Enzymes

Question 1

How do enzymes increase the rate of metabolic reactions in the body

Answer 1

They are biological catalysts- they catalyse reactions by creating an alternative reaction pathway with a lower activation energy.
They do this by binding the substrate to the enzymes active site to form an enzyme-substrate complex. This bends/weakens bonds in the substrate meaning a lower activation energy is then needed to break these bonds.

Question 2

What conditions must be satisfied for a metabolic reaction to take place

Answer 2

• the molecules must collide with sufficient energy to alter their structures
• the free energy of the products must be less than the substrates
• there must be the activation energy present

Question 3

Describe the features of an enzyme

Answer 3

• enzymes are biological catalysts that catalyse metabolic reactions
• enzymes are globular proteins
• enzymes are highly specific due to their tertiary structure
• enzymes can affect structures in an organism as well as functions.

Question 4

What is activation energy

Answer 4

The minimum energy required for a chemical reaction to occur

Question 5

What is the enzyme-substrate complex

Answer 5

An enzyme-substrate complex is formed when the substrate has bonded to the enzymes active site. It is held in place by bonds that temporarily form between certain amino acids on the active site and chemical groups on the substrate molecule.

Question 6

Define active site

Answer 6

the active site is a depression on the surface of an enzyme where the substrate binds.

Question 7

Describe the lock and key model of enzyme action

Answer 7

• enzymes have an active site with a rigid structure that is complementary to the substrate they bind to
• this forms an enzyme substrate complex which forms products from the substrate.

Question 8

Describe the induced fit model of enzyme action

Answer 8

• the substrate collides with the enzymes active site.
• the tertiary structure (shape) of the enzymes active site is specific and combines with complementary substrates.
• When the substrate binds to the active site, the active site changes shape slightly to better fit the substrate.
• this is held in position by oppositely charge R groups and an enzyme-substrate complex forms.
• The change in shape of the active site puts strain on the substrate bonds and bens/weakens them which lowers the activation energy needed to then break them.

Question 9

What is the key limitation of the lock and key model of enzyme action

Answer 9

That the enzyme was considered to have a rigid structure

Question 10

What are the properties of an enzyme and how do these relate to its tertiary structure

Answer 10

• enzymes are proteins
• the shape of the active site is determined by the enzymes tertiary structure.
• the tertiary structure is determined by the enzymes primary structure (base sequence of amino acids)
• if the tertiary structure of an enzyme were to change, this could change the shape of the enzymes active site and this could prevent it from carrying out its function as the active site would no longer be complementary to the substrate.

Question 11

What is the equation for rate of reaction

Answer 11

Rate of reaction= Amount of reactant used or amount of product formed/ time

Question 12

List the four factors that affect the rate of enzyme action

Answer 12

• Temperature
• Concentration of substrate
  -Concentration of enzyme
• Inhibitors
• PH

Question 13

Describe the affect of temperature on enzyme action

Answer 13

• A rise in temperature increases the kinetic energy of the molecules which means that in an enzyme catalysed reaction, the enzyme and the substrate collide more often in a given time.
• This increases the number of successful collisions and therefore more enzyme-substrate complexes are formed and so the rate of the reaction increases.
• However, the rise in temperature also causes the hydrogen bonds and disulphide bridges that hold the tertiary structure of the enzyme together to begin to break which causes the tertiary structure of the enzyme to change- meaning the enzyme changes shape.
• At first this slows the rate of reaction as the substrate fits less easily into the active site but eventually the enzymes shape has changed so much that it stops working altogether- this is called denaturation.
• At first increase temperature increases rate of reaction but then it decreases- giving each enzyme an optimum temperature to function at and creating the shape of a curve on a graph representing this information.

Question 14

In humans, when do the hydrogen bonds and disulphide bridges first begin to break and alter the shape of an enzyme

Answer 14

At around 45 degrees Celsius

Question 15

In humans, at what temperature do enzymes usually become fully denatured

Answer 15

60 degrees Celsius

Question 16

What is the optimum temperature for many enzymes in the human body

Answer 16

40 degrees Celsius

Question 17

Why has the human body evolved to have a body temperature of 37 degrees (not 40 degrees that is optimum for many enzymes)

Answer 17

• Although slightly higher body temperatures would increase the metabolic rate slightly more energy would be required to maintain this.
• Other proteins, apart form enzymes, may denature at higher temperatures
• At higher temperatures of 40 degrees, any increase in temperature, for example during illness, may denature the enzymes.

