Enzymes

Question 1

What are enzymes?

Answer 1

Enzymes are biological catalysts that speed up the rate of reactions, but are unused by the reaction.
Enzymes act within cells, and are also involved in making the structural parts of the human body, such as muscles, bone and connective tissue.
Enzymes work best at a specific temperature (37°C for human enzymes), normal pressure and specific pH.

Question 2

What are intracellular enzymes?

Answer 2

Some enzymes are intracellular. This means that they work inside the cell and catalyse metabolic reactions.

Question 3

What type of enzyme is catalase?

Answer 3

Intracellular

Question 4

What does catalase do?

Answer 4

Catalase breaks down harmful hydrogen peroxide in the liver, forming non-harmful products - oxygen and water.

Hydrogen peroxide -> oxygen + water

Question 5

How do enzymes affect activation energy?

Answer 5

Enzymes work by lowering the amount of activation energy needed to carry out the reaction.
When the activation energy is lowered, the reaction can be carried out at a much lower temperature.

Question 6

What are extracellular enzymes?

Answer 6

Extracellular enzymes work outside of the cells. They are made inside cells and then secreted. Many of our digestive enzymes are extracellular, for example, amylase and trypsin.

Question 7

What type of enzyme is amylase?

Answer 7

Extracellular

Question 8

What type of enzyme is trypsin?

Answer 8

Extracellular

Question 9

What does amylase do?

Answer 9

Amylase is found in saliva in the mouth, and breaks down starch into maltose. It is also found in the small intestine.

Question 10

What does trypsin do?

Answer 10

Trypsin is found in the small intestine and breaks down protein into amino acids.

Question 11

What type of protein is an enzyme?

Answer 11

Globular protein with an active site

Question 12

What is an enzyme-substrate complex?

Answer 12

When a substrate is bound to the enzyme’s active site

Question 13

What is an enzyme-product complex?

Answer 13

The substrate becomes a product while still bound to the enzyme.

Question 14

What is the lock and key hypothesis?

Answer 14

The active site of the enzyme is like a “lock” and the substrate is like a “key”. The two fit together in a complementary way. A particularly substrate will only fit into a particular enzyme, in the same way that a key fits into a lock. The substrate is then broken into product and leaves active site.

Question 15

What is the induced fit hypothesis?

Answer 15

Research into enzyme structure before and after substrate binding, suggests that the shape of the enzyme is not rigidly fixed. The active site of the enzyme will only accept a particular substrate, and after the substrate binds the enzyme moulds around it to allow a tighter fit.

Question 16

What happens to enzymes if the temperature increases above the optimum?

Answer 16

The hydrogen bonds break in the tertiary structure of the active site. The enzyme denatures, and the substrate can no longer bind to the active site.

Question 17

What happens to enzymes below the optimum temperature?

Answer 17

Lower temperature, so less kinetic energy and less frequent collisions between enzyme and substrate. Rate of reaction decreases.

Question 18

What happens to enzymes that are above or below their optimum pH?

Answer 18

Hydrogen bonds holding together the tertiary structure of the active site are broken. The enzyme is denatured. The substrate is no longer able to bind to the active site.

Question 19

What are cofactors?

Answer 19

Cofactors are non-protein, inorganic substances that have to be present in order for the enzyme reaction to occur.
Cofactors can act as co-substrates. They bind to an inactive protein and allow the substrate to bind to the active site.
Cofactors can also alter the charge on the surface of the substrate or active site of the enzyme.
Cofactors are usually mineral ions. They do not permanently bind to the enzyme, but they temporarily bind to the enzyme-substrate complex, to make its formation easier.

Question 20

What are coenzymes?

Answer 20

Coenzymes are a type of small, organic cofactor. They are used up by the enzyme reaction, and have to be recycled. Coenzymes help the enzyme by binding temporarily to the active site with the substrate and carrying chemical groups between enzymes.

Question 21

What are may coenzymes derived from?

Answer 21

Vitamins

Question 22

What is an apoenzyme?

Answer 22

An enzyme without its cofactor

Question 23

What is a holoenzyme?

Answer 23

When an enzyme is bound to its cofactor

Question 24

What are prosthetic groups?

Answer 24

Prosthetic groups are cofactors that are a permanent part of the enzyme. Some examples of prosthetic groups include:

• Haem group ( includes Fe*2+)
• Magnesium ion (Mg*2+) in haemoglobin
• Zinc ion (Zn*2+) in carbonic anhydrase

Question 25

What are competitive inhibitors?

Answer 25

Competitive inhibitors bind to the active site of the enzyme. They compete with the enzyme for access to the active site. Once the inhibitor is bound, the substrate cannot bind.
Most competitive inhibition is reversible. When the substrate is in excess, the substrate out-competes the inhibitor and rate of reaction returns to normal.

Question 26

What are non-competitive inhibitors?

Answer 26

Non-competetive inhibitors bind to the enzyme’s allosteric site. When the inhibitor binds, it alters the tertiary structure of the enzyme and distorts the active site so that the substrate can no longer binds.
Many non-competetive inhibitors bind permanently to the enzyme so their effect is irreversible. Therefore, the enzyme is denatured.

Question 27

How can inhibitors act as poisons?

Answer 27

Some poisons work by binding to enzymes. E.g. cyanide is a non-competetive inhibitor of the enzyme cytochrome oxidase. This enzyme is essential in aerobic respiration.
Cyanide poisoning is reversible, but the antidote must be given quickly.

Question 28

How can inhibitors act as medicines?

Answer 28

Many medical drugs, such as blood pressure medicines, work by inhibiting enzymes.
Angiotensin converting enzyme (ACE) inhibitors inhibit ACE. This prevents ACE from taking part in a metabolic pathway that would increase blood pressure.