B4 - Enzymes

Question 1

What are enzymes?

Answer 1

• globular proteins that interact with substrate molecules
• they act as catalysts to metabolic reactions in living organisms

Question 2

What is a metabolic reaction?

Answer 2

• it is the sum of an anabolic (building up) and a catabolic (breaking down) reaction
• can only take place as a result of the control/order imposed by enzymes

Question 3

Example of an anabolic reaction

Answer 3

• cellulose forming the walls of plant cells
• long protein molecules forming contractile filaments of muscles in animals

Question 4

Example of a catabolic reaction

Answer 4

• growth (living processes)
• energy released from large organic molecules (glucose) in metabolic pathways

Question 5

What is ‘Vmax’?

Answer 5

• maximum initial velocity of the enzyme catalysed reaction
• enzymes can only increase the rates of reaction up to this point

Question 6

What are intracellular enzymes?

Answer 6

• a type of enzyme that aids the metabolic processes inside the cells that secreted them

Question 7

What is an example of an intracellular enzyme?

Answer 7

• catalase ensures that hydrogen peroxide is broken down into oxygen and water
• benefits = it is time efficient so it minimizes the risk of damage

Question 8

What are extracellular enzymes?

Answer 8

• a type of enzyme that works outside of the cell that secreted them
• cells require substrates (raw materials) to make products needed by the organism
• the nutrients are often in forms of large molecules and need to be broken down
• enzymes are released from the cells to break down these large nutrient molecules

Question 9

What are examples of extracellular enzymes?

Answer 9

• amylase (salivary glands/pancreas) is a digestive enzyme that breaks down starch polymers into maltose
• maltose is then broken down into glucose by maltase
  ‎
• trypsin (protease) catalyses the digestion of proteins into smaller peptides
• produced in pancreas and released into small intestine
• amino acids are produced and absorbed into the bloodstream

Question 10

What are the properties of enzymes?

Answer 10

• tertiary structure (interactions between R-groups)
• active site is complementary to the shape of substrate molecule
• have the ability to reduce the activation energy required for a reaction to occur
• activity is affected by temp., pH, enzyme conc. and substrate conc.

Question 11

What is the lock and key hypothesis?

Answer 11

• idea that only a specific substrate will fit the active site of an enzyme
• forms an enzyme-substrate complex
• the substrates react and form an enzyme-product complex (products are then released)
• the right atom-groups and the R-groups (within active site) will react/interact with the substrate, forming temporary bonds

Question 12

What is the induced-fit hypothesis?

Answer 12

• recent research evidence has shown that the active site of an enzyme slightly changes its shape as substrate enters
• the initial interaction is relatively weak but they rapidly induce changes in the enzyme’s tertiary structure
• this weakens a bond/bonds in the substrate and lowers the activation energy

Question 13

What factors affect enzyme activity?

Answer 13

• temperature
• pH
• substrate/enzyme conc.

Question 14

How does temperature affect enzyme activity?

Answer 14

• increase = increases K.E. of particles which causes them to collide more frequently (increases rate of reaction)
• Q10 is a measure of how much the rate of reaction increases with a 10 d.C. rise
  ‎
• denaturation:
  
  • at extremely high temperatures bonds holding the protein together vibrate
  • this increases until they strain/break causing a change in the tertiary structure
  • the change in shape means they have denatured so the active site is no longer complementary to the substrate
    ‎
• optimum:
  
  • highest rate of activity takes place
  • human body = 40 d.C
  • ** once enzymes have denatured past the opt. temp., the decrease in rate of reaction is rapid **
    ‎
• extremes:
  
  • cold = enzymes controlling metabolic activities of organisms
  • they tend to have more flexible structures and are less stable than enzymes working at higher temps.
    ‎
  • thermophiles = organisms adapted to living in very hot environments
  • enzymes are much more stable as they have an increased no. of bonds (H and S)
  • their active sites and shapes are more resistant to temp. rises

Question 15

How does pH affect enzyme activity?

Answer 15

• change in pH = change in hydrogen ion conc.
  
  • acidic = more
  • alkaline = less
• this causes a change in the hydrogen and ionic bonds between the amino acid R-groups
• optimum pH = active site is in the right shape
  ‎
• renaturation = when pH returns to opt. and protein/active site resumes its normal shape to catalyse the reaction again
• a more significant change will irreversibly alter the structure and enzymes will become denatured
  ‎
• low/high pH = less R-group interaction
• enzyme will only function within a narrow pH range

Question 16

How does substrate/enzyme conc. affect enzyme activity?

Answer 16

• substrate:
  
  • increase = more frequent successful collisions with enzyme active sites
  • so more enzyme-substrate complexes are produced
  • rate of reaction increases til Vmax as all active sites are occupied by substrate particles
• enzyme:
  
  • increase = more available active sites for the substrate
  • increases rate of reaction so reaction rate rises towards higher Vmax

Question 17

How is metabolic activity controlled within cells?

Answer 17

• enzymes are activated with cofactors and inactivated with inhibitors
• inhibitors = molecules that prevent enzymes from carrying out their normal function (catalysis)

Question 18

What is competitive inhibition?

Answer 18

• a molecule that is similar to the substrate complementary to the enzyme’s active site
  
  • blocks substrate from entering the active site so reaction cannot take place
    ‎
• substrate and inhibitors compete to bind too the active site
• this slows down the rate of reaction as less substrate molecules are bound in a given time
  ‎
• e.g. statins inhibit the synthesis of cholesterol
• helps to reduce blood cholesterol concs.

Question 19

How does competitive inhibition affect rate of reaction?

Answer 19

• reduces rate of reaction but does not change the Vmax of the enzyme
• increasing substrate conc. will allow for original Vmax to be reached

Question 20

What is non-competitive inhibition?

Answer 20

• inhibitor binds to an alternative area on the enzyme (allosteric site)
• causes the tertiary structure of the protein to change, changing the active site
  
  • so it is no longer complementary to the substrate
• this prevents the enzyme from carrying out its function
  ‎
• e.g. organophosphates (insecticides, herbicides) irreversibly inhibit the enzyme for nerve impulse transmission

Question 21

How does non-competitive inhibition affect rate of reaction?

Answer 21

• increasing enzyme/substrate conc. will not overcome its effect
• increasing the conc. of the inhibitor will further decrease the rate of reaction

Question 22

What is end-product inhibition?

Answer 22

• where the product of a reaction acts as an inhibitor to the enzyme that produces it
• example of a negative feedback control mechanism
• excess products are not made/resources are not wasted

Question 23

What is the difference between a cofactor and a coenzyme?

Answer 23

• cofactor:
  
  • transfer atoms/groups from one reaction to another (multi-step pathway)
  • inorganic = minerals like iron, chloride, calcium, zinc ions
    ‎
• coenzymes:
  
  • similar but it is an organic molecule
  • derived from vitamins (B5, B3)

Question 24

What are prosthetic groups?

Answer 24

• they are cofactors which are required to carry out a catalytic function
• tightly bound to the enzyme protein to form a permanent feature

Question 25

What is precursor activation?

Answer 25

• inactive precursor enzymes = produced in an inactive form
  
  • particularly enzymes that can cause damage within cells
• they need to change their tertiary structure in order to be activated
  
  • precursor protein is added first (apoenzyme)
  • holoenzyme = when cofactor is added and enzyme has been activated