Metabolic pathways

What controls each step of a metabolic reaction?

A particular enzyme

What codes for each enzyme in a metabolic reaction?

A particular gene

What do metabolic pathways consist of?

Several stages involving the conversion of one metabolite to another

What happens when the appropriate enzyme is present in a metabolic reaction?

The pathways proceeds

What happens when the appropriate enzyme is not present in a metabolic reaction?

The pathway stops

Metabolic pathways

Integrated and controlled pathways of enzyme-catalysed reactions within a cell

Metabolism

Biochemical reactions that take place in cells

What are the two types of metabolic pathways?

Anabolic reactions and catabolic reactions

Anabolic reactions

The build up of larger molecules from smaller molecules, this requires energy

Catabolic reactions

The break down of larger molecules into smaller molecules, this releases energy

Irreversible routes

The molecules are permanently changed

Reversible routes

The molecules can be converted back into its previous form

Example: reversible route

Anaerobic respiration (fermentation) in animal cells:

Alternative routes

A metabolic pathway that has more than one ‘route’ of getting the substrate to the product(s)

Where are metabolic enzymes found?

Embedded on the surface of membranes

What are the three proteins found embedded in the phospholipid membrane?

• (Metabolic) enzymes

• Protein pores

• Pumps

Diagram: phospholipid membrane

(Metabolic) enzymes

Enzymes that organise the sequence of reactions and control/speed them up without chemically changing the enzyme after the reaction

Protein pores

Holes in the membrane that are surrounded by proteins that allow small molecules through by diffusion

Pumps

Proteins in the membrane that allow small molecules through by active transport

What happens when a mutation is present in a metabolic reaction?

The enzyme coded for by the gene would be faulty and so the pathway would be blocked

The ‘lock and key’ mechanism

Enzymes have a specific shape and so their active site is shaped to complement that of their specific substrate(s)

The active site and its substrate(s)

The active site has a high affinity to these

The active site and its product(s)

The active site has a low affinity to these

How does the active site affect the activation energy of a reaction?

The active site helps to orientate the reactants, which lowers the activation energy of a reaction as the substrate(s) bind

Induced fit model

This is when the shape of the active site changes shape to better fit the substrate, after the substrate binds to ensure the active site comes into very close contact with the substrate

Diagram: induced fit

Activation energy

The energy required to initiate a reaction

What happens before a substrate becomes a product?

The substrate must overcome an energy barrier (the activation energy)

What do enzymes do in living cells besides organise the sequence of reactions and control/speed them up?

Reduce the activation energy needed for a reaction to occur

Inhibitors

Substances that prevent enzyme activity

What are the three types of metabolic inhibitors?

• Feedback inhibitors

• Competitive inhibitors

• Non-competitive inhibitors

Feedback inhibitors

When the end-product reaches a critical concentration, the end-product can inhibit an earlier enzyme in the metabolic pathway (stops the enzyme from working)

How are feedback inhibitors beneficial?

They shut down the whole pathway, preventing the formation of additional end-products, which allows the process to control itself

Competitive inhibitors

These inhibitors compete with the substrate to fit into the active site of an enzyme, which they can do because they have a similar structure to that of a substrate

Diagram: competitive inhibitors

Why are competitive inhibitors affected by substrate concentration?

The substrate(s) will ‘win’ the competition to combine with the active site of the enzyme

Non-competitive inhibitors

These inhibitors bind elsewhere on the enzyme (the allosteric site), which causes the active site to change shape, preventing any substrates from binding

Diagram: non-competitive inhibitors

Why are non-competitive inhibitors unaffected by substrate concentration?

The inhibitor can bind elsewhere on the enzyme (the allosteric site)