1.4.2 Enzymes

Question 1

Why are enzymes proteins?

Answer 1

Only proteins have such a wide variety of shapes because proteins are made up of 20 amino acids, all with different combinations and tertiary bonds determining their specific 3D shape

Question 2

What is the structure of enzymes?

Answer 2

• Tertiary proteins
• Globular shape
• has an active site (pocket or cleft area which has a specific 3D shape) where the reaction takes place

Question 3

What is the function of enzymes?

Answer 3

• biological catalysts - Proteins that speed up metabolic reactions without being used up
• provide an alternative pathway during a reaction

Question 4

The specific substrate has a ………….. shape to the active site

Answer 4

The specific substrate has a complementary shape to the active site

Question 5

What is enzymes role in digestion?

Answer 5

They hydrolyse polymers into monomers
E.g polypeptides into amino acids

Question 6

An individual cell contains over a ….. different enzymes which are involved in ……… reactions such as hydrolysis reactions in digestion and ……………. and respiration

Answer 6

An individual cell contains over a 1000 different enzymes which are involved in metabolic reactions such as hydrolysis reactions in digestion and photosynthesis and respiration

Question 7

what is the equation linking substrate and product

Answer 7

Question 8

how can you tell if something is an enzyme from the name?

Answer 8

the name of the enzyme is usually derived from its substate and it has the suffix ‘ase’

Question 9

Name the reaction and biological importance for the following enzymes:
- lactase
- catalase
- RUBISCO
- ATPsynthase
- Glycogen synthetase

Answer 9

Question 10

What is activation energy?

Answer 10

The extra energy that is required to enable a reaction to occur
(Often supplied through heat in a laboratory reaction)

Question 11

How do enzymes speed up the rate of a reaction?

Answer 11

Enzymes are biological catalysts that are able to lower the activation energy of a reaction allowing it to proceed more quickly at lower temperatures in the body so that molecules can be broken down and new ones formed. Enzymes produced and alternative route during a chemical reaction, lowering the activation energy

Question 12

What is the lock and key model?

Answer 12

Enzymes have a specific shaped active site which is complementary to the shape of the substrate molecules being used in the reaction

Question 13

What is the induced fit model?

Answer 13

• the substrate, not complementary, collides and binds with the active site
• the active site shape changes slightly to fit more closely and becomes complementary to the substrate molecule and is held in position by oppositely charged R groups - the tertiary structure of the enzymes active site changes
• an enzyme-substrate complex is formed
• a change in the enzymes active site shape places a strain on the ionic/hydrogen bonds in the substrate molecule (so they are more likely to break or form) allowing the reaction to occur more easily
• this strain lowers the activation energy needed for the reaction to occur
• an enzyme-product complex now forms
• the product no longer fits into the active site
• product is released. The enzyme can catalyse another reaction

Question 14

What is the rate of reaction like at low temperatures?

Answer 14

Rates of reactions are very slow at low temperatures as enzyme and substrate molecules have little kinetic energy and few enzyme substrate collisions take place

Question 15

What happens to the rate of reaction as temperature increases and why?

Answer 15

• The kinetic energy of substrate and enzyme molecules increases
• more successful collisions between enzyme and substrate
• meaning more enzyme - substrate complexes form
• rate of reaction increases initially

Question 16

What is the v max (regarding temperature)?

Answer 16

When the rate of reaction reached its maximum - the optimum temperature of the enzyme reaction

Question 17

What happens to the rate of reaction if you increase the temperature further after v max is reached?

Answer 17

• Molecules vibrate more which causes weaker bonds (hydrogen and ionic) holding the enzyme 3D structure to break
• tertiary shape of the enzyme will be altered which may alter the shape of the active site
• fewer enzyme- substrate complexes will form and rate of reaction will decrease
• the enzyme is described as being denatured

Question 18

Different enzymes have ……… optimum temperatures. The optimum temperature is likely to be related to the organisms ………… temperature

Answer 18

Different enzymes have different optimum temperatures. The optimum temperature is likely to be related to the organisms environmental temperature.

Question 19

What is pH?

Answer 19

A measure of hydrogen ion concentration.
• The greater the concentration of H+ ions the lower the pH
• The greater the concentration of OH- the higher the pH

Question 20

What is the optimum pH?

Answer 20

The pH value at which the date of an enzyme controlled reaction is at its maximum

Question 21

What happens to the rate of reaction above and below the optimum pH and why?

