2.4 - Enzymes

Question 1

what are enzymes

Answer 1

Biological catalysts. Globular proteins that interact with substrate molecules, causing them to react at much faster rates by lowering the activation energy. Enzymes make reactions possible or more efficient by increasing the rate of successful collisions

Question 2

What is metabolism

Answer 2

the sum of all the different reactions and reaction pathways happening in a cell or organism

Question 3

Anabolic reactions

Answer 3

Endothermic reactions building up smaller molecules into larger molecules e.g. photosynthesis

Question 4

Catabolic reactions

Answer 4

Exothermic reactions breaking down larger molecules into smaller molecules e.g. respiration

Question 5

Intracellular enzymes

Answer 5

Enzymes in cells
e.g. catalase (breaks down hydrogen peroxide, a toxic product of many metabolic pathways, into oxygen and water)

Question 6

Extracellular enzymes

Answer 6

Enzymes outside cells.
Nutrients in the form of polymers (e.g. proteins, polysaccharides) cannot enter the cell directly, so therefore need to be broken down by enzymes before entering the cell.
Extracellular enzymes are released by unicellular and multicellular enzymes (into digestive system or immediate environment)

Question 7

Lock and key hypothesis

Answer 7

The substrate molecule binds to the complementary active site forming an enzyme-substrate complex. Substrate reacts and an enzyme-product complex is formed. Products are then released from the enzyme and it leaves enzyme unchanged to take part in subsequent reactions

Question 8

Active site

Answer 8

Area within the tertiary structure of an enzyme that has a shape that binds to a specific substrate molecule

Question 9

Induced fit hypothesis

Answer 9

The active site is similar but not complementary to the substrate, so the initial interaction between the enzyme and the substrate is weak. Weak interactions changes the enzyme’s tertiary structure , which strengthens binding. This puts strain on the substrate, which weakens bonds and lowers activation energy. The products are released and the enzyme returns to its original shape

Question 10

Starch digestion

Answer 10

Involves 2 enzymes, amylase and maltase. Begins in the mouth and continues in the small intestine.
- amylase produced in salivary glands and pancreas
- starch (polysaccharide) is partially broken down into maltose (disaccharide)
- maltase is present in the small intestine and breaks maltose into glucose (monosaccharide)
- glucose is small enough to be absorbed by cells in the lining of the small intestine and then the blood stream

Question 11

Digestion of proteins

Answer 11

Protease = a type of enzyme that catalyses the digestion of proteins into smaller peptides.
Trypsin is a protease produced in the pancreas and released into the small intestine. Peptides can then be broken down into amino acids by other processes and then absorbed into the bloodstream

Question 12

Saprotrophic nutrition

Answer 12

Enzymes are secreted outside of the cell into food material followed by absorption of the products.
Fungal hyphae use this to decay and digest organic matter

Question 13

What factors affect the rate of enzyme activity?

Answer 13

• temperature
• pH
• substance concentrations
• enzyme concentration
• concentration of inhibitors

Question 14

How does temperature affect the rate of enzyme activity?

Answer 14

Increasing the temperature increases the kinetic energy of the particles, resulting in more frequent successful collisions between substrate and enzyme, increasing rate of reaction.

Question 15

What is the temperature coefficient (Q10)

Answer 15

A measure of how much the rate of reaction increases with a 10 degree rise in temperature. In enzyme controlled reactions, this is usually 2 (rate of reaction doubles when temperature rises by 10 degrees)
Q10 = R2/R1

Question 16

What is the optimum temperature and denaturation?

Answer 16

The optimum temperature is the temperature at which the enzymes have the highest rate of activity (human body = 40). when enzymes exceed the optimum temperature they begin to denature, as the bonds holding the protein together in the enzyme vibrate more until the bonds strain and break. This results in a change in the tertiary structure, meaning the substrate can no longer bind to the enzyme, so it can no longer function as a catalyst

Question 17

Enzymes in thermophiles

Answer 17

More stable than other enzymes due to an increased number of bonds in their tertiary structures (e.g. H bonds, sulfide bridges). Means they are more resistant to high temperatures

Question 18

What are thermophiles

Answer 18

Organisms adapted to living in very hot environments such as hot springs and deep hydrothermal vents

Question 19

How do Siamese cats provide visual evidence of the effect of temperature on enzyme activity?

Answer 19

Siamese cats produce a denatured form of the enzyme that catalyses the production of . This means it is inactive at normal body temperature, but the extremities of the cat are too cold to denature the enzyme, leading to colouration of just the extremities of the cats

Question 20

How does pH affect the rate of enzyme activity?

Answer 20

Enzymes have a narrow optimum pH range. A change in pH means a change in hydrogen ion concentration. At low pH, there will be more hydrogen ions, at a high pH, there will be fewer hydrogen ions. At a too high or too low pH, the hydrogen ion concentration will cause the tertiary structure to change shape, denaturing the enzyme

Question 21

What is the Vmax

Answer 21

The rate of enzyme activity remains constant because either the active sites or the substrate concentration becomes the limiting factor

Question 22

How does substrate concentration and enzyme concentration affect the rate of enzyme activity?

