Enzymes

Question 1

What are enzymes

Answer 1

Proteins that act as biological catalysts for intra and extracellular reactions to determine structure and function
Therefore affect metabolism of cells and whole organism
Specific tertiary structure determines shape of active site, complementary to a specific substrate
Formation of enzyme-substrate complexes lowers activation energy of metabolic reactions

Question 2

Give an example of an enzyme that catalyses intercellular reactions

Answer 2

Catalase: catalyses decomposition of hydrogen peroxide (which causes oxidative stress) into water and oxygen

Question 3

Give two examples of enzymes that catalyse extracellular reactions

Answer 3

Amylase: carbohydrase catalyses digestion of starch to maltose in saliva / small intestine lumen
Trypsin: pancreatic endopeptidase catalyses hydrolysis of peptide bonds in small intestine lumen

Question 4

Explain the induced fit model of enzyme action

Answer 4

Shape of active sit is not directly complementary to substrate and is flexible
Conformational change enable ES complexes to form when substrate absorbs
This puts strain on substrate bonds lowering activation energy. Bonds in enzyme-product complex are weak so product desorbs

Question 5

Explain the lock and key model of enzyme action

Answer 5

Suggests that active site has rigid shape determined by tertiary structure so is only complementary to 1 substrate. Formation of ES complex lowers activation energy
Bonds in enzyme-product complex are weak , so product desorbs

Question 6

Name 5 factors which affect the rate of enzyme controlled reactions

Answer 6

• Enzyme concentration
• Substrate concentration
• Concentration of inhibitors
• pH
• Temperature

Question 7

How does substrate concentration affect rate of reaction?

Answer 7

Given that enzyme concentration is fixed, rate increases proportionally to substrate concentration
The rate levels off when maximum number of ES complexes form at any given time

Question 8

How does enzyme concentration affect the rate of reaction

Answer 8

Given that substrate is in excess, rate increase proportionately to enzyme concentration
Rate levels off when maximum number of ES complexes form at any given time

Question 9

How does temperature affect the rate of enzyme-controlled reactions

Answer 9

Rate increases as kinetic energy increases and peaks at optimum
Above optimum, ionic and H-Bonds in tertiary structure break = active site no longer complementary to substrate (denaturation)

Question 10

What is the temperature coefficient

Answer 10

Q10 measures the change in rate of reaction per 10*C temperature increases
Q10 = R2/R1 (where R represents rate)

Question 11

How does pH affect rate of reaction

Answer 11

Enzymes have a narrow optimum pH range
Outside range H+ / OH- ions interact with H-Bonds and ionic bonds in tertiary structure = denaturation

Question 12

How do competitive inhibitors work

Answer 12

Bind to active site since they have similar shape to substrate. Temporarily prevent ES complexes forming until released.
Increasing substrate concentration decreases their effect

Question 13

How do non-competitive inhibitors work

Answer 13

Bind at allosteric binding site
Trigger conformational change of active site
Increasing substrate concentration has no impact on their effect

Question 14

What is end-product inhibition

Answer 14

One of the products of a reaction acts as a competitive or non-competitive inhibitor involved in the pathway
Prevents further formation of products

Question 15

What are irreversible inhibitors

Answer 15

Permanently prevent formation of ES complexes
Heavy metal ions e.g. mercury, silver cause disulphide bonds in tertiary structure to break
Bind to enzymes by strong covalent bonds e.g. cyanide binds to cytochrome c

Question 16

Define metabolic poison

Answer 16

Substance that damages cells by interfering with metabolic reactions
Usually an inhibitor

Question 17

Give some examples of metabolic poisons

Answer 17

Respiratory inhibitors include:
- Cyanide: non-competitive, irreversible, inhibits cytochrome c oxidase
- Malonate: Competitive, inhibits succinate dehydrogenase
- Arsenic: Competitive, inhibits pyruvate dehydrogenase

Question 18

What are inactive precursors in metabolic pathways

Answer 18

To prevent damage to cells, some enzymes in metabolic pathways are synthesised as inactive precursors e.g. proteases
One part of the precursor acts an inhibitor.
ES complexes form when it is removed

Question 19

What are cofactors

Answer 19

Non-protein compounds required for enzyme activity:
- Coenzymes
- Inorganic cofactors
- Prosthetic groups

Question 20

What are coenzymes

Answer 20

Organic cofactors. Do not bind permanently
Often transport molecules or electrons between enzymes
Frequently derived from water-soluble vitamins e.g. the hydrogen acceptor NAD is derived from niacin

Question 21

What are inorganic cofactors? Give an examples

Answer 21

Facilitate temporary binding between substrate and enzyme. Often metal ions e.g. Cl- is the cofactor for amylase

Question 22

What are prosthetic groups? Given an example

Answer 22

Tightly bound cofactors act as a permanent part of enzyme’s binding site e.g. Zn2+ for carbonic anhydrase

Question 23

Suggest how a student could produce a desired concentration of solution from a stock solution

Answer 23

Volume of stock solution = required concentration x final volume needed / concentration of stock solution
Volume of distilled water = final volume needed - volume of stock solution