Lipid Synthesis and Degradation

Question 1

How are fats found in the body?

Answer 1

Fats are either:
• Obtained from the diet.
• Made de novo (made anew) from carbohydrates.

Question 2

What 4 essential roles do fats play in the body?

Answer 2

1. A role in membranes
2. Uptake of lipid soluble vitamins
3. As precursors of steroid hormones
4. Energy store

Question 3

Why is fat such an important store of energy?

Answer 3

The energy content of fat per gram is over twice that of either carbohydrates or proteins, making an important energy source

Question 4

When is the synthesis of fat triggered?

Answer 4

• When our caloric intake exceeds that of consumption, the excess is laid down as fat.
• Some tissues, such as cardiac muscle, use fats as their preferred energy source.
• However, dietary carbohydrate is the most common source, although amino acids can also be used

Question 5

Where are fats stored and where are they synthesised?

Answer 5

• Fats are stored in adipose tissue

* But majority are synthesised in the liver.

Question 6

What are the molecules we should consider when discussing lipid metabolism?

Answer 6

1. Fatty acids
2. Triglycerides or Neutral Fats
3. Cholesterol

Question 7

Describe the structure of fatty acids.

Answer 7

• hey are chains of methyl groups, with a terminal carboxyl group.
• If double bonds are present, it is in a cis formation.

Question 8

Can humans make double bond positions in fatty acid?

Answer 8

Yes, BUT: Humans are unable to make double bonds at positions less than position 9.
• That’s why there are ‘essential’ fatty acids that we have to obtain from our diets, as they cannot be made in our bodies.
• Palmitic acid (C16) makes up the majority of fatty acids made. – Enzymes can modify this into other fatty acids. E.g. enzymes can add of remove carbons or C=C.

Question 9

Where does fatty acid synthesis take place?

Answer 9

Fatty acid synthesis takes place in the cytosol of hepatocytes and it requires:
• Acetyl-CoA
• NADPH – majority from pentose phosphate pathway.
• ATP
It (FA synthesis) involves the sequential addition of 2 carbon units derived from Acetyl-CoA.

Question 10

Describe the citrate-malate antiport system and why it is needed.

Answer 10

1. Pyruvate is transported from the cytosol to the inside of the mitochondrion.
2. There, it is converted to Oxaloacetate (by pyruvate carboxylase), where Acetyl CoA is added to it to create Citrate.
3. The Citrate is brought out of the mitochondrion back into the cytosol, an Acetyl CoA molecule is released from it, turning it back into Oxaloacetate.
4. Oxaloacetate is converted to Malate w/addition of NADH
5. Malate is converted back to Pyruvate by the removal of NADPH.
   o This process provides 40% of NADPH so additional 60% NADPH is provided by the Pentose Phosphate Pathway.
   o This is done to transport Acetyl CoA outside of the mitochondrion as it can’t pass through the membrane.

Question 11

Write up the equation in the first step of fatty acid synthesis.

Answer 11

Acetyl-CoA + ATP + HCO3- ——-> Malonyl-CoA + ADP + Pi

Question 12

Describe the first step of fatty acid synthesis

Answer 12

Acetyl-CoA + ATP + HCO3- ——-> Malonyl-CoA + ADP + Pi
• It is where Acetyl-CoA (2C) has a carbon molecule added to it by HCO3-, with the help of ATP, changing it to Malonyl-CoA (3C).
o This is done by the enzyme Acetyl-CoA carboxylase.
• It is an important, irreversible, regulatory step activated by Citrate (positive feedforward) and inhibited by Palmitic Acid (negative feedback).
• It also requires the vitamin Biotin (vitamin B7).

Question 13

What inhibits Acetyl-CoA carboxylase, and what increases/ decreases the expression of Acetyl-CoA?

Answer 13

• The enzymes is inhibited by phosphorylation – glucagon stimulates phosphorylation and therefore inhibits the enzymes.
• Expression of Acetyl-CoA (2C) carboxylase is increased by high carbohydrate and low fat.
• Expression of Acetyl-CoA carboxylase is decreased by low carbohydrate and high fat.

Question 14

Describe the second step of fatty acid synthesis.

