Lecture 1: Introduction to Metabolism

Question 1

Define metabolism.

Answer 1

Metabolism occurs through a series of enzyme catalysed reactions, each involving a small, specific chemical change. Although the entire pathway must be thermodynamically favourable, it is not necessary that every reaction be favourable.

Question 2

Give 3 methods of regulating metabolism.

Answer 2

1) Change the amount of enzyme
2) Change the activity of the enzyme
3) Change the amount/availability of the substrate

Question 3

Which is the slowest method of metabolic regulation. Explain why this method is the slowest.

Answer 3

Changing the amount of enzyme. This requires a change in the rate of transcription of the enzyme or a change in the rate of enzyme degradation (turnover). This method can take an hour or two to produce a change in the rate of a metabolic pathway.

Question 4

Give four examples of ways of changing the activity of the enzyme to regulate metabolism and identify the fastest method.

Answer 4

1) Associate the enzyme with a regulatory protein
2) Allosteric activation/inhibition (fastest)
3) Covalent modification
4) Sequestration (keeping enzyme away from the substrate)

Question 5

Describe the process of allosteric activation.

Answer 5

The allosteric enzyme undergoes a conformational (shape) change, as a result of binding of a regulatory molecule (binds to a site distinct to the active site - not competitive). The conformational change changes the shape/structure of the active site, so the enzyme’s affinity for the substrate increases. The enzyme is said to be activated.
During allosteric inhibition, the changed active site has a lower affinity for the substrate and the enzyme is deactivated.

Question 6

Explain why phosphofructokinase is a key regulatory enzyme in glycolysis.

Answer 6

Phosphofructokinase is the enzyme responsible for catalysing the reaction in which fructose-6-phosphate is converted to fructose-1,6-bisphosphate.
Phosphofructokinase is allosterically inhibited by ATP and citrate and is allosterically activated by AMP.

Question 7

What is catabolism?

Answer 7

Catabolism is the removal of energy and reducing power from energy-containing nutrients, producing energy-depleted ‘waste’ products.

Question 8

What is anabolism?

Answer 8

Anabolism is the synthesis of cell macromolecules, e.g. proteins, from precursor molecules e.g. amino acids, using energy and reducing power.

Question 9

Give 3 examples of energy-depleted ‘waste’ products produced by catabolism.

Answer 9

NH3, CO2, H2O

Question 10

Can a catabolic reaction be reversed into an anabolic reaction using the same enzymes?

Answer 10

No. There are distinct enzymes for the catabolic and anabolic pathways comprised of the same reactions.

Question 11

Are catabolic and anabolic reactions usually localised to the same or different compartments in the cell?

Answer 11

Different compartments.

Question 12

Give an example of a biologically important dehydration/condensation reaction.

Answer 12

The conversion of two amino acids into a dipeptide.

Question 13

Does reduction involve the loss or gain of electrons?

Answer 13

Gain. Remember OIL RIG: Oxidation is Losing electrons, Reduction is Gaining electrons.

Question 14

What is the product if an alcohol is a) oxidised, b) reduced?

Answer 14

a) aldehyde (primary alcohol) or a ketone (secondary ketone)

b) alkene

Question 15

What is the product is a carboxylic acid is oxidised?

Answer 15

Carbon dioxide

Question 16

Give an example of an isomerisation reaction in glycolysis.

Answer 16

1) Glucose-6-phosphate is isomerised to Fructose-6-phosphate.
2) 3-phosphoglycerate is isomerised to 2-phosphoglycerate

Question 17

Give 5 examples of common types of reaction in metabolic pathways.

Answer 17

1) Hydrolysis/dehydration
2) Oxidation/Reduction
3) Isomerisation
4) C-C cleavage
5) Group transfer

Question 18

Is glycolysis common to all organisms?

Answer 18

Yes

Question 19

When is glycolysis most important?

Answer 19

In anaerobic conditions, when the TCA cycle and the ETC stop, glycolysis is the only source of ATP.

Question 20

Why is glycolysis particularly important in red blood cells?

Answer 20

Red blood cells have no mitochondria, so cannot perform the TCA cycle or the ETC and so rely on glycolysis for ATP production.

Question 21

Is the TCA cycle catabolic or anabolic?

Answer 21

Catabolic. It is the breakdown of complex molecules into simpler ones, with the removal of energy and reducing power.

Question 22

Where do carbohydrates enter the TCA cycle?

Answer 22

Via pyruvate, via glucose and glycolysis. Starch, sucrose and glycogen are all converted to glucose and undergo glycolysis to enter the TCA via the pyruvate route?

Question 23

How do lipids enter the TCA cycle?

Answer 23

As Acetyl-CoA. Fatty acids undergo beta oxidation to become Acetyl-CoA.

Question 24

How do amino acids enter the TCA cycle?

Answer 24

As pyruvate, alpha-ketoglutarate, fumarate and oxaloacetate.

Question 25

From which TCA cycle intermediate are fatty acids and sterols synthesised through anabolism?

Answer 25

Citrate.

Question 26

Which type of molecules can be synthesised from TCA cycle intermediate Succinyl-CoA?

Answer 26

Porphyrins (contain haem group)

Question 27

From which TCA cycle intermediates can amino acids be synthesised through anabolism?

Answer 27

Alpha-ketoglutarate and oxaloacetate.

Question 28

Fill the gap. Regulation only occurs at ______ reactions.

Answer 28

Irreversible

Question 29

Give the enzymes involved in regulating glycolysis and the regulatory molecules which they are activated or inhibited by.

Answer 29

Hexokinase (inhibited by glucose-6-phosphate, the product of its reaction)
Phosphofructosephosphate (allosterically inhibited by ATP and citrate and activated by AMP)
Pyruvate kinase (allosterically inhibited by ATP and Acetyl CoA and activated by Fructose-1,6-bisphosphate (a substrate earlier in glycolysis) in a feed forward stimulation)