8

Part I : Digestion, absorption and transport of dietary fats

What happens after monoacylglycerol / diacylglycerol / fatty acids / cholesterol diffuses through epithelial cells and are inside the epithelial cells?

Answer 8

• Monoacylglycerol / diacylglycerol and fatty acids combine back to form TAGs and are packaged into lipoproteins called chylomicrons
• FAs that do not recombine to form TAGs bind to fatty acid binding proteins (FABP) in intestine cells → solubility of FAs increases + protect epithelial cells from FAs (cause inflammation and damage cells)

Question 9

Part I : Digestion, absorption and transport of dietary fats

What are lipoproteins made of?

Answer 9

Lipoproteins are made of :
1. Hydrophilic outer coating → phospholipids (single layer) + apolipoproteins
2. Hydrophobic core containing lipids → TAGs + cholesterol

Note : Monoacylglycerol (MAG) and diacylglycerol (DAG) are NOT typically found in lipoproteins—they are intermediates in TAG metabolism inside cells.

Question 10

Part I : Digestion, absorption and transport of dietary fats

What are the 5 different types of lipoproteins, and explain their densities and particle size.

Answer 10

1. Chylomicrons
2. VLDL → very low density lipoproteins
3. IDL → intermediate density lipoproteins
4. LDL → low density lipoproteins
5. HDL → high density lipoproteins

From chylomicrons to HDL, density increases and particle size decreases.

Question 11

Part I : Digestion, absorption and transport of dietary fats

State the functions of the 5 different types of lipoproteins.

Answer 11

1. Chylomicrons → transport dietary (exogenous) TAG and cholesterol from intestine to tissues and liver
2. VLDL → transport synthesised (endogenous) cholesterol / TAGs from liver to tissues. As TAGs are delivered to tissues, VLDL becomes IDL and LDL (as TAGs get digested, gets released as FA and uptaken by muscle and adipose cell)
3. IDL
4. LDL
5. HDL → transport remaining / excess cholesterol from tissues to liver

Question 12

Part I : Digestion, absorption and transport of dietary fats

Out of all lipoproteins, HDL has the highest density because it has the highest proportion of lipids (TAGs, cholesterol etc). True or False?

Answer 12

False.
- All lipoproteins have the same outer coating (phospholipid + protein), which is higher in density than lipids.
- In HDL, since density is high, it means that the proportion of TAGs is the low ;; proportion of outer coating is high

Thus, HDL has the lowest proportion of lipids → have less lipids (lower density) → can pack closer → higher density and smaller particle size

Question 13

Part I : Digestion, absorption and transport of dietary fats

After chylomicrons are formed in intestinal epithelial cells, they are absorbed into blood capillaries for transport to other parts of the body. What happens to chylomicrons in the blood capillaries?

Answer 13

Chylomicrons undergo de-lipidation, where
- Lipases digest TAGs inside the chylomicrons into FA. Cholesterol remains undigested.

• FA are uptaken by muscle cells or adipose tissues (FA used for energy metabolism in muscle ;; and FA stored as TAG in adipose tissue)
• Chylomicron remnants, which are cholesterol-rich, are transported back to liver.

Question 14

Part I : Digestion, absorption and transport of dietary fats

State the main routes on how lipids are transported in the body (between liver and other tissues)

Start from absorption into epithelial cells

Answer 14

1. After absorption into epithelial cells, chylomicrons containing dietary TAGs / cholesterol are formed, which are then absorbed into the bloodstream.
2. In blood bessels, chylomicrons are de-lipidated, where TAGs are digested by lipoprotein lipases into FAs. FAs are absorbed by muscle tissues (for energy) and adipose tissues (for storage)
3. Chylomicron remnants which are cholesterol-rich are transported back to liver
4. Cholesterol from chylomicron remnants, as well as biosynthesised cholesterol + TAGs are packaged into lipoproteins known as VLDLs, which are then transported from liver to extrahepatic cells (cells outside of liver)
5. TAGs in VLDL can be digested into lipases by lipoprotein lipase, releasing FA for uptake by muscle and adipose tissue. VLDL is degraded into IDL → LDL
6. LDL is uptaken by extrahepatic cells (excl muscle and adipose), and TAGs and cholesterol are uptaken from LDL → forming HDL
7. HDL scavenges for excess cholesterol from cell surfaces and transport remaining TAG / cholesterol from cells back to liver.

Question 15

Part I : Digestion, absorption and transport of dietary fats

How is LDL uptaken extrahepatic cells? (mechanism, and explain briefly on how it works)

Answer 15

Receptor-mediated endocytosis.
- LDL particles bind to surface cell receptors, cluster and bud inwards to from vesicles

Question 16

Part I : Digestion, absorption and transport of dietary fats

Is HDL uptaken by liver cells via the same mechanism as LDL (receptor mediated endocytosis)? If not, how is HDL uptaken into liver cells

Answer 16

No.
HDL binds to SR-BI (Scavenger recpetor class B type I) receptor on liver cells, and contents within HDL (remaining cholesterol and TAGs) are directly released into the liver.

Question 17

Part I : Digestion, absorption and transport of dietary fats

Orlistat is a drug to treat obesity. How does it work and what is its side effect?

Answer 17

Orlistat inhibits gastric and pancreatic lipase through competitive inhibition. Thus, TAGs cannot be digested by lipases and fats are not absorbed by the body, reducing calorie intake.

