Carbohydrates 3 & Lipid energy production

Question 1

Describe the roles of the tricarboxylic acid cycle (TCA cycle) in metabolism

Answer 1

The TCA cycle is a central pathway in the catabolism of sugars, fatty acids, ketone bodies, alcohol and amino acids. Occurs in mitochondria and requires NAD+, FAD and oxaloacetate. Main function is to break C-C bond in acetate and to oxidise C atom to CO2. Produces NADH, GTP (ATP), FADH2 and CO2.
The anabolic role is to provide intermediates for the synthesis of various important molecules such as haem, fatty acids, glutamate and aspartate.

Question 2

How is the TCA cycle regulated

Answer 2

The oxidation of acetyl CoA linked to the reduction of NAD+ and FAD by the TCA cycle is essential for the generation of ATP in all tissues containing mitochondria. The rate of ATP utilisation is therefore the primary factor in driving the TCA cycle. Two major signals feed information on the rate of utilisation of ATP to the TCA cycle:
-ATP/ADP ratio
-NADH/NAD+ ratio
One of the irreversible steps of the TCA cycle (catalysed by isocitrate dehydrogenase), is allosterically inhibited by the high-energy signal NADH and activated by the low-energy signal ADP).

Question 3

Describe the key features of oxidative phosphorylation

Answer 3

Oxidative phosphorylation is how ATP is actually produced. By the end of stage 3 of catabolism, all the C-C bonds have been broken and the C- atoms oxidised to CO2, and all the C-H bonds have been broken and the H-atoms transferred to NAD+ and FAD
NADH and FAD2H contain high energy electrons that can be transferred to oxygen through a series of carrier molecules with the release of large amounts of energy. This occurs in two processes:
-Electron transport
-ATP synthesis

Question 4

Explain the processes of electron transport and ATP synthesis and how they are coupled

Answer 4

Electron transport - Electrons in NADH and FAD2H are transferred through a series of carrier molecules to oxygen with the stepwise release of free energy. This process therefore requires oxygen (final electron acceptor).
ATP synthesis - The free energy released in electron transport is used to drive ATP synthesis from ADP + pi. Energy causes H ions to be transported across the membrane to the intermembral space. As H ions go back, they go through ATP synthase

Question 5

Describe how, when and why uncoupling of these processes occurs in some tissues.

Answer 5

Usually, Electron transport and ATP synthesis are tightly coupled and controlled such that one does not occur without the other.
Some substances, (e.g., dinitrophenol) increase the permeability of the inner mitochondrial membrane to protons. This enables protons pumped out of the mitochondria by electron transport to re-enter the matrix without driving ATP synthesis, i.e., the two processes are uncoupled. Under these conditions, ET continues, ATP does not occur, and an excessive amount of heat is generated

Question 6

What are the features of lipids

Answer 6

Lipids are:
-structurally diverse
- generally insoluble in water (hydrophobic),
-most only contain C H O (phospholipids contain P and N).
-They are more reduced than carbohydrates, so they release more energy when oxidised and complete oxidation requires more O2

Question 7

Describe the various classes of lipids

Answer 7

1) Fatty acid derivatives
Faty acids - Fuel molecules
Triglycerols (triglycerides) - Fuel storage and insulation
Phospholipids - Components of membranes and plasma lipoproteins
Elcosanoids - Local mediators
2) Hydroxy-methyl-glutaric acid derivatives (C6 compound):
Ketone bodies(C4) - water soluble molecules
Cholesterol (C27) - Membranes and steroid hormone synthesis
Cholesterol esters -cholesterol storage
Bile acids and salts (C24) - Lipid digestion
3)Vitamins A, D, E and K

Question 8

What is the structure of triacylglycerols (TAG)

Answer 8

-Triacylglycerols are hydrophobic
-Stores in an anhydrous form
-Stored in specialised tissue - adipose tissue
-Utilised in prolonged exercise, ‘starvation’ during pregnancy
-Storage/mobilisation under hormonal control

Question 9

Describe how dietary triacylglycerols are processed to produce energy (stage 1)

Answer 9

Stage 1) Metabolism of triacylglycerols
Triacylglycerols are hydrolysed by pancreatic lipase in the small intestine to release glycerol and fatty acids. This requires bile salts and colipase (protein factor).
-Glycerol enters the blood stream and transported to the liver to be metabolised.
-Fatty acids are most commonly hydrophobic and highly reduced molecules making them great for storage

