Action of enzymes

Question 1

What are enzymes?

Answer 1

Biological catalysts!

• They catalyse metabolic reactions both at a cellular level (e.g. respiration), and at the level of the whole organism (e.g. mammalian digestion)
• They can affect structures and functions
• They can act within cells (intracellular) or outside of them (extracellular)

Question 2

Catalase is an example of an intracellular enzyme. What reaction does it catalyse?

Answer 2

Hydrogen peroxide –> oxygen + water

Note: hydrogen peroxide is a by-product of many cellular processes, and needs to be broken down as it can kill cells

Question 3

Amylase and trypsin are both extracellular enzymes. What reactions do they catalyse? (Hint: they are found in the digestive system)

Answer 3

Amylase is found in saliva, and is secreted by salivary glands. It catalyses the hydrolysis (breakdown) of starch into maltose (a sugar) in the mouth.

Trypsin is produced in teh pancreas and secreted into the small intestine. It catalyses the hydrolysis (breakdown) of peptide bonds so turns big polypeptides into smaller ones, which can then be broken down to amino acids by other enzymes.

Question 4

What type of proteins are enzymes - globular or fibrous?

Answer 4

Globular

Question 5

What is the site on the enzyme that the substrate binds to called?

Answer 5

The active site - it has a specific shape for a specific substrate. This means that enzymes work for very few substrates, usually only one.

Question 6

Enzymes work by reducing the activation energy. What is this?

Answer 6

This is the amount of energy that needs to be supplied to the chemicals (usually as heat) before the reaction can start.

By reducing it, reactions can often happen at a lower temp than they could without an enzyme, speeding up the rate of reaction.

The formation of the enzyme substrate complex lowers the activation energy:

• if two molecules need to be joined it brings them close together, reducing any repulsion between them
• if teh enzyme is catalysing a breakdown reaction, fitting in the active site puts strain on the bonds that need to be broken in teh substrate.

Question 7

The ‘induced fit’ model is a better theory than the ‘lock and key’ model. What is the difference?

Answer 7

Induced fit says that, as the substrate binds to the active site of the enzyme, the active site is not yet a perfect fit as in the lock and key model, but it instead changes shape slightly to fit the substrate easily. The enzyme-substrate complex is then changed to the enzyme-product complex, and then the product(s) is released.