1.4 Enzymes

Question 1

define metabolism

Answer 1

the sum of all the enzyme controlled chemical reactions taking place in a cell.

Question 2

state the two main types of reactions that make up metabolism.

Answer 2

Anabolic and Catabolic reactions.

Question 3

what is anabolism?

Answer 3

a set of metabolic pathways that synthesise complex molecules from smaller, simpler molecules.

Question 4

what is catabolism?

Answer 4

a set of metabolic pathways that breakdown complex molecules into smaller, simpler molecules.

Question 5

what is an enzyme?

Answer 5

a biological catalyst used to speed up the rate of intracellular or extra cellular biochemical reactions

not used up or permanently altered

an enzyme is proteins compromised of amino acids linking together in one or more polypeptide chain

Question 6

what is an intracellular enzyme?

Answer 6

an enzyme that acts within cells e.g. catalase.

Question 7

what is an extracellular enzyme?

Answer 7

an enzyme that is secreted by cells and functions outside of cells e.g. amylase.

Question 8

what is the active site of an enzyme?

Answer 8

A region on an enzyme that is complimentary to the shape of a specific substrate. The substrate binds and the reaction takes place.

Question 9

why is the active site described as ‘specific’?

Answer 9

the 3D structure of each enzyme (including active site) is unique due to the presence of different side chains and branches.

only specific substrates complementary to the active site can bind.

Question 10

define activation energy

Answer 10

the minimum amount of energy needed for a reaction to take place

Question 11

what is catalysis?

Answer 11

an increase in the rate of a chemical reaction using a catalyst (such as an enzyme)

the catalyst lowers the activation energy of the reaction.

Question 12

describe the ‘lock and key’ model.

Answer 12

substrate(s) and the active site of the enzyme come into contact

substrate(s) binds, enzyme-substrate complex forms

reaction takes place, product(s) formed in an enzyme-product complex

product(s) released from the active site. the active site is now free to bind to another substrate.

Question 13

what is the induced-fit hypothesis?

Answer 13

a model of enzyme action which states that once a specific substrate binds to the active site, the enzyme undergoes subtle conformational changes. This puts a strain on the substrate, lowering the activation energy for the reaction.

Question 14

what factors affect the rate of an enzyme-controlled reaction?

Answer 14

temperature
pH
substrate concentration
enzyme concentration

Question 15

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

Answer 15

as temp increases molecules have more KE

molecules moves faster and collide more frequently

more enzyme-substrate complexes form

rate of reaction increases

rate peaks at the optimum temp

Question 16

explain how increasing temperature above the optimum affects the rate of an enzyme-controlled reaction.

Answer 16

temp increases above optimum

increased vibrations break hydrogen and ionic bond in tertiary structure

active site changes shape,enzyme is denatured

no more enzyme-substrate complexes can form

rate of reaction decreases

Question 17

how does pH affect the rate of enzyme-controlled reactions?

Answer 17

enzymes have an optimum pH
pH shifts from the optimum
hydrogen and ionic bonds in tertiary structure are altered
interaction if polar and charged R-groups changes
active site changes shape, enzyme is denatured
rate of reaction decreases

Question 18

what is a buffer?

Answer 18

a molecule that maintains a constant pH in a solution when small amounts of acid (H+) or a base (OH-) are added.

Question 19

how does substrate concentration affect the rate of an enzyme-controlled reaction?

Answer 19

if enzyme concentration is fixed, the rate of reaction increases proportionally to the substrate concentration.

Once all active sites become full, the rate of reaction becomes constant - graph plateaus (enzyme concentration is a limiting factor)

Question 20

how does enzyme concentration affect the rate of an enzyme-controlled reaction?

Answer 20

if substrate concentration is fixed, the rate of reaction increases proportionally to the enzyme concentration.

When all of the substrates occupy active sites, the rate of reaction remains constant - graph plateaus (substrate concentration is a limiting factor)

Question 21

what is a competitive inhibitor?

Answer 21

a molecule which binds to the active site of an enzyme, blocking it and preventing the substrate from binding.

Question 22

is competitive inhibition temporary or permanent?

Answer 22

competitive inhibition is generally temporary. However, in some cases (e.g. aspirin) it may be permanent.

Question 23

how does increasing substrate concentration affect competitive inhibition?

Answer 23

increase in substrate concentration
more substrate than inhibitor
rate of reaction increases

Question 24

what is a non-competitive inhibitor?

Answer 24

an inhibitor which binds to a different part of an enzyme, the allosteric site.
the tertiary structure of the enzyme (including the active site) changes shape.
the active site is no longer complimentary to the substrate. The substrate cannot bind and the enzyme is inhibited.

Question 25

is non-competitive inhibition temporary or permanent?

