Enzymes

Question 1

Are all cofactors required for enzyme activity?

Answer 1

All apart from co-enzymes

Question 2

What are the three types of cofactors?

Answer 2

Inorganic cofactors, co-enzymes, prosthetic groups

Question 3

Where are inorganic cofactors obtained from?

Answer 3

via the diet as minerals usually ions

Question 4

Do inorganic cofactors bind temporarily or permanently?

Answer 4

may be either as may bind temporarily or may be a permanent part of an enzyme

Question 5

Where are co-enzymes derived from?

Answer 5

vitamins, organic molecules

Question 6

Do co-enzymes bind temporarily or permanently to an enzyme?

Answer 6

temporarily

Question 7

What is the role of a co-enzyme?

Answer 7

transfer substances between molecules to increase the rate of reaction/enzyme activity

Question 8

Do prosthetic groups bind permanently or temporarily to an enzyme?

Answer 8

permanently bound to an enzyme to form a permanent feature of an enzyme

Question 9

How may inorganic cofactors and prosthetic groups increase an enzyme’s rate of activity?

Answer 9

• they change the shape/tertiary structure of the active site
• they may affect the charges of the active site
• they could bind to or interact with the substrate

Question 10

An enzyme’s active site is a specific shape created by which level of protein structure?

Answer 10

tertiary

Question 11

Is catalase an intracellular or extracellular enzyme?

Answer 11

intracellular

Question 12

Is trypsin an intracellular or extracellular enzyme?

Answer 12

extracellular

Question 13

What is the cofactor for carbonic anhydrase?

Answer 13

Zn2+

Question 14

What are enzymes?

Answer 14

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 15

Give an example of an enzyme that catalyses intracellular reactions.

Answer 15

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

Question 16

Give two examples of enzymes that catalyse extracellular reactions

Answer 16

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 17

Explain the induced fit model of enzyme action

Answer 17

• shape of active site is not directly complementary to substrate and 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 18

Explain the lock and key model of enzyme action

Answer 18

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 19

Name 5 factors that affect the rate of enzyme-controlled reactions

Answer 19

• enzyme concentration
• substrate concentration
• concentration of inhibitors
• pH
• temperature

Question 20

How does substrate concentration affect rate of reaction?

Answer 20

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 21

How does enzyme concentration affect rate of reaction?

Answer 21

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 22

How does temperature affect the rate of enzyme-controlled reactions?

Answer 22

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

Question 23

What is the temperature coefficient?

Answer 23

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

Question 24

How does pH affect rate of reaction?

Answer 24

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

Question 25

How do competitive inhibitors work?

Answer 25

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 26

How do non-competitive inhibitors work?

Answer 26

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

Question 27

What is end-product inhibition?

Answer 27

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 28

What are irreversible inhibitors?

Answer 28

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 29

What are reversible inhibitors?

Answer 29

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 30

Define metabolic poison.

Answer 30

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

Question 31

Give some examples of metabolic poisons

Answer 31

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

Question 32

How do some medicinal drugs act as inhibitors?

Answer 32

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 33

What are inactive precursors in metabolic pathways?

Answer 33

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

Question 34

What are cofactors?

Answer 34

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

Question 35

What are coenzymes?

Answer 35

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 36

What are inorganic cofactors? Give an example

Answer 36

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

Question 37

What are prosthetic groups? Give an example

Answer 37

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

Question 38

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

Answer 38

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