16. Ketogenesis, Phospholipids/sphingolipids, Eicosanoids

Question 1

Part I : ketogenesis

Where does ketogenesis occur / where are ketone bodies synthesised?

Organ and organelle

Answer 1

It occurs in the mitochondria of the liver

Question 2

Part I : ketogenesis

List the 3 step mechanism for ketone body generation, stating the enzymes that catalyse each reaction

Answer 2

1) 2 acetyl CoA → acetoacetyl CoA
- Catalysed by thiolase

2) Acetoacetyl CoA + acetyl CoA → Hydroxymethylglutaryl-CoA (HMG-CoA, 6C)
- Catalysed by HMG-CoA synthase

3) Hydroxymethylglutaryl-CoA → acetoacetate (4C) + acetyl CoA (2C)
- Catalysed by HMG-CoA lyase

Question 3

Part I : ketogenesis

What is the intermediate in the ketogenesis pathway that is also an intermediate in cholesterol synthesis?
Which organelle does cholesterol synthesis occur?

Answer 3

Intermediate : hydroxymethylglutaryl-CoA (HMG-CoA)
Cholesterol synthesis occurs in the cytosol

Question 4

Part I : ketogenesis

Explain why HMG-CoA lyase is only expressed in the mitochondria (of liver), and not in cytosol

Answer 4

HMG-CoA is an important intermediate for cholesterol synthesis in the cytosol. If HMG-CoA lyase was expressed in the cytosol, it would be broken down, thus interfering with cholesterol synthesis

Question 5

Part I : ketogenesis

After acetoacetate are generated in the mitochondria in the liver, explain how it is converted into other ketone bodies and how it is utilised in the mitochondria of other tissues (heart, brain, muscle) for energy.

Answer 5

In the mitochondria of the liver:

1. Acetoacetate (4C) is reduced into β-hydroxybutyrate (4C) by NADH
   - β-hydroxybutyrate is transported to mitochondria of other organs (heart, brain, muscle).
   - β-hydroxybutyrate → acetoacetate → acetoacetyl CoA → 2 acetyl CoA
   - Acetyl CoA enters TCA cycle to generate energy

2) Acetoacetate → acetone through decarboxylation of COO^- group
- transported into mitochondria of brain, heart, muscle cells, and converted back into Acetyl CoA
- Acetyl CoA enters TCA

Question 6

Part I : ketogenesis

What condition can ketone bodies lead to?

Answer 6

Ketoacidosis, where blood pH is low (~5)

Question 7

Part I : ketogenesis

How does prolonged fasting / starvation lead to ketosis (burning ketones for energy)?

Answer 7

• During fasting, gluconeogenesis in the liver (&kidney) occurs to produce glucose to maintain blood glucose levels and for energy production in tissues (esp brain).
• As gluconeogenesis carries on, oxaloacetate is being depleted, acetyl CoA in liver cannot enter TCA cycle
• In addition, fat mobilisation increases as an alternative source of energy, increasing [acetyl-CoA]
• Since there is not enough oxaloacetate, acetyl-CoA cannot enter TCA cycle, leading to ketogenesis and ketosis

Question 8

Part I : ketogenesis

What other metabolic condition leads to ketoacidosis? Explain.

Answer 8

Type I diabetes, where there is insulin resistance.
- There is high blood glucose in blood, but low intracellular glucose since tissues (incl liver) cannot uptake glucose as an energy source.
- The low availability of glucose in liver cells means less pyruvate is produced, leading to reduced oxaloacetate (OAA) levels, and TCA cycle cannot be utilised efficiently.
- At the same time, fats are burned to produce energy, increase [acetyl CoA], leading to accumulation of acetyl CoA
- acetyl CoA cannot enter TCA → ketogenesis → ketoacidosis

Question 9

What are some symptoms of diabetic ketoacidosis? [2]

Answer 9

1. Decrease in blood pH, causing tissues to malfunction (e.g. central nervous system)
2. Acetone breath (sweet and fruity smell)

Question 10

Part II : structural lipids - sphingolipids and phospholipids

How does the structure of membrane lipids (glycerolipids, sphingolipids) differ from that of triacylglycerols?

Answer 10

1) TAG : glycerol backbone + 3 FA
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2) (Glycero) Phospholipids : glycerol backbone + 2 FA + carbohydrate sugar grp / phosphate ester
- If R3 = carbohydrate → glyceroglycolipid ;; if R3 = phosphate ester → glycerophospholipid
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3) Sphingolipid : sphingosine backbone + R1 (built-in FA) + R2 (FA) + R3
- If R3 = carbohydrate → sphingoglycerolipid ;; if R3 = phosphate ester → sphingophospholipid

Question 11

Part II : structural lipids - sphingolipids and phospholipids

In structural lipids (phospholipid, sphingolipid), the polar head group is always on which carbon?

Answer 11

C3

Question 12

Part II : structural lipids - sphingolipids and phospholipids

How are fatty acids esterified to form TAG? State the reactants needed.

Answer 12

Activated fatty-acyl CoA (e.g. palmitoyl CoA, 16C) + glycerol-3-phosphate → TAGs

Question 13

Part II : structural lipids - sphingolipids and phospholipids

Which other pathway’s intermediate does glycerol-3-phosphate originate from?

Answer 13

Dihydroxyacetone phosphate (DHAP) from glycolysis can be reduced by NADH (catalused by glycerol-3-phosphate dehydrogenase) to form glycerol-3-phosphate.

Question 14

Part II : structural lipids - sphingolipids and phospholipids

What are 2 important TAG precursors that are also precursosrs for glycerophospholipids synthesis?

