L1 - Introduction To Metabolism

Question 1

What is metabolism?

What are the two types of metabolic reactions?

Answer 1

Metabolism - a series of enzyme reactions which involve fuel molecules being used to produce energy. This involves synthesis of new molecules, breakdown of molecules/ interconversion of molecules.

There are two types of metabolic reactions; anabolic and catabolic.

Question 2

What is anabolism?
Is energy required?
Name a few examples.
Where these reactions commonly occur in cell.

Answer 2

• Anabolic reactions involve the build up of macromolecules from monomers.
• This is an energy requiring process (ATP, GTP, or UTP).
• E.g. Glycogenesis, gluconeogenesis and lipogenesis.
• This tends to occur in the cytosol.

Question 3

What is catabolism?
Is energy required?
Name a few examples.
Where these reactions commonly occur in cell.

Answer 3

• Catabolism is the break down of macromolecules into smaller molecules.
• This produces energy in the form of ATP or NADH.
• E.g. lipolysis, glycolysis, and glycogenolysis.
• These reactions commonly occur in the mitochondria.

Question 4

An example of integration of metabolic pathways: Glycolysis, TCA and OXPHOS

Answer 4

The complete oxidation of glucose occurs via glycolysis, TCA, and OXPHOS - this provides energy to tissues which need this e.g. brain, muscles etc.

Question 5

Summary diagram showing the catabolism of sugar, fat and a.a:

Answer 5

This occurs in three stages.

1. Glucose —— Glycolysis ——> Pyruvate
   FA ———— ß-oxidation ——> Acetyl CoA
   A.A ———— Transamination —> carbon skeleton
2. These then enter into the TCA cycle where they are converted to CO2 and NADH/FADH2.
3. This can then participate in OXPHOS to produce energy (ATP).

Question 6

Where do the main metabolic pathways occur (glycolysis, TCA, OXPHOS, ß-oxidation, FA synthesis):

Answer 6

Glycolysis and FA synthesis occurs in the cytosol.

The rest occur in the mitochondria. This has a double membrane where in OXPHOS, electrons are transferred from complex to complex and protons are pumped into the IMS. This generates a proton gradient which is needed for ATP synthesis. H2O is also produced where oxygen combines with H.

Question 7

Transport of FAs and Glucose in blood plasma and into cells:

Answer 7

FAs:
FAs are lipid soluble so are transported in blood attached to albumin. To enter cells, they can either freely diffuse across or can use transporters on membrane. Once in cell, they can either be rapidly metabolised or stored where they are bound to FABP (FA binding protein).

Glucose:
This is water soluble so can freely diffuse in plasma. However, in order to enter cells, they require transporters (GLUTs).

Question 8

What is ATP:

Answer 8

ATP is an energy carrier molecule as this contains high energy phosphate bonds. When hydrolysed, it generates 30.5 kJ/mol of energy. ATP —> ADP + Pi + energy + H+

This is chemically stable in pH 6-9 and its structural features can be recognised by proteins and enzymes allowing it to be used for metabolic reactions.

Question 9

What are the functions of ATP:

Answer 9

1. ATP hydrolysis can be coupled to thermodynamically unfavourable reactions —> makes them favourable!
2. Cell motility and contraction
3. Active transporters (e.g. Na/K pump)
4. Metabolic/ allosteric modulators - phosphorylation of enzyme using ATP hydrolysis can alter enzyme activity (activate/inhibits)
5. Needed for metabolic reactions to allow production of metabolic intermediates e.g. Glucose + ATP —> G-6-P + ADP

Question 10

General Features of Metabolic Pathways:

Answer 10

Reactions in metabolic pathways are linked to one another where product of one reaction is the substrate of the next pathway.

First enzyme tends to be the control enzyme as this determines whether a reaction occurs or not.

Reactions can either be reversible or irreversible.

Question 11

Enzymatic reactions - how do enzymes affect the rate and equilibrium of a reaction?

Answer 11

Enzymes speed up the R.O.R but has no effect on equilibrium of a reaction. The latter is dependent on the properties of the chemical molecules. R.O.R is dependent on enzyme activity.

Question 12

What are endergonic and exergonic reactions?

How does enzyme catalysis affect activation energy (Ea)?

Answer 12

Endergonic - requires energy
Exergonic - releases energy

Enzyme reduces the activation energy making the reaction more favourable i.e. less energy is required to overcome this barrier and allow product formation.

Question 13

How does the rate of an enzyme catalysed reaction be regulated:

Answer 13

1. Enzyme activity can be altered by the availability of substrate - more substrate means enzyme catalysis is greater - greater rate of reaction.
2. Depends on amount of enzyme present - this can be altered by gene transcription. Induction/repression of gene will increase/decrease [enzyme].
3. Allosteric regulation/ covalent modification of enzyme - phosphorylation can either activate or inhibit enzyme. Another allosteric enzyme can bind to allosteric site on enzyme —> this affects enzyme catalysed reaction. The regular hyperbolic curve won’t be seen; instead it would be sigmoidal where velocity is independent to [substrate].

Question 14

Metabolic reactions require three things:

Answer 14

• Substrates i.e. fuel molecules
• Enzymes
• Cofactors

Cofactors can either be metal ions e.g. Mg2+ or can be coenzymes/prosthetic groups.

Question 15

Example of enzyme cofactors:

Answer 15

ATP is an high energy cofactor where its hydrolysis is usually coupled to thermodynamically unfavourable reactions.

Have other high energy nucleotide cofactors that are present e.g. UTP and GTP.

UTP - needed for formation of complex sugars (glycogen)
GTP - needed for protein synthesis