Answer 14

The second part would be fatty acid elongation.
1. The malonyl residue from Malonyl-CoA is transferred to the Acyl Carrier Protein (ACP) part of the multienzyme complex of fatty acid synthase (exists as a dimer).
2. A second acetyl molecule from Acetyl CoA is then transferred to ACP where the two condense to form Acetoacetyl-ACP (C4). (condensation involves decarboxylation, release of CO2).
3. The Acetoacetyl-ACP is then reduced and dehydrated to Butyryl-ACP (this requires 2 NADPH molecules). It then combines with another Malonyl-ACP molecule to form the final 6C molecule (and a CO2 molecule).
• The intermediates in this reaction are covalently linked to ACP. All enzymes required form a multi-functional complex called Fatty Acid Synthase. This exists as a dimer.
• (Product binding to fatty acid synthase – passed from one active site to another. The end result is the addition of 2Cs and passed to start of adjacent enzyme).

Question 15

Describe the structure and functions of cholesterol

Answer 15

• It is a rigid, hydrophobic molecule that is virtually insoluble in water.
• It is an important membrane component, and a precursor of:
1. Sterols
2. Steroids
3. Bile salts.
• It is transported in the circulation as cholesteryl esters.
• It can’t be oxidised to O2 or H2O, so it provides no energy.
• Cholesterol imbalance can lead to significant health issues, such as gallstones and atherosclerosis.

Question 16

Briefly, describe cholesterol synthesis.

Answer 16

• Cholesterol is synthesised in the ER (with over 30 steps involved).
• It starts with the activation of Acetate, (made from?) Acetyl-CoA.
• The major regulatory step is the conversion of 3-hydroxyl-3-methyl-glutaryl CoA (HMG-CoA) to Mevalonate.
• Cholesterol inhibits HMG-CoA reductase, an enzyme involved in its own synthesis.
• It’s difficult to remove circulating cholesterol by diet alone as endogenous synthesis is increased.

Question 17

Describe the structure and functions of cholesterol

Answer 17

• It is a rigid, hydrophobic molecule that is virtually insoluble in water.
• It is an important membrane component, and a precursor of:
1. Sterols
2. Steroids
3. Bile salts.
• It is transported in the circulation as cholesteryl esters.
• It can’t be oxidised to O2 or H2O, so it provides no energy.
• Cholesterol imbalance can lead to significant health issues, such as gallstones and atherosclerosis.

Question 18

Briefly, describe cholesterol synthesis.

Answer 18

• Cholesterol is synthesised in the ER (with over 30 steps involved).
• It starts with the activation of Acetate, (made from?) Acetyl-CoA.
• The major regulatory step is the conversion of 3-hydroxyl-3-methyl-glutaryl CoA (HMG-CoA) to Mevalonate.
• Cholesterol inhibits HMG-CoA reductase, an enzyme involved in its own synthesis.
• It’s difficult to remove circulating cholesterol by diet alone as endogenous synthesis is increased.

Question 19

What are the three steps of Fatty Acid Degradation?

Answer 19

Release of energy from reserves stored in adipose tissues.

1. Mobilisation – in adipocytes
2. Activation – in liver cystol
3. Degradation – in liver mitochondria

Question 20

Describe Fatty Acid Mobilisation

Answer 20

1. G-protein coupled receptor is activated, stimulating adenylate cyclase to produce cAMP from ATP.
2. The cAMP then activates protein kinase.
3. This Protein Kinase phosphorylates the enzyme Triacylglycerol Lipase(only in adipocytes).
4. This converts Triacylglycerol to Diacylglycerol. (by removal of a Fatty acid)
5. The Diacylglycerol is then broken down into Glycerol and Fatty Acids by other lipases.
6. The FFA can pass the membrane to travel to the liver, or it can be converted to glycerol.

Question 21

What becomes of the glycerol mobilised in Fatty Acid Mobilisation?

Answer 21

It is absorbed by the liver. In there, it follows a series of steps to convert it to GAP (Glyceraldehyde-3-phosphate):
1. Glycerol is phosphorylated to Glycerol-3-Phosphate.
2. Glycerol-3-Phosphate is oxidised to Dihydroxyacetone Phosphate.
3. Dihydroxyacetone Phosphate is isomerised to Glyceraldehyde-3-Phosphate (GAP).
The majority of GAP goes towards Gluconeogenesis, and a bit of it goes towards Glycolysis.