• Side effect : oily loose stools, as TAGs are excreted wihtout being chemically changed (not digested or anything)

Question 18

Part 2 : De novo synthesis of fatty acids

What is the main precursor and the main donor for fatty acid synthesis?

Answer 18

Main precursor : acetyl CoA
Main donor : malonyl CoA

Question 19

Part 2 : De novo synthesis of fatty acids

Where does de novo synthesis of fatty acids occur? (organ, organelle)

Answer 19

Organ : Liver
Organelle : cytosol

Question 20

Part 2 : De novo synthesis of fatty acids

Acetyl CoA is mainly present in the mitochondria. Thus, how is it shuttled out and regenerated in the cytosol for FA synthesis ?

Answer 20

• Pyruvate → oxaloacetate by pyruvate carboxylase
• Acetyl-CoA combines with oxaloacetate to form citrate
• Citrate shuttles through the tricarboxylate transport system into the cytosol

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In cytosol:
- Citrate → oxaloacetate + acetyl CoA (consumption of ATP to break bond)
- Oxaloacetate → malate → pyruvate, which then shuttles through inner mitochondrial membrane through a transport protein back into the mitochondria

Question 21

Part 2 : De novo synthesis of fatty acids

When acetyl CoA is regenerated in the cytosol for use in FA synthesis, oxaloacetate → malate → pyruvate. Why can’t malate directly go through the malate shuttle, and gets converted into pyruvate first?

Answer 21

The conversion of malate → pyruvate generates NADPH, which is needed in FA synthesis

Question 22

Part 2 : De novo synthesis of fatty acids

What are the 2 key enzymes in FA synthesis?

Answer 22

1. Acetyl-CoA carboxylase
2. FA synthase

Question 23

Part 2 : De novo synthesis of fatty acids

What is the first committed step in FA synthesis and what enzyme catalyses it? List out reaction equation.

Answer 23

First committed step : Acetyl-CoA + HCO₃- + ATP → Malonyl-CoA + ADP + P_i

Question 24

Part 2 : De novo synthesis of fatty acids

How does Acetyl CoA carboxylase add a C atom to acetyl CoA to form malonyl CoA? (What coenzyme is responsible for this?)

Answer 24

Biotin (vit B7) is responsible for donating C atom in the form of HCO₃- to be added to acetyl CoA to form malonyl CoA

Question 25

# **Part 2 : De novo synthesis of fatty acids** Fatty acid synthase is a multifunctional enzyme with ___ domains and thus, ___ catalytic sites. FA synthase can synthesise one FA at a time. True or False?

Answer 25

7 False, FA synthase is a dimer

Question 26

# **Part 2 : De novo synthesis of fatty acids** List out the general outline for FA synthesis. (For reduction / dehydration reactions, state electron carriers produced, if any)

Answer 26

1) Fatty acid synthase (FAS) is primied by loading ACP (acetyl carrier protein) with acetyl-CoA 2) Acetyl group is transferred to cysteine residue in KS domain ;; second priming by loading ACP with malonyl-CoA 3) ACP is flexible and moves towards KS, acetyl CoA + malonyl CoA → acetoacetyl-CoA (4C) on ACP - COO^- group on malonyl CoA is lost as CO₂ 4) Reduction of acetoacetyl-ACP (C=O) → (-OH) group to form D-β-hydroxybutyrl-ACP ;; with oxidation of NADPH to NADP⁺ 5) Elimination reaction : C(OH)H - CH2 to C=C 6) Reduction reaction : C=C to CH₂-CH₂ ;; with oxidation of NADPH to NADP⁺ 7) Butyrl group is transferred to cysteine in KS, new malonyl-CoA added to ACP protein *Process repeats till a palmitoyl-ACP is synthesized* (Palmitate, 16C carbon)

Question 27

# **Part 2 : De novo synthesis of fatty acids** How many rounds of addition reactions are needed to synthesise palmitate (16C) in the cytosol?

Answer 27

After round 1 : 2+2 = 4C After round 2 : 2+2(2) = 6C After round 3 : 2+3(2) = 8C No of rounds = (16-2)/2 = 7

Question 28

# **Part 2 : De novo synthesis of fatty acids** The cytosol can only produce FA up to 16C long (palmitate). To produce longer FA (18C or more), which organelles could it be transported to?

Answer 28

Mitochondria / smooth ER

Question 29

# **Part 2 : De novo synthesis of fatty acids** What is the difference between the elongation of FA chain in the cytosol and in the ER?

Answer 29

Elongation of FA in smooth ER is similar to elongation by FA synthase in cytosol (react with malonyl CoA), just that FA-CoA instead of FA-ACP is the derivative form of the FA.

Question 30

# **Part 2 : De novo synthesis of fatty acids** What enzymes in mamallian body help to form unsaturated FA? (state the position of desaturation too)

Answer 30

4-/5-/6-/9- fatty acyl-CoA desaturases

Question 31

# **Part 2 : De novo synthesis of fatty acids** What kinds of desaturases do plants have?

Answer 31

12- and 15- desaturases

Question 32

# **Part 2 : De novo synthesis of fatty acids** How are triacylglycerides (TAGs) synthesized from de novo synthesised FA?

Answer 32

De novo synthesised FA, such as palmtate (C16), is activated into palmitate acyl CoA (fatty acyl CoA) Fatty acyl CoA forms ester bonds with glycerol-3-phosphate to form TAGs