Metabolism Session 3 - Energy production from Carbs and Lipids

Question 1

List the catabolic roles of the TCA cycle

Answer 1

The catabolic role is to oxidise the acetyl group (CH3CO-) of acetylCoA to two molecules of carbon dioxide, with the concomitant reduction of NAD & FADH and formation of GTP. The NADH and FADH2 are subsequently oxidised via the respiratory chain to generate ATP by oxidative phosphorylation.

Question 2

List the anabolic roles of the TCA cycle

Answer 2

The anabolic role is to provide intermediates for the synthesis of various important molecules such as haem, fatty acids, glutamate and aspartate

The TCA cycle plays also an important role in the conversion of several glucogenic amino acids to glucose.

Question 3

What three things does the TCA cycle require?

Answer 3

NAD+, FAD and oxaloacetate.

Question 4

What is added to NAD+ and FAD?

Answer 4

H+ and e-

Question 5

Why are there no known defects in the TCA cycle?

Answer 5

They would be lethal

Question 6

How molecules of ATP does the TCA cycle produce?

Answer 6

32

Question 7

What do the C4, C5 and C6 carbon intermediates of the TCA cycle served as intermediates for?

Answer 7

C5 and C4 intermediates used for the synthesis of non-essential amino acids
C4 intermediates used for the synthesis of haem and glucose
C6 intermediates used for the synthesis of fatty acids

Question 8

How is the TCA cycle regulated?

Answer 8

ATP/ADP ratio
NADH/NAD+ ratio

Inhibited by high energy signals, activated by low energy signals

Question 9

Give an example of an enzyme which is rate limiting in the TCA cycle, and explain how it is regulated

Answer 9

Isocitrate dehydrogenase, inhibited by high energy NADH, activated by low energy ADP

Question 10

Give two eventual results of stage 3 catabolism

Answer 10

All C-C bonds have been broken, and C-atoms oxidised to CO2

All C-H bonds have been broken, and H-atoms (H+ and e-) transferred to NAD+ and FAD.

Question 11

Where does the energy from stage 3 catabolism go?

Answer 11

ATP/GTP formation (2 in glycolysis, 2 in the TCA cycle)

Chemical bond energy of the e- in NADH/FAD2H

Question 12

What is important about NADH and FAD2H?

Answer 12

contain high energy electrons that can be transferred to oxygen through a series of carrier molecules, releasing large amounts of free energy.

Question 13

What two things occur in oxidative phosphorylation?

Answer 13

Electron Transport, electrons in NADH and FAD2H are transferrerd through a series of carrier molecules to oxygen, releasing free energy.
ATP synthesis, the free energy released in electron transport drives ATP synthesis from ADP + Pi

Question 14

Describe the key features of electron transport and explain how the proton motive force (p.m.f) is produced

Answer 14

• In electron transport electrons are transferred from NADH (and FAD2H) sequentially through a series of multi-component complexes to molecular oxygen with the release of free energy. Three of them also serve as proton translocating complexes.
• The free energy is used to move protons from the inside to the outside of the inner mitochondrial membrane.
• The membrane itself is impermeable to protons and as electron transport proceeds the proton concentration on the outside of the inner membrane increases.
• The chemical bond energy of the electrons is transformed into an electro-chemical potential difference of protons. This is known as the proton motive force (p.m.f).

Question 15

How much pmf energy is required to generate one molecule of ATP?

Answer 15

+31KJ

Question 16

Where does the energy required to generate ATP from the ATP synthases enzyme come from?

Answer 16

This energy is derived from the pmf that has been produced across the inner mitochondrial membrane by electron transport.
Protons can normally only re-enter the mitochondrial matrix via the ATP synthase complex, driving the synthesis of ATP from ADP and Pi.

Question 17

What is the ratio of NADH to ATP production?

Same for FAD2H

Answer 17

The oxidation of 2 moles of NADH gives 5 moles of ATP

The oxidation of 2 moles of FAD2h gives 3 moles of ATP

Question 18

What does it mean to say ET and ATP synthesis are tightly couple?

Answer 18

One does not occur without the other

Question 19

How ATP concentration being high inhibit further ATP synthesis?

Answer 19

When ATP concentration is high:
The ADP concentration is low and the ATP synthase stops (lack of substrate)
This prevents H+ transport back into the mitochondria
The H+ concentration outside increases to a level that prevents more protons being pumped to the outside
In the absence of proton pumping, electron transport stops

Question 20

What causes uncoupling, and what is it? Why is it important in the body?

Answer 20

Some substances (eg dinitrophenol, dinitrocresol) increase the permeability of the inner mitochondrial membrane to protons. Therefore protons being pumped out by electron transport can re-enter without passing through the ATP synthase complex.

The two processes become uncoupled so the p.m.f. is dissipated as heat.
Proton leak is physiologically important and accounts for 20-25% of the BMR.

