Chapter Four: Enzymes

Question 1

what are enzymes?

Answer 1

• Proteins that act as biological catalysts for intra & extracellular reactions to determine structure & function.
  Therefore affect metabolism of cells & whole organism.
• Specific tertiary structure determines shape of active site, complementary to a specific substrate.

Question 2

Give an example of an enzyme that catalyses
intracellular reactions.

Answer 2

• Catalase: catalyses decomposition of
  hydrogen peroxide (which causes
  oxidative stress) into water + oxygen.

Question 3

Give 2 examples of enzymes that
catalyse extracellular reactions.

Answer 3

• Amylase: carbohydrase catalyses digestion of
  starch to maltose in saliva/ small intestine
• trypsin: hydrolyses proteins in the small intestines

Question 4

what is activation energy? what can enzymes do to it?

Answer 4

• the amount of energy needed before a reaction can occur
• enzymes lower the activation energy needed for the reaction to occur when attached to the substrate which speeds up the reaction

Question 5

describe the lock and key model hypothesis.

Answer 5

• enzyme is like a lock, substrate is like a key that fits into it because of the enzymes specific tertiary structure resulting in a complementary shape
• enzyme active site is a fixed shape so when random collisions occur, the substrate can collide and attach to the enzyme, creating an enzyme-substrate complex
• the charged groups within the active site are said to distort the substrate which lowers the activation energy

Question 6

describe the induced fit model.

Answer 6

• suggests that the enzyme is like a glove and the substrate is like your hand
• induced fit is when the enzyme active site is induced, (slightly changes shape) allowing it to mould around the substrate
• the enzyme-substrate complex puts strain on the bonds which lowers the activation energy

Question 7

why are enzymes sensitive to certain conditions? what factors affect the rate of enzyme-controlled reactions?

Answer 7

• because they’re globular proteins

1. temperature
2. pH
3. enzyme concentration
4. substrate concentration

Question 8

describe how temperature affects the rate of enzyme-controlled reactions.

Answer 8

• temp too low = insufficient kinetic energy for successful collisions
• temp increase = increase in frequency of successful collisions
• temp too high = enzymes denature, active site changes shape, enzyme-complexes can’t form
  • high temps causes bonds to break which changes the tertiary structure, causing a change in active site shape

Question 9

what is the Q10 temperature coefficient? what is the formula?

Answer 9

• a measure of the rate of change of an enzyme-controlled reaction as a result of increasing the temperature by 10°C
• formula: Q10 = R2/R1
  R1 = rate of reaction at a temp of X°C (initial temp)
  R2 = rate of reaction a temp (X + 10)°C

Question 10

describe how pH affects the rate of enzyme-controlled reactions.

Answer 10

• too high or too low a pH will affect the charges in the amino acids in the active site which causes (ionic/hydrogen) bonds to break
• bonds breaking alters tertiary structure and changes the shape of the active site and the enzyme denatures
• enzymes have different optimal pH levels that they work at

Question 11

describe how enzyme concentration affects the rate of enzyme-controlled reactions.

Question 12

describe how substrate concentration affects the rate of enzyme-controlled reactions.

Answer 12

• low conc. of substrates, reaction rate will be lower due to the lower frequency of successful collisions between the enzyme and substrate
• increase in substrate conc = increase in frequency of successful collisions, increasing rate of reaction
• at high substrate concs the rate of reaction will plateau because all the enzyme active sites are in use, (enzymes are saturated) decreasing the frequency of successful collisions

Question 13

describe how enzyme concentration affects the rate of enzyme-controlled reactions.

Answer 13

• low enzyme conc = lower rate of reaction - less frequent successful collisions
• increase in enzyme conc increase the rate of reaction because more enzyme-substrate complexes can form due to the increase in frequency of successful collisions
• unless unlimited substrate is added, at high enzyme concentrations, the rate of reaction will plateau since there isn’t enough substrate to bind with the amount of enzymes present, lowering the frequency of successful collisions

Question 14

describe competitive inhibitors.

Answer 14

• similar in shape to the substrate and complementary in shape to the active site, allowing them to bind to the active site
• they prevent the substrate from binding, allowing enzyme-inhibitor complexes to form instead and the reaction rate is lower
• most competitive inhibitors are reversible (can be removed) if a high enough concentration of substrate is added, the substrate can knock out the inhibitor which increase the rate of reaction because of the increase in frequency of successful collisions

Question 15

describe non- competitive inhibitors.

Answer 15

• bind to the enzyme away from the active site at the allosteric site, causing the active site to change shape
• change in active site shape causes the substrate to not be able to bind, regardless of how much substrate is added
• enzyme-substrate complexes form less frequently causing the rate of reaction to be much lower

Question 16

describe end-product inhibition

Answer 16

• the products of some reactions are reversible inhibitors for the enzymes involved in controlling the reaction which allows the reactions to be controlled
• if a lot of product is present, it will inhibit the enzyme, causing the reaction to slow or stop, preventing resources from being wasted

Question 17

what are coenzymes, cofactors, and prosthetic groups?

Answer 17

• they are an additional non-protein molecule that catalyse a reaction. they are needed in some enzyme-controlled reactions

Question 18

describe coenzymes and cofactors.

Answer 18

• some reactions need atoms to be carried from one reaction to the next in multi-step pathway reactions
• some enzymes require a non-protein molecule to bind to the active site to make it complementary to the substrate
  
  • these molecules are cofactors and coenzymes

Question 19

what is the difference between cofactors and coenzymes?

Answer 19

• coenzymes = organic
• cofactors = inorganic

Question 20

give examples of cofactors and coenzymes.

Question 21

describe prosthetic groups. what is precursor activation?

Answer 21

• a type of cofactors that is permanently attached to the enzyme by covalent or non covalent bonds
• precursor activation is when enzymes are inactive and require activation by a cofactor
• this prevents enzymes from causing damage within cells and makes sure enzymes are only used when needed
• enzymes activated by the binding of a cofactor causes a change in the tertiary structure so the active site becomes complementary in shape to its substrate
• precursor protein (the inactive enzyme) = apoenzyme. when activated by the binding of the cofactor = holoenzyme