Answer 22

Increased number of substrates:
- higher collision rate with active site
- more enzyme-substrate complexes formed
Increased number of enzymes:
- higher number of active sites available
- formation of enzyme-substrate complexes at a faster rate

Question 23

Enzymes in organisms living in extreme cold environments

Answer 23

Enzymes have more flexible active sites (fewer H bonds and sulfide bridges) so there are more successful collisions

Question 24

What are inhibitors

Answer 24

A molecule that prevents enzymes from carrying out their normal function of catalysis, or slow them down

Question 25

What are the two types of inhibitors

Answer 25

• competitive inhibitors
• non-competitive inhibitors

Question 26

Competitive inhibition

Answer 26

• a molecule or part of a molecule that has a similar shape to the substrate of an enzyme can fit into the active site
• tis blocks the substrate from entering the active site
• the enzyme cannot carry out its function, so is inhibited
• this slows the rate of reaction
• most competitive inhibitors are reversible as they only bind temporarily

Question 27

Non-competitive inhibition

Answer 27

• the inhibitor binds to the enzyme at a location other than the active site (allosteric site)
• causes the tertiary structure to change, so the active site changes shape
• enzyme cannot carry out its function so is inhibited
• often irreversible

Question 28

Examples of competitive inhibitors

Answer 28

Statins:
- reduce blood cholesterol as they inhibit an enzyme involved in the synthesis of cholesterol
- reduce risk of heart disease
Aspirin:
- irreversibly inhibits COX enzymes
- prevents synthesis of thromboxane and prostaglandins, chemicals responsible for pain and fever

Question 29

Examples of non-competitive inhibitors

Answer 29

Organophosphates:
- insecticides and herbicides
- irreversibly inhibit acetyl cholinesterase, responsible for nerve impulse transmission
- can lead to paralysis and death if ingested
Proton pump inhibitors (PPIs):
- used to treat long-term indigestion
- irreversible block enzyme responsible for secreting H+ ions into the stomach
- reduces the production of excess stomach acid

Question 30

End-product inhibition

Answer 30

Inhibition that occurs when the product of a reaction acts as an inhibitor to the enzyme that produces it. It is a negative feedback mechanism that ensures excess products are not made and resources are not wasted

Question 31

Why do enzymes need to be controlled?

Answer 31

• prevent a build up of products (some waste products are toxic)
• conserve resources/substrate
• maintain optimum conditions
• so enzymes are only active when or where required
  e.g. protease could break down cell organelles

Question 32

Example of end-product inhibition

Answer 32

Respiration:
- produces ATP
- initial breakdown of glucose is catalysed by enzyme PFK, which is then competitively inhibited by ATP
- as ATP is used up, less binds to PFK and the enzyme is able to carry out its function, leading to the production of more ATP

Question 33

What is a metabolic poison

Answer 33

A substance that damages cells by interfering with metabolic reaction. Usually an inhibitor

Question 34

What are some examples of metabolic poisons

Answer 34

Cyanide:
- non-competitive and irreversible
- binds to c oxidase, inhibiting it
- no ATP is made
Arsenic:
- competitive
- inhibits pyruvate dehydrogenase

Question 35

What are cofactors?

Answer 35

Non-protein compounds required for enzyme activity
- inorganic cofactors
- prosthetic groups
- coenzymes (organic cofactor)

Question 36

What are coenzymes

Answer 36

Organic cofactors
- do not bind permanently
- often transport molecules or electrons between enzymes e.g. NAD
- frequently derived for water-soluble vitamins found in the diet e.g. vitamin B3

Question 37

What are inorganic cofactors?

Answer 37

• obtained via the diet as minerals
• often metal ions
  e.g. amylase contains a chloride ion (Cl-) that is necessary for the formation of a correctly shaped active site to catalyse the breakdown starch

Question 38

What are prosthetic groups?

Answer 38

• tightly bound cofactors acting as a permanent part of the enzyme. e.g. Zinc ions form an important part of carbonic anhydrase, necessary for the metabolism of carbon dioxide

Question 39

Precursor activation

Answer 39

• many enzymes are produced in inactive form (inactive precursor enzymes)
• they often need to undergo a change in their tertiary structure (to change active site) in order to be activated
• this can be achieved by the addition of a cofactor

Question 40

Why are many enzymes produced as inactive precursor enzymes?

Answer 40

• particular enzymes can cause damage inside the cells producing them or to tissues where they are released
• enzymes that need to be controlled and only activated under certain conditions

Question 41

What are inactive precursor enzymes without a cofactor attached called?

Answer 41

apoenzyme

Question 42

What is the inactive precursor enzymes called when a cofactor is added and it is activated?

Answer 42

holoenzyme

Question 43

How are precursor enzymes sometimes activated without the use of a cofactor?

Answer 43

• The action of another enzyme such as protease which cleaves certain bonds in the molecule
• a change in conditions such as pH or temperature

Question 44

What are precursor enzymes which are activated without the use of a cofactor called?

Answer 44

zymogens or proenzymes

Question 45

what is an example of a zymogen/proenzyme

Answer 45

Pepsinogen is inactive, then is released into the stomach to digest proteins. The acidic pH transforms pepsinogen into the active enzyme pepsin. This protects other body tissues against the digestive actions of pepsin