Question 21

Describe the roles of UCP-1, -2 and -3

Answer 21

UCP1 - (previously known as thermogenin) is expressed in brown adipose tissue and involved in non-shivering thermogenesis enabling mammals to survive the cold.
UCP2 – Quite widely distributed in the body. Research suggest it is linked to diabetes, obesity, metabolic syndrome and heart failure.
UCP3 – Found in skeletal muscle, brown adipose tissue and the heart. It appears to be involved in modifying fatty acid metabolism and in protecting against ROS damage.

Question 22

How does noradrenaline increase uncoupling?

Answer 22

stimulates lipolysis releasing fatty acids to provide fuel for oxidation in brown adipose tissue. NADH and FAD2H are formed as a result of -oxidation of the fatty acids. NADH and FAD2H drive ET and increase p.m.f. However, noradrenaline also activates UCP1, allowing protons to cross the inner mitochondrial membrane without passing through the ATP synthase complex. The higher p.m.f. is dissipated as heat.

Question 23

Give four key features of oxidative phosphorylation IMPORTANT KEEP ON YELLOW LEVEL MAX

Answer 23

Requires membrane associated complexes
(inner mitochondrial membrane)
Energy coupling occurs indirectly through generation and subsequent utilisation of a proton gradient (p.m.f.)
Cannot occur in the absence of oxygen
Major process for ATP synthesis in cells that require large amounts of energy

Question 24

Give four key features of substrate level phosphorylation IMPORTANT KEEP ON YELLOW LEVEL MAX

Answer 24

Requires soluble enzymes.
(Cytoplasmic and mitochondrial matrix)
Energy coupling occurs directly through formation of a high energy of hydrolysis bond (phosphoryl-group transfer)
Can occur to a limited extent in absence of oxygen
Minor process for ATP synthesis in cells that require large amounts of energy

Question 25

Give four fatty acids derivatives and their functions

Answer 25

Fatty Acids – Fuel molecules
Triacylglycerols – Fuel storage and insulation
Phospholipids – Components of membranes and plasma lipoproteins
Eicosanoids – Local mediators

Question 26

Give four hydroxymethylglutaric acid derivatives and outline their roles

Answer 26

Ketone bodies (C4) – Water soluble fuel molecules
Cholesterol (C27) – Membranes and steroid hormone synthesis
Cholesterol esters – Cholesterol storage
Bile acids and salts (C24) – Lipid Digestion

Question 27

Name the four fat soluble vitamins

Answer 27

ADEK

Question 28

How are tags digested, and what do they release?

Answer 28

Pancreatic lipases in the small intestine, and they release glycerol and fatty acids.
Requires BILE SALTS and a protein factors called COLIPASE

Question 29

How is glycerol metabolised in the liver?

Answer 29

Phosphorylated by glycerol kinase, and then either enters TAG synthesis or become DHAP, entering glycolysis in process.

Question 30

Why are fatty acids excellent energy stores?

Answer 30

hydrophobic and highly reduced molecules

Question 31

Give an important dietary polyunsaturated fat, and explain why it is essential in the diet

Answer 31

Arachidonic acid is the starting point for the synthesis of the eicosanoids. Cannot be synthesised in the body.

Question 32

How are fatty acids metabolised?

Answer 32

Via beta oxidation

Question 33

How are fatty acids activated?

Answer 33

Linked to CoA via S-atom linkage

Question 34

How do fatty acids enter mitochondrion

Answer 34

Special transport system that ultilises carnitine to shuttle the fatty acid across the inner mitochondrial membrane.

Question 35

What inhibits transport of fatty acids into mitochondrion, and why?

Answer 35

Malonyl CoA - by product of fatty acid synthesis. Prevents fatty acids newly synthesised in cytoplasm from being oxidised in mitochondria.

Question 36

How does Beta oxidation proceed?

Answer 36

Removal of 2 carbons from fatty acid at a time (acetate) to form acetyl CoA

Question 37

Name three ketones produced in body

Answer 37

Acetoacetate – CH3COCH2COO-
Acetone – CH3COCH3 (Spontaneous non-enzymatic decarboxylation of above)
-hydroxybutyrate – CH3CHOHCH2COO-

Question 38

When does ketone synthesis occur?

Answer 38

D Glucose U Glucagon U Lyase U Ketones and vice versa

Question 39

What two things are required for ketone bodies to be produced?

Answer 39

Fatty acids to be available for oxidation in the liver following excessive lipolysis in adipose tissue – this supplies the substrate.
The plasma insulin/glucagon ratio to be low, usually due to a fall in plasma insulin – this activates the lyase and inhibits the reductase.

Question 40

Name enzyme for each arrow

Acetyle coA –> hydroxymethylglutaryl coA –> Ketone

Answer 40

Synthase andLyase

Question 41

What enzyme is responsible for conversion of hydroxmethylglutarl coA to cholesterol?

Answer 41

Reductase

Question 42

How is acetyle coA produced?

Answer 42

by the catabolism of:
Fatty Acids
Sugars
Alcohol
Certain amino acids

Question 43

What is acetyl coA an important intermediate in?

Answer 43

lipid biosynthesis