Answer 21

• H+ will cluster around negatively charged R groups and disrupt the hydrogen bonds and ionic bonds
• the tertiary structure of a protein changes and therefore the shape of the active site changes
• fewer successful collisions
• fewer enzyme- substrate complexes form
• this will reduce the rate of an enzyme controlled reaction

Question 22

Describe the process of an enzyme denaturing with ph

Answer 22

• in extremes of pH, the enzyme denatures
• H+ concentration alters the charges around the active site
• the hydrogen/ ionic bonds break
• the tertiary structure is altered- changing the shape of the active site
• the substrate can no longer bind to the active site and no enzyme- substrate complexes can form
• the rate of reaction falls to 0

Question 23

Do didfernt enzymes have the same or differnt optimum pH conditions?

Answer 23

Differnt
eg pepsin is a protease found in the stomach which has a pH of 2

Question 24

When measuring the effect of pH on an enzyme controlled reaction, what is used?

Answer 24

pH buffer solutions of different pHs can be used as pH needs to be controlled

Question 25

How do you calculate the pH of a solution?

Answer 25

pH = -log10[H+] e.g. a h+ concentration of 1x10^-9 has a pH of… pH = -log10( 1x10^-9) = 9

Question 26

What are the 2 main ways of measuring rate of reaction?

Answer 26

After a fixed time interval measure either: 1. The concentration of product formed 2. Substrate used up

Question 27

How can the rate of reaction be calculated from the gradient of a graph (showing volume of product formed or substrate catalysed against time) ?

Answer 27

- Draw a tangent at the point mentioned in the question - make a triangle (the bigger the more accurate) - calculate the gradient using the change in Y / change in x

Question 28

What is the initial rate of a reaction?

Answer 28

When the rate of reaction is at its highest at the first few seconds, when the concentration of substrate molecule did the highest and therefore more enzyme- substrate collisions and so more enzyme- substrate complexes are being formed

Question 29

How do you calculate the initial rate?

Answer 29

Draw a tangent at zero then read off a point

Question 30

As more substrate molecules are converted into product there are ….. substrate molecules to bind with the enzymes active sites and the reaction will get ….. until it finally ….

Answer 30

As more substrate molecules are converted into product there are **fewer** substrate molecules to bind with the enzymes active sites and the reaction will get **slower** until it finally **stops**

Question 31

How can the rate be calculated measuring the decrease in substrate over time?

Answer 31

Question 32

What is a limiting factor?

Answer 32

A variable that prevents V max from being achieved. If it is increased, then the rate of the process will increase *can only ever be one limiting factor*

Question 33

What is the effect of increasing substrate concentration on the rate of an enzyme catalysed reaction?

Answer 33

• If the conc of enzyme remains constant and conc of substrate increases the number of collisions between the enzyme and substrate molecules will increase • more ESC forming • rate of reaction increases • a point will be reached when all the enzymes active sites are occupied at all times, so increasing the substrate concentration will not increase the rate of reaction any further so it remains at a maximum rate (V max) • the concentration of enzymes is said to be limiting the rate of reaction

Question 34

What is the effect of increasing enzyme concentration on the rate of an enzyme catalysed reaction?

Answer 34

If conc of substrate is kept constant and the conc of enzyme is increased each time the experiment is carried out then the rate of the reaction will increase and then remain constant at a maximum rate

Question 35

Why would the initial rate of reaction increase if you increased enzyme concentration?

Answer 35

• more active sites available • greater chance that the substrate will collide with active site • more ESC form • all the substrate molecules are occupying active sites • initial rate of reaction will increase

Question 36

Why would a further increase in enzyme concentration when all substrate molecules are occupying active sites not increase the rate of reaction anymore?

Answer 36

The concentration of substrate limits any further increase - V max is achieved

Question 37

What do enzyme inhibitors do? What are the two types?

Answer 37

Reduce the rate of the reaction Two types: 1. Competitive 2. Non - competitive

Question 38

Explain how competitive inhibitors work

Answer 38

• have a similar shape to the substrate molecule and **must be complementary** to the active site of the enzyme • they bind with active site forming an enzyme- inhibitor complex and prevent the normal substrate from entering by blocking the active site • the number of ESC are reduced and rate of reaction falls

Question 39

For competitive inhibitors: what happens to the level of inhibition If the concentration of substrate is increased?

Answer 39

Decreases since this will increase the chance of normal substrate binding with the active site

Question 40

Explain how non competitive inhibitors work

Answer 40

• inhibitor (can be a completely different shape to substrate) attaches at the allosteric site • distorting the tertiary shape of the enzyme leading to a **change in the shape of the active site** • substrate no longer fits the active site • ESCs cannot form • rate of reaction decreases

Question 41

For non competitive inhibitors: what happens to the level of inhibition If the concentration of substrate is increased?

Answer 41

Will not increase the rate of reaction

Question 42

Why are some inhibitors reversible?

Answer 42

Many inhibitors do not permanently bind with the active or allosteric sites whereas some do and are therefore non reversible