Answer 25

permanent

Question 26

how does increasing substrate concentration affect non-competitive inhibition?

Answer 26

increasing the substrate concentration will not overcome the effect of the non-competitive inhibitor.

Question 27

what are immobilised enzymes?

Answer 27

enzymes which are attached to an inert, insoluble material over which the substrate passes and the reaction takes place.

Question 28

give an example of an application of immobilised enzymes.

Answer 28

Biosensors

Question 29

Why are immobilised enzymes important in industrial processes?

Answer 29

enables enzymes to be reused
improved enzyme stability in variable/extreme temperatures and pH
increases the efficiency of reactions

Question 30

What is the theory of induced fit as illustrated by lysozyme?

Answer 30

1. The induced fit theory is an alternative theory of enzyme action - lysozyme is proposed to function in this way.
2. In this,the active site and substrate are not fully complementary in shape.
3. Reactive groups in these areas align and the substrate forces its way into the active site.
4. Both areas change structure slightly, the bonds in the substrate weakens and the reaction occurs at a lower activation energy.

Question 31

What are the four factors that affect enzyme activity?

Answer 31

1. Enzyme concentration
2. Substrate concentration
3. Temperature
4. pH

Question 32

How is a control set up in a practical measuring enzyme activity?

Answer 32

Replace the enzyme solution with distilled water or boiled enzyme solution.

Question 33

Outline the practical procedure used to find the effect of substrate concentration on enzyme activity, using hydrogen peroxide and catalase.

Answer 33

1. Grind a piece of potato cylinder with 5 cm³ of distilled water to make a paste.
2. Transfer 10 cm³ of hydrogen peroxide to a test tube.
3. Dip a filter paper disc into the enzyme solution, and place the disc into the hydrogen peroxide solution and measure the time that taken to float up to the surface from touching the bottom.
4. Remove the disc from the tube using forceps and discard.
5. Repeat steps 1-4 using at least 4 other concentrations of hydrogen peroxide.

Question 34

State the word equation for the action of catalase on hydrogen peroxide.

Answer 34

Hydrogen peroxide → Oxygen + Water

Question 35

How is the rate of reaction calculated from time?

Answer 35

Rate of reaction = 1/time

Question 36

What is the effect of enzyme concentration on enzyme activity?

Answer 36

As the concentration of enzyme increases, successful collisions to form ES complexes increase, so the rate of reaction increases to an optimum.

Beyond the optimum, the rate plateaus as substrate concentration becomes limiting.

Question 37

What is the effect of substrate concentration on enzyme activity?

Answer 37

Enzyme activity increases initially as substrate concentration increases, as substrate concentration is limiting, and higher concentration results in more successful collisions to form ES
complexes.

Beyond a certain substrate concentration, enzyme activity plateaus, as all the enzyme active sites are saturated and enzyme concentration is limiting.

Question 38

How does phenolphthalein indicate the rate of enzyme activity (using milk and lipase)?

Answer 38

As lipase digests lipids in milk into fatty acids (and glycerol), the pH of the solution decreases, causing a colour change from pink to colourless at the end point. The shorter the time, the faster the rate.

Question 39

How is the effect of temperature on lipase activity measured?

Answer 39

Prepare water baths with a range of temperatures. Place 1 cm³ of lipase solution, 5 cm³ of milk and 7 cm³ of sodium carbonate in a water bath in separate test tubes.

Leave for 10 minutes.

Mix together. Start the stopwatch and measure the time taken for the pink colour to disappear. Repeat for at least 4 other temperatures.

Question 40

What is the effect of temperature on enzyme activity?

Answer 40

Increasing temperature increases enzyme activity to an optimum.

Enzyme and substrate molecules gain kinetic energy and move faster, so there are more successful collisions to form ES complexes.

Beyond the optimum temperature, enzyme activity decreases as the high temperature disrupts the tertiary structure of enzymes and denatures them.

Question 41

How is the effect of pH on enzyme activity investigated?

Answer 41

Add a fixed volumes of buffer solutions with a range of pH values to 1 cm³ of trypsin and 2 cm³ of milk, both of a fixed concentration.

Record the absorbance immediately and every 15 seconds for 5 minutes.

Question 42

What is the effect of pH on enzyme activity?

Answer 42

Enzyme activity is highest at the optimum pH.

Above or below the optimum pH, enzyme activity decreases as the unsuitable pH disrupts its tertiary structure and changes the shape of its active site, causing partial denaturation.

Complete denaturation may occur at extreme pH values.

Question 43

State a hazard and safety precaution involved in this practical.

Answer 43

Students may have allergic reactions to enzymes, so avoid contact with skin and eyes, wear eye protection.