2.1.4 - ENZYMES

Question 1

What is an enzyme?

Answer 1

A biological catalyst (speeds up chemical reactions without being used up in the reaction itself)
GLOBULAR PROTEINS

Question 2

What is an active site?

Answer 2

The area of the enzyme where the reaction with the substrate takes place

Question 3

State an example of an intracellular enzyme

Answer 3

• Catalase breaks down hydrogen peroxide into harmless oxygen and water
  -Hydrogen peroxide is the toxic by-product of several cellular reactions (if built up, can kill cells)

Question 4

State an example of an extracellular enzyme

Answer 4

• Amylase and Trypsin work in the human digestive system
• Amylase in saliva catalyses the hydrolysis of starch into maltose
  Trypsin catalyses the hydrolysis of peptide bonds - turning big polypeptides into smaller ones (which get broken down into amino acids by other enzymes)

Question 5

Describe the ‘Lock and Key’ model

Answer 5

• Enzymes only work with complementary substrates that fit their active site

Question 6

Describe the ‘Induced Fit’ model

Answer 6

• The substrate binds to the active site + active site changes shape to fit the substrate more closely

Question 7

What is an enzyme-substrate complex?

Answer 7

A molecule formed when an enzyme comes into perfect contact with its complementary substrate

Question 8

Explain why enzymes are so specific.

Answer 8

• Different enzymes have different shapes active sites, determined by their tertiary structures since they’re proteins
• For the enzyme to work, the substrate has to fit into the active site (be complementary)

Question 9

List factors affecting enzyme activity

Answer 9

• Enzyme concentration
• Substrate concentration
• pH
• Temperature

Question 10

Explain temperature as a factor affecting enzyme activity

Answer 10

• Rate of reaction increases up to optimum temperature
• Even if higher than optimum temperature, rate of reaction will begin to decrease and enzymes will denature

Question 11

Explain pH as a factor affecting enzyme activity

Answer 11

• All enzymes have an optimum pH
• Most work best at pH 7
• Above + below optimum pH, rate of reaction will decrease

Question 12

Explain enzyme concentration as a factor affecting enzyme activity

Answer 12

• Rate of reaction increases as enzyme concentration increases as there are more active sites for substrates to bind to
• However, after a certain point, rate of reaction will plateau + won’t increase simply because even if you add more enzymes, there aren’t any more substrates to bind to them

Question 13

Explain substrate concentration as a factor affecting enzyme activity

Answer 13

• Rate of reaction increases as substrate concentration increases as more enzyme-substrate complexes are formed
• However, beyond a certain point the rate of reaction no longer increases as enzyme concentration becomes the limiting factor

Question 14

What is an inhibitor?

Answer 14

A substance which slows down or stops a reaction by affecting the binding of substrates to enzymes
- reversible or irreversible
- competitive or non-competitive

Question 15

What is a reversible inhibitor?

Answer 15

reversible inhibitors bind to the active site through hydrogen bonds and weak ionic interactions –> don’t bind permanently

Question 16

What is an irreversible inhibitor?

Answer 16

irreversible inhibitors include heavy metal ions (e.g. mercury + silver) which cause disulphide bonds within the protein structure to break (changes shape of active site)

Question 17

What is a competitive inhibitor?

Answer 17

Similar in shape to substrate molecules
They compete with substrate molecules to bind to the active site - but no reaction takes place
They just block the active site so substrates cant bind

Question 18

What is a non-competitive inhibitor?

Answer 18

Bind to enzyme away from active site and instead at ALLOSTERIC SITE
- Causes enzyme’s active site to change shape so substrates can no longer bind to it

Question 19

What is a cofactor?

Answer 19

• Some ions require inorganic ions to function properly
• They help the enzyme and substrate bind together
• Aren’t used up or changed in any way (don’t participate in the reaction)

Question 20

What is a coenzyme?

Answer 20

• Larger organic (carbon-containing) cofactors
• Participate in the reaction + changed by it (like a second substrate)
• Can carry chemical groups or electrons between enzymes
• VITAMINS AREA SOURCE OF COENZYMES

Question 21

Give an example of a cofactor.

Answer 21

• Chloride ions act as a cofactor for amylase
• For amylase to break starch down into maltose, chloride ions must be present

Question 22

What is a prosthetic group?

Answer 22

A cofactor that is tightly bound to an enzyme + permanent

Question 23

Give an example of a prosthetic group.

Answer 23

Haemoglobin contains a prosthetic haem group which contains iron permanently bound to the molecule, which serves as a means of binding oxygen