Question 18

Describe how PH affects rate of enzyme action

Answer 18

Similar to temperature, each enzyme me has an optimum PH and any move away from this (increase or decrease) decreases the rate of enzyme action by changing the enzymes shape.
If the change in PH is extremely drastic, it can fully denature the enzyme.

Question 19

Describe how a change in PH can decrease the rate of an enzyme catalysed reaction

Answer 19

• The PH of a solution is a measure of its hydrogen ion concentration.
• The arrangement of the active site is partly determined by the hydrogen and ionic bonds between -NH2 and -COOH groups of the polypeptides that make up the enzyme. The change in H+ ions affects this bonding, causing the active site to change shape.
• A change in PH alters the charges on the amino acids that make up the active site of the enzyme. As a result, the substrate can no longer bind to the active site and so the enzyme-substrate complex cannot be formed.
• Depending on how significant the change in PH is, it may cause the bonds maintaining the enzymes tertiary structure to break, therefore changing the shape of the enzyme and its active site.

Question 20

Why is an enzymes denaturation unlikely to be due to a change in PH

Answer 20

Because the PH fluctuations in organisms are usually very small, meaning they are far more likely to reduce an enzymes activity than denature it.

Question 21

Explain why enzymes can work well at low concentrations

Answer 21

Enzymes are catalysts and therefore are not used up in the chemical reaction. This means that after the enzyme has acted on a substrate, it can repeat the process again and again. This means a small amount of enzymes can act on many substrates.

Question 22

How does increase in enzyme concentration affect the rate of an enzyme catalysed reaction when there is excess substrate.

Answer 22

When there is excess substrate, increasing enzyme concentration causes a proportionate increase in the rate of the reaction. This is because when there is excess substrate, there are more substrate molecules than active sites that can act on them. Therefore increasing enzyme concentration allows some of the excess substrate to be acted on and so the rate of the reaction increases.

Question 23

How does increasing enzyme concentration affect the rate of an enzyme catalysed reaction when the substrate is limiting.

Answer 23

When the substrate is limiting, increasing enzyme concentration has no affect on the rate of the reaction. This is because all of the substrates would already be being acted on by an enzyme so increasing enzyme concentration would have no affect. This causes the rate of reaction to stabilise and the graph to level off.

Question 24

Describe the affect of substrate concentration on the rate of an enzyme controlled reaction

Answer 24

• If the concentration of an enzyme is fixed and substrate concentration is slowly increased, the rate of reaction increases in proportion to the increase in substrate because at low substrate concentrations, enzymes only have a limited number of substrate molecules to collide with, and therefore the active sites of the enzymes are not working at full capacity.
• As more substrate is added, the active sites become more and more filled until the rate of reaction is at its maximum (Vmax)
• After that, the addition of more substrate has no affect on the rate of the reaction.
• When there is excess substrate, the rate of reaction levels off.

Question 25

What is an enzyme inhibitor

Answer 25

A substance that reduces an enzymes activity by interfering with how the active site functions.

Question 26

What are the two types of enzyme inhibitor

Answer 26

Competitive
Non-competitive

Question 27

Describe how competitive inhibitors affect enzyme action

Answer 27

• Competitive inhibitors have a molecular shape similar to that of the substrate.
• This allows them to occupy the active site of an enzyme and they therefore compete with the enzyme for the available active sites.
• It is the difference between the concentration of the inhibitor and the concentration of the substrate that determines the affect this has on enzyme activity.

Question 28

How do you reduce the affect of a competitive inhibitor

Answer 28

Increase substrate concentration

Question 29

Describe how non-competitive inhibitors affect enzyme action

Answer 29

• non-competitive inhibitors attach to the enzyme at a binding site that is not the active site.
• Upon attaching to the enzyme, the inhibitor alters the shape of the enzyme and thus its active site in such a way that substrate molecules can no longer occupy it and therefore the enzyme cannot function.

Question 30

Can the affect of non-competitive inhibitors be reduced by increasing substrate concentration

Answer 30

No. As the substrate and inhibitors are not competing for the same site, an increase in substrate concentration does not decrease the effect of the inhibitor.