CB4 & CB5 Enzyme 1 & 2

Question 1

Explain what enzymes are, and why they are necessary

Answer 1

Enzymes are biological catalysts - molecules that can lower a reaction’s activation energy
• enzymes are never consumed/irreversibly altered by the reaction
• enzymes accelerate the rate of an already favourable reaction
•Increase reaction rates by several orders of magnitude

Question 2

Give examples of different enzyme mechanisms

Answer 2

• Enzymes accelerate the rate of an already favourable reaction
• increase reaction rates by several orders of magnitude

Question 3

Outline now an enzyme shows specificity towards certain substances

Answer 3

• Enzymes are highly specific for theirsubstrate & catalyse particular reactions
• substrate binds to the enzyme’s active site
• specific residues involved in substrate binding and orientation into position within the active site

Question 4

Outline factors that influence the rate of an enzyme

Answer 4

• Temperature
• pH
• time
• conc. of substrate & enzyme

Question 5

Define ‘co-enzyme’ & provide examples

Answer 5

Co-enzyme= small, non-protein molecules used to perform key functions, assist the enzyme’s catalytic function
e.g. NADH, FADH

Question 6

Define the term prosthetic group

Answer 6

A prosthetic group is a tightly bound, specific non-polypeptide unit required for the biological function of some proteins

Question 7

Define the term haloenzyme

Answer 7

Enzyme plus cofactor = haloenzyme

Question 8

Define the term apoenzyme

Answer 8

Enzyme alone (inactivated)

Question 9

Describe the allosteric control of example enzymes, distinguishing between homotropic & heterotrophic mechanisms

Answer 9

Allosteric regulation- induces conformational changes in the protein that can influence protein function

Homotropic allostery:
• protein has multiple effector binding sites on different subunits
• binding at one site positively alters binding at the others
• binding is co-operative
• e.g. O2 binding to haemoglobin

Heterotrophic allostery:
• binding sites for different effectors
• binding at one site can positively /negatively alter the binding of the other
.E.g. H+, CO2 allosteric modulation of O2 binding to haemoglobin

Question 10

Describe what enzyme kinetics can tell us

Answer 10

Enzyme kinetics- taking multiple measurements of enzyme activity over time & under specific conditions
• measure rate of substrate consumption / product formation - easiest way to look at enzyme activity
• allows us to understand an enzyme’s characteristics & substrate preferences
• helps determine type of enzyme inhibition present

Question 11

Explain the michaelis - menton equation & how it may be represented graphically

Answer 11

• Michaelis-mention & lineweaver Burke graphs are used to show key kinetic parameters
• relationship between substrate conc. & reaction velocity

Question 12

Explain what the kinetic measurements km & vmax represent

Answer 12

KM = conc. Of substrate required for enzyme to reach 1/2 Vmax
Vmax = maximum velocity of the enzyme

Question 13

Demonstrate how km & Vmax are determined from a double- reciprocal plot

Answer 13

1 / Vmax where line intercepts y-axis
- 1/ km where line intercepts x-axis

Question 14

Distinguish between the different types of enzyme inhibition

Answer 14

Competitive: inhibitor binds at active site, reversible, Vmax unchanged, km greater
Non-competitive: inhibitor binds at allosteric site, irreversible, vmax reduced, km unchanged
Uncompetitive: inhibitor only binds to enzyme complexed with substrate, irreversible, Vmax reduced, km reduced

Question 15

Describe the requirements for control of metabolism in unicellular & multicellular organism

Answer 15

Enzyme regulation (inhibition/feedback/activation) is key for efficient cell function & control of complex pathways

Question 16

How do enzymes increase the rate of kinetically favourable reactions?

Answer 16

• Decrease the kinetic barrier to the reaction
• decrease the activation energy of the reaction
• orientate the substrates in proximity to catalytic residues
• stabilise the transition state of the reactants
• orientate the substrates so reaction is more favourable

Question 17

Which type of reaction involves abstraction/donation of hydrogen ions?

Answer 17

• Acid /base catalysis
• oxidation/reduction reactions

Question 18

How might allosteric mechanisms influence enzyme activity?

Answer 18

• By inducing conformational changes that increase substrate binding
• By inducing conformational changes that enhance enzyme catalysis

Question 19

How do enzymes achieve substrate selectivity?

Answer 19

• By forming distinct points of contact with the substrate in the active cleft
• through electrostatic interactions between amino acid side chains & the substrate

Question 20

What is the effect of a competitive inhibitor on an enzyme’s kinetic parameters?

Answer 20

Increase km & no change in vmax

Question 21

What is the effect of a non-competitive inhibitor on an enzyme’s kinetic parameters?

Answer 21

Decreased vmax & no change in km