Question 10

Describe how dietary triacylglycerols are processed to produce energy (stage 2)

Answer 10

Stage 2) Catabolism of fatty acids
When the body is subjected to stress situations triacylglycerols are hydrolysed by lipase to release fatty acids and glycerol during lipolysis. the fatty acids are carried to tissues via the blood stream. The glycerol is transported to the liver where it can be oxidised, converted to glucose or use in the synthesis of triacylglycerols. Fatty acids can be used as a source of energy. The process where fatty acids are oxidised to release energy is known as beta- oxidation and occurs in the mitochondria. This means that cells that do not contain mitochondria such as red blood cells cannot oxidise fatty acids

Question 11

Explain how, when and why ketone bodies are formed.

Answer 11

The liver constantly generates ketone bodies from acetyl CoA such as:
-Acetoacetate (CH3COCH2COO-)
-Acetone (CH3COCH3)
-Beta hydroxybutyrate (CH3CHOHCH2COO-)
Ketone bodies are synthesised in liver mitochondria by the actions of lyase and reductase enzymes that are reciprocally controlled by the insulin/glucagon ratio. When the ratio falls, the lyase is activated and the reductase is inhibited, and ketone body formation is activated. When the ratio increases, the lyase is inhibited and the reductase is activated, and cholesterol synthesis occurs.

Question 12

What does the synthesis of ketone bodies require

Answer 12

-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 activatesn the lyase and inhibits the reductase
The rate of utilisation is proportional to the plasma concentration. They are converted to acetyl CoA and this is oxidised via stage 3 of catabolism

Question 13

Describe the central role of acetyl CoA in metabolism

Answer 13

Acetyl CoA is produced by the catabolism of fatty acids, sugars, alcohol and certain amino acids and can be oxidised via stage 3 of catabolism. It is an important intermediate in lipid biosynthesis.

Question 14

What is glucose 6-phosphate dehydrogenase and why is the enzyme important to cellular function

Answer 14

It is a rate limiting enzyme and an important source of NADPH, which is required for the reducing power for biosynthesis and maintenance of GSH levels. It is also useful for detoxification reactions.

Question 15

Why does an absence of glucose 6-phosphate dehydrogenase cause sensitivity to primaquine

Answer 15

An absence means that there is reduce NADPH which means that there are low levels of reduced glutathione, so they are more susceptible to oxidative damage. Primaquine increases the levels of oxidative stress which is usually maintained due to NADPH. Due to the G6PD deficiency, this cannot occur

Question 16

How are Heinz bodies produced

Answer 16

Haemoglobin becomes cross linked by disulphide bonds due to the oxidative damage. These form insoluble aggregates called Heinz bodies, and this occurs because of the G6PD deficiency

Question 17

How is the relationship between electron transport and ATP synthesis altered during thermogenesis

Answer 17

The inner membrane of brown adipose tissue has in addition to the ATP synthase complex, a special proton conductance protein (thermogenin), that allows the controlled re-entry of protons in the matrix without driving ATP synthase. The protein is used to activate heat production.
In response to cold, noradrenaline is released from the sympathetic nervous system and stimulates lipolysis releasing fatty acids to provide fuel for oxidation in brown adipose tissue.

Question 18

Compare oxidative phosphorylation and substrate level phosphorylation

Answer 18

Oxidative - Produces ATP from ADP and Pi. requires membrane associated complexes (inner mitochondrial membrane). Energy coupling occurs through a proton gradient. Cannot occur in the absence of oxygen. 33% efficient. Major process for ATP synthesis for cells that require large amounts of energy.
Substrate - Produces ATP from ADP and phosphorylated organic compound. Requires soluble enzyme. Energy coupling occurs directly through formation of high energy of hydrolysis bond. Can occur to a limited extent in absence of oxygen. 60% efficient. Minor process for ATP synthesis in cells that require large amounts of energy,

Question 19

Explain beta oxidation of fatty acids

Answer 19

The oxidation of fatty acids occurs via a sequence of reactions (beta oxidation pathway), that oxidises the fatty acid and removes a C2 unit (acetate). The shortened fatty acid is cycled through the reaction sequence repeatedly removing a C2 unit each turn until only 2 carbons remain.
Reaction requires NAD+ and FAD and can’t occur without oxygen. All intermediates in the pathway are linked to coenzyme A and the C atoms of the fatty acid cid are converted to acetyl CoA