2.1.4 Enzymes

Question 1

What are enzymes?

Answer 1

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

Question 2

Give an example of an enzyme that catalyses intracellular reactions.

Answer 2

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

Question 3

Give 2 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 site is not directly complementary to substrate & is flexible.
● Conformational change enables ES complexes to form when substrate adsorbs.
● 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 a 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 that 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. Given that enzyme concentration is fixed, rate increases proportionally to substrate concentration.

Question 8

How does enzyme concentration affect rate of reaction?

Answer 8

Given that substrate is in excess, rate increases proportionally 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 & peaks at optimum temperature. Above optimum, ionic & H-bonds in 3rd 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 increase.
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 & ionic bonds in 3° 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 from forming until released. Increasing substrate concentration decreases their effect.

Question 13

How do non-competitive inhibitors work?

Answer 13

Bind at allosteric binding site. Bind at allosteric binding 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 for an enzyme 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

What are reversible inhibitors?

Answer 16

May be competitive or non-competitive.
Bind to enzyme temporarily e.g. by
H-bonds or a few ionic bonds. ES complexes can form after the inhibitor is released.

Question 17

Define metabolic poison.

Answer 17

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

Question 18

Give some examples of metabolic poisons.

Answer 18

Respiratory inhibitors include:
● cyanide: non-competitive, irreversible, inhibits cytochrome c oxidase
● malonate: competitive, inhibits succinate dehydrogenase
● arsenic: competitive, inhibits pyruvate
dehydrogenase

Question 19

How do some medicinal drugs act as inhibitors?

Answer 19

Penicillin: non-competitive inhibitor of
transpeptidase to prevent formation of
peptidoglycan cross-links in bacterial cell wall.
Ritonavir: inhibits HIV protease to prevent assembly of new virions.

Question 20

What are inactive precursors in metabolic pathways?

Answer 20

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 21

What are cofactors?

Answer 21

Non-protein compounds required for
enzyme activity:
● coenzymes
● inorganic cofactors
● prosthetic groups

Question 22

What are coenzymes?

Answer 22

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 23

What are inorganic cofactors? Give an example.

Answer 23

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

Question 24

What are prosthetic groups? Give an example.

Answer 24

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