Question 22

What is Fatty Acid Activation

Answer 22

• It is when fatty acids are transported to the liver and activated by Acyl-CoA Synthase in the cytoplasm. – allows it to enter the mitochondria.
• The Acyl-CoA produced is transported across the inner mitochondrial membrane bound to the alcohol Carnitine

Question 23

Describe Fatty Acid Activation.

Answer 23

• Happens in the liver.
1. In the cytoplasm, the Acyl-CoA will react with carnitine to give Acyl-Carnitine, which can then be transported across the membrane, in to the mitochondrial matrix for oxidation, through the action of the enzyme Translocase.
2. The Acyl-Carnitine is then broken down back into Carnitine, releasing the Acyl to combine with the CoA, remaking Acyl-CoA in the mitochondria. This allows the Carnitine molecule to get across the membrane.
• Carnitine deficiency can cause muscle weakness or even death.
• The transport is also inhibited by Malonyl-CoA (present when there is XS glucose), which is a step in the synthesis of fatty acids.
• So if it builds up, we will be moving towards synthesis, meaning this degradation transport process is inhibited.
• (reciprocal regulation – if one process happens the other cannot happen.)

Question 24

Describe Fatty Acid beta-Oxidation, and what happens to its products.

Answer 24

• It occurs in the mitochondria of the liver.
• It is when the Acyl-CoA is degraded by sequential removal of two carbon units.
• Acyl-CoA is oxidised, hydrated and goes under thiolysis. This produces FADH2, NADH, Acetyl-CoA and Acyl-CoA (-2C)
• The FADH2 and NADH form ATP.
• The Acetyl-CoA (from β-oxidation) will enter the citric acid cycle, but only in the presence of glycolysis.
• Odd chain lengths will yield Propionyl-CoA in the last round of oxidation.
• Even and odd double bond chains are removed by reductase and isomerase (odd by isomerase, even by both)
• In the liver, the main production of fatty acid oxidation is for generation of Acetyl-CoA which can be fed into citric acid cycle to generate energy, but it doesn’t tend to be fed this way.
• Rather, ACoA tends to be converted to ketone bodies.

Question 25

What are the three ketone bodies produced?

Answer 25

1. Acetoacetate
2. 3-β-Hydroxybutyrate
3. Acetone

Question 26

Describe Ketogenesis.

Answer 26

1. Acetyl-CoA from breakdown of fatty acids is converted to Acetoacetyl-CoA
2. Acetoacetyl-CoA is converted to HMG-CoA
3. HMG-CoA is converted to Acetoacetate
   o Acetoacetate can be reduced to 3-β-Hydroxybutyrate or non-enzymatically to Acetone.
   These are the three ketone bodies and the two former ones are the important ones (esp. first).

Question 27

How are ketone bodies regulated?

Answer 27

• Synthesis of ketone bodies are regulated by the insulin/glucagon ratio
• Ketogenesis is high when the ratio is low insulin: glucagon, as this inhibits Acetyl-CoA carboxylase (rate limiting step in FA synthesis).

Question 28

What is the fate of ketone bodies?

Answer 28

The acetoacetate is converted in the non-hepatic tissue back to Acetyl-CoA – this can then be used to generate ATP. This happens by a 2 step process:
1. Acetoacetate is converted to Acetoacetyl CoA – by CoA transferase and succinyl CoA is converted to succinate.
2. Acetoacetyl CoA is converted to 2 Acetyl CoA – by combing with CoA and with the help of the enzyme thiolase.
• Ketone bodies are a major energy source for cardiac muscle and renal cortex. – dependent on the flow of carbohydrate in glycolysis.
• During starvation or diabetes, 75% of the brains energy is derived from acetoacetate – the production of ketones increases with decreased food intake.

Question 29

How does Insulin regulate metabolism?

Answer 29

1. increases glycolysis in the liver
2. increases fatty acid synthesis in the liver
3. increases TG in adipose tissue
4. decreases β-oxidation