3.1.4.2 many proteins are enzymes

Question 1

What is an enzyme?

Answer 1

Enzymes are globular, tertiary structure proteins, which act as biological catalysts in all organisms.

Question 2

What is an active site?

Answer 2

The part of an enzyme that binds to the substrate. It has a specific and complementary shape to the shape of the substrate it binds to. They bind to form an enzyme substrate complex.

Question 3

What determines the shape of the active site?

Answer 3

-This is due to the specific folding and bonding in the tertiary structure of the protein. -The tertiary structure is determined by the sequence of amino acids, as they determine where bonds form, and therefore how the polypeptide chain folds.
-The sequence of amino acids form hydrogen bonds during folding, which cause it to further fold into a unique 3D shape

Question 3

Define activation energy

Answer 3

Activation energy is the minimum energy required in order for a reaction to take place.

Question 3

How do catalysts speed up the rate of reaction?

Answer 3

-all reactions require enough activiation energy to take place
Catalysts, and therefor enzymes, provide an alternative reaction pathway which has a lower activation energy.

Question 3

Describe the lock and key model

Answer 3

-the enzyme active site is a fixed and complementary shape to the substrate
-the active site binds to the substrate through random collisions
-this forms an enzyme-substrate complex
-the enzyme catalyses the reaction, but the enzyme is left unchanged to be used again.

Question 4

Why are enzymes important?

Answer 4

They catalyse a wide range of intra and extra cellular reactions that determine structure and function from cellular to organism level.

Question 5

How do enzymes speed up the rate of reaction in organisms?

Answer 5

-holding two molecules together closely reduces repulsion between them so they bond more easily
-fitting into the active site of an enzyme puts strain on the bonds in the substrate so it breaks up more easily.

Question 6

Describe the induced fit model (accepted model for how enzymes function)

Answer 6

-induced fit is when the enzymes active site is induced, or slightly changes shape, to mould around the substrate. It undergoes conformational changes.
-when the enzyme-substrate complex occurs and the enzyme moulds around the substrate, it puts strain on the hydrogen and ionic bonds, which lowers the activation energy
-the products are removed, and then enzymes active sites are returned to the original shape

Question 7

Name the 5 factors that affect enzyme activity?

Answer 7

-temperature
-pH
-substrate concentration
-enzyme concentration
-inhibitors

Question 7

Describe how low temperatures affect enzyme activity

Answer 7

-If the temperature is too low, there is not enough kinetic energy for successful collisions between the enzymes active site and substrate
-this means that the frequency of successful collisions decreases
-therefore fewer enzyme-substrate complexes form
-this means the rate of enzyme activity decreases

Question 8

Describe how high temperatures affect enzyme activity?

Answer 8

-if the temperature is too high, the enzymes active site denatures (changes shape) so that it is no longer complementary to the substrate
-this means that enzyme substrate complexes do not form
-this decreases the rate of enzyme activity

Question 9

How does denaturation occur due to temperature?

Answer 9

-denaturation occurs when the hydrogen bonds between amino acids that hold the enzyme in its tertiary structure are broken
-this causes the tertiary structure to permanently change shape
-denaturation has occurred if the substrate can no longer bound to the active site

Question 10

Describe how a low pH affects enzyme activity

Answer 10

-a low pH (acidic) means that the solution has an excess of H+ ions, that are able to break the ionic/hydrogen bonds between amino acids that hold the tertiary structure of the enzyme together (by interfering with the charges)
-this causes the enzymes active site to denature, so its shape is no longer complementary to the substrate
-an enzyme substrate complex cannot be formed
-this decreases the rate of enzyme activity

Question 11

Describe how a high pH affects enzyme activity

Answer 11

-a high pH (alkali) means that the solution has an excess of OH- ions, that are able to break the ionic/hydrogen bonds between amino acids that hold the tertiary structure of the enzyme together (by interfering with the charges)
-this causes the enzymes active site to denature, so its shape is no longer complementary to the substrate
-an enzyme substrate complex cannot be formed
-this decreases the rate of enzyme activity

Question 12

Describe how substrate concentration affects enzyme activity

Answer 12

-the greater the concentration of substrate, the greater the likelihood of successful collisions occurring
-this means that the frequency of successful collisions increases, so more enzyme-substrate complexes are formed
-this means the rate of enzyme activity increases

-Once the substrate concentration is increased past a certain point, all available active sites become saturated (full).
-therefore, past the saturation point, the rate of enzyme activity no longer increases, because the added substrate molecules have nowhere to bind to as all active sites are saturated.

Question 13

Describe how enzyme concentration affects enzyme activity

Answer 13

-the higher the concentration of enzymes in a reaction mixture, the greater the number of active sites available to bind to the substrate
-this means more enzyme-substrate complexes form
-this is also due to more frequent successful collisions between the substrate and enzymes active site
-therefore the rate of enzyme activity increases

-however, past a certain point, there will be insufficient substrate, meaning the rate of enzyme activity remains the same
-this is because the enzyme is in excess, meaning there is not enough substrate for the active site to bind to
-therefore, no more enzyme-substrate complexes are formed, so the rate of enzyme activity remains constant

Question 14

What is a competitive inhibitor?

Answer 14

Competitive inhibitors are the same shape as the substrate, and can bind to the enzymes active site, which prevents the substrate from binding to it, to form an enzyme-substrate complex.

Question 15

Describe how competitive inhibitors affect the rate of enzyme activity

Answer 15

-inhibitors slow down the rate of enzyme activity, because there are less available active sites for the substrates to bind to
-fewer enzyme-substrate complexes are formed, which slows the rate of enzyme activity

-however, by increasing the substrate concentration, the rate of reaction increases
-this is because a higher concentration of substrate means the frequency of successful collisions increases, as a high concentration of substrate can knock the inhibitors out of the active sites.
-this means more enzyme substrate complexes are formed, which increases the rate of enzyme activity

Question 16

What is a non-competitive inhibitor?

Answer 16

Non-competitive inhibitors bind to the enzyme at a site other than the active site (allosteric site). This causes the active site to change shape, preventing the substrate from binding to it, to form an enzyme-substrate complex.

Question 17

How do non-competitive inhibitors affect enzyme activity?

Answer 17

-non-competitive inhibitors decrease the rate of enzyme activity, regardless of how much substrate is added
-the additional substrate does not knock out the inhibitor, so the active site shape remains altered, and not complementary to the shape of the substrate
-means no more enzyme-substrate complexes can be formed
-therefore the rate of reaction remains the same

Question 18

Describe what end-point inhibition is

Answer 18

End-point inhibition is the process whereby non-competitive inhibitors can act as regulators in the control of metabolic pathways. By acting as an inhibitor, the end product of a series of reactions can prevent its own production.

Question 19

Describe the process of end-point inhibition

Answer 19

-enzyme 1 binds to substrate to produce product 1
-enzyme 2 binds to product 1 to produce product 2
-enzyme 3 binds to product 2 to produce product 3 (inhibitor)
-product 3 binds to the alternative site of enzyme 1, which inhibits the whole reaction from taking place again (as it changes shape of the active site of enzyme 1)

NOTE: this is a reversible reaction as the inhibitor can detach from the enzyme, so the active site reforms and becomes active again