C1.1 Enzymes

Question 1

To which biological group of molecules do enzymes belong to

Answer 1

Proteins

Question 2

Why are enzymes called ‘biological catalysts’

Answer 2

They speed up chemical reactions in living organisms without being used in the process

Question 3

Enzymes are specific. What does this mean

Answer 3

One enzyme only binds to one substrate. An enzyme can only catalyze one specific reaction.

Question 4

Most recent hypothesis on enzyme action

Answer 4

Induced fit model: suggests that the active site is flexible and only assumes its catalytic conformation after the substrate molecules bind to the site.

Question 5

What is activation energy

Answer 5

Energy required to start a reaction

Question 6

How do enzymes affect activation energy of a reaction

Answer 6

Enzymes reduce activation energy

Question 7

Why do reactions occur more slowly at low temp

Answer 7

Less kinetic energy = less chance of successful collisions

Question 8

What does ‘optimum conditions’ mean

Answer 8

Conditions in which an enzyme works its fastest rate

Question 9

What happens to an enzyme if its temperature rises above the optimum value

Answer 9

Enzyme denatures and the active site loses its shape

Question 10

Why does a competitive inhibitor slow down an enzyme action

Answer 10

They compete with the substrate for the active site, thus reducing the rate.

Question 11

Define Metabolism

Answer 11

• The complex network of interdependent and interacting chemical reactions occurring in living organisms. Most of these reactions are enzyme controlled/catalyzed.

Question 12

Types of chemical reactions

Answer 12

• Almost all chemical reactions in living organisms are either:
• Anabolic: enzymes bring the substrate molecules together to build larger molecules (E.g. condensation reactions: protein synthesis, glycogen formation and photosynthesis)
• Catabolic: enzymes’ active sites affects the bonds in substrates so they are easier to break to smaller molecules (E.g. Hydrolysis reactions in digestion and oxidation of substrates in respiration)

Question 13

Describe the enzyme structure

Answer 13

• Enzymes are globular proteins with a specifically shaped active site, determined by the tertiary structure.
• They are large molecules but only a small section called the active site is involved in catalysis.

Question 14

Explain enzyme action

Answer 14

• Enzymes move around due to kinetic energy and collide with substrates. If the substrate is complementary to the enzyme and the alignment is correct, a reaction will occur and the substrate will bind to the active site forming an enzyme-substrate complex.

Question 15

Explain Denaturation

Answer 15

• Enzymes are proteins and can be denatured.
• Can be caused by pH, heat and the presence of heavy metals.
• Destroys the tertiary or quaternary structure.
• Occurs when the active site of an enzyme changes shape and can no longer bind with its complementary substrate.
• May be reversible if only a minor increase in temp or change in pH.

Question 16

Factors affecting enzyme activity

Answer 16

• Substrate concentration
• Enzyme concentration

Question 17

Rate of reaction calculation

Answer 17

Rate of reaction = product formed or reactant used up/ time

Question 18

Define and explain Metabolic pathway

Answer 18

• A metabolic pathway is any chain or cycle of linked reactions catalyzed by enzymes (E.g. Respiration and photosynthesis)
• Can be either: Linear or cyclical.

Question 19

Define metabolic rate and state one factor that affects it

Answer 19

• An organism’s metabolic rate is the amount of energy needed by that organism in a given time period
• One factor is body mass: positive correlation

Question 20

Explain Competitive inhibitors

Answer 20

• similar to the substrate and binds to the active site of the enzyme.
• If substrate concentration increases, it will reduce the effect of the inhibitor: more substrate molecules = more chance of finding active site and leaving fewer occupied by the inhibitor.

Question 21

Explain Non-competitive inhibitors

Answer 21

• Bind to the enzyme at a site different from the active site (Allosteric site).
• This alters shape of the enzyme molecule including active site so that the substrate can no longer bind to the active site.
• Bind to different sites therefore, rate of reaction unaffected by substrate concentration
• Binding is reversible

Question 22

Explain end-product inhibition (Feedback inhibition)

Answer 22

• In order to control metabolic pathways, the end product of the pathway may sometimes inhibit the activity of the first enzyme: End product acts like a non-competitive inhibitor and binds to the allosteric site of the first enzyme, altering its active site and disabling it.
• As concentration of end product build up, more chance of inhibition.
• Avoids excessive production and build up of the intermediate chemicals in a pathway.

Question 23

Explain mechanism-based inhibition (suicide inhibition)

Answer 23

• Caused by irreversible binding of inhibitor to the active site of a specific enzyme through a covalent bond.
• Creates a stable inhibitor-enzyme complex, in which the enzyme is permanently inactive.
• Inhibitor is very similar to substrate and are inactive until they bind to the active site
• This makes them great medicines which have specific targets and result in little to no side effects.

Question 24

Where can enzymes act

Answer 24

• Intracellularly (inside cells: E.g. glycolysis and Krebs cycle)
• Extracellularly (outside cells: E.g. chemical digestion in the gut)

Question 25

Explain the generation of heat energy by the reactions of metabolism

Answer 25

• Transformation of energy is never 100% efficient: heat is always lost (second law of thermodynamics). Warm blooded animals use this heat to keep warm and maintain constant body temperature

Question 26

Example of end-product inhibition (Feedback inhibition)

Answer 26

Threonine-isoleucine pathway:
- Bacteria can synthesize the amino acid isoleucine from threonine in a metabolic pathway
- When end product isoleucine accumulates, its concentration increases, and binds to the allosteric site of threonine deaminase. The active site is altered, preventing threonine from binding.

Question 27

Example of mechanism-based inhibition

Answer 27

Penicillin antibiotic:
- Penicillin is a group of antibiotic chemicals.
- They have a very similar shape to the peptide substrate’s terminal ends, allowing them to bind to the active site of transpeptidase instead.
- Penicillin irreversibly forms covalent bond with the enzyme, inactivating transpeptidase.
- As peptidoglycan synthesis is halted, its cell wall is compromised, which cause cell to be unable to maintain osmotic pressure, leading to lysis and death.

Question 28

Example of Competitive inhibition

Answer 28

Statins:
- Medicines that are used to treat high blood cholesterol, a contributor to heart disease
- Reduces production of cholesterol in liver: the product of a linear metabolic pathway
- Due to similar shape, statins compete with HMG-CoA and can also bind to the active site of HMG-CoA reductase, reducing rate of reaction and amount of cholesterol being produced

Question 29

Example of non-competitive inhibition

Answer 29

ACE inhibitors:
- Medicines that are used to treat hypertension (high blood pressure)
- Precent vasoconstriction (contraction of blood vessels)
- Bind to allosteric site of ACE and changes its active site to prevent Angiotensin I conversion.

Question 30

Define and Explain Immobilised enzymes

Answer 30

• Enzyme attached to an inert, insoluble material
• Beneficial as it improves enzyme stability and can provide a better environment for enzyme activity