Answer 14

1. Phosphatidic acid
2. Diacylglycerol

Question 15

Part II : structural lipids - sphingolipids and phospholipids

What 2 different pathways can synthesis of glycerophospholipids be formed?

Answer 15

1. Activation of head group (CDP-head group), and diacylglycerol attacks this activated head group to form glycerophospholipid (activate head group first)
2. Activation of diacylglycerol to form CDP- diacylglycerol, and head group attacks this activated body to form glycerophospholipid(activate phospholipid group first)

Question 16

Part II : structural lipids - sphingolipids and phospholipids

Ceramide (N-acyl sphingosine) can be synthesised from sphingosine, and cermamide can be subsequently used as a backbone for formation of other sphingolipids. What is the difference between ceramide and sphingosine?

Answer 16

Sphingosine : only has an in-built acyl group on C1
N-acyl sphingosine (ceramide) : has in-built acyl group on C1 + fatty acyl group on C2 (R2)

Question 18

Part III : Eicosanoids

What are eicosanoids?

Answer 18

Lipid-based signalling hormones which are synthesised from C20 fatty acid known as arachidonic acid

Question 19

Part II : structural lipids - sphingolipids and phospholipids

What 2 initial reactants produces ceramides (N-acyl sphingosine)?

Answer 19

1. Palmitoyl-CoA
2. Serine

Question 20

Part II : structural lipids - sphingolipids and phospholipids

What are the 3 classes of sphingolipids (sphingophospholipids / sphingoglycolipids) synthesised from ceramide + polar head group? What are their functions, what kind of sphingolipids they are and the polar head groups?

Answer 20

1) Sphingomyelins (sphingophospholipids)
- Component of myelin sheaths, which insuate nerve axons in CNS

2) Cerebrosides (sphingoglycolipids) : ceramides with simple sugar residue as the polar head group
- Important structural components of animal muscle and nerve cell membranes

3) Ganglioside (sphingoglycolipids) : ceramides + oligosaccharides containing at least 1 acidic side chain residue as polar head group
- Significant fraction of brain lipids

Question 21

Part II : structural lipids - sphingolipids and phospholipids

Explain how diseases arise in relation to sphingolipid degradation / storage.

Answer 21

• Due to hereditary defects, some people do not have enzymes to degrade sphingolipids.
• Accumulation of substrate lipids causes huge phenotypic consequences which increase over time (more of chronic disease)

Question 22

Part II : structural lipids - sphingolipids and phospholipids

People with sphingolipid storage diseases offer suffer from what kinds of symptoms / outcomes ? [2]

Answer 22

1. Mental retardation
2. Death in infancy (die early on in life)

Question 23

Part II : structural lipids - sphingolipids and phospholipids

What is Tay Sach’s disease?

Answer 23

It is a hereditary defect in enzyme hexoaminidase A.
It leads to :
- accumulation of ganglioside g_m2
- causing shell-like g_m2 inclusions in neuronal cells

Question 24

Part III : Eicosanoids

What is the important precursor for the production of eicosanoids?

Answer 24

Arachidonic acid (C20 fatty acid).

Question 25

# **Part III : Eicosanoids** Arachidonic acid is an important precursor for production of eicosanoids, which are lipid-based signalling hormones. How is arachidonic acid stored in the body, and how is it released for eicosanoid synthesis?

Answer 25

- Arachidonic acid is stored in phospholipids at the C2 position (R2). - Arachidonic acid is released by phospholipase A2

Question 26

# **Part III : Eicosanoids** All eicosanoids are inflammatory in nature. True or False?

Answer 26

False, some are anti-inflmmatory too

Question 27

# **Part III : Eicosanoids** What are the different categories of eicosanoids? State whether there are inflammatory or not.

Answer 27

1. Prostaglandins : inflammatory 2. Leukotrienes : inflammatory 3. Lipoxins : anti-inflammatory

Question 28

# **Part III : Eicosanoids** ____ is a type of prostaglandin and a precursor for other biologically significant molecules.

Answer 28

Prostaglandin H2 (PGH2)

Question 29

# **Part III : Eicosanoids** What is an importnant enzyme that is involved in converting archidonic acid into PGH₂? How many isoforms are there, and which type expressed in response to inflammation stimuli?

Answer 29

Cyclooxygenase (COX). It has 3 isoforms - COX-1 : expressed in most tissues - COX-2 : expressed in response in **inflammation stimuli** - COX-3 : expressed in CNS

Question 30

# **Part III : Eicosanoids** Most prostaglandins derived from PGH2 are inflammatory, thus drugs are designed to target COX, especially COX-2 to counter inflammation. However, some drugs are non-specific COX inhibitors. What undesirable side effects could occur?

Answer 30

These non-specific COX inhibitors not only inhibit COX-2, but also inhibit COX-1 expressed in most tissues. COX-1 enzymes are important in catalysing synthesis of other prostaglandins, which play a role in inhibiting gastric acid secretion. - Thus, when COX-1 is inhibited, prostaglandins synthesis is inhibited, gastric acid is secreted and may cause gastrointestinal ulceration (especially on an empty stomach)

Question 31

# **Part III : Eicosanoids** COX-2 specific inhibitors can reduce inflammation without gastric effects. Howeve, they have been banned. What problem can COX-2 specific inhibitors cause?

Answer 31

Blood clotting and constricting of blood vessels, increasing the risk of cardiovascular disease. ## Footnote By inhibiting COX-2, prostacyclin synthesis is inhibited (important for vasodilation and reducing blood clotting). However, synthesis of thromboxane is not inhibited (important for vasoconstriction and increasing blood blotting. Thus, thromboxane synthesis not inhibited + prostacyclin synthesis inhibited = non-balanced production of both eicosanoids = increased blood clotting and vasoconstriction