Enzyme Activity

Question 1

What is a chemical reaction?

Answer 1

The conversion of a substrate to a product and vice-versa by making/breaking bonds.

Question 2

What is meant by the term transition state?

Answer 2

High energy intermediate that lies between S and P.

Question 3

What is meant by the term activation energy?

Answer 3

Minimum energy S must have to allow reaction.

Question 4

How can the rate of a reaction be increased and why are catalysts required to do this?

Answer 4

• Can be enhanced by increasing:
  
  • temperature: increases no. Of molecules with activation energy
  • concentration: increases chance of molecular collisions
• But homeostasis requires temp and conc to be kept stable - so need to use catalysts to increase reaction rate.

Question 5

What are enzymes?

Answer 5

Biological catalysts (usually proteins) that increase the rate of reaction by lowering the activation energy - facilitates formation of the transition state.
Enzymes are:
- highly specific
- unchanged after reaction
- do not affect the reaction equilibrium
- may require associated co-factors

Question 6

What are active sites?

Answer 6

Small cleft where substrates bind via multiple weak bonds and where the chemical reaction occurs.
Have a complementary shape to the substrate - ‘lock and key’ hypothesis or ‘induced fit’ hypothesis.

Question 7

What is the ‘induced fit’ hypothesis?

Answer 7

Active site only forms a complementary shape after binding of the substrate.

Question 8

What is V0?

Answer 8

The initial rate of reaction

Question 9

What is the theory behind the Michaelis-Menten equation? What is V0 according to this model?

Answer 9

• A specific complex between the enzyme and substrate is a necessary intermediate in catalysis.
• V0 = Vmax [S] / Km + [S]

Question 10

Which shape will the plot of V0 vs [S] have according to the Michaelis-Menten model?

Answer 10

• Rectangular hyperbola

Question 11

What is Vmax?

Answer 11

Maximal rate when all enzyme active sites are saturated with substrate.

Question 12

What is Km?

Answer 12

• Substrate concentration that gives half maximal velocity.

- Measure of affinity of an enzyme for its substrate?

Question 13

Is a high or low Km indicative of high enzyme-substrate affinity?

Answer 13

• Low Km = high substrate affinity

- High Km = low substrate affinity

Question 14

What is the relationship between reaction rate and enzyme concentration?

Answer 14

• Reaction rate is proportional to the enzyme concentration, but not the standardised rate (per L or g).

Question 15

What is the linear plot version of the Michaelis-Menten equation called?

Answer 15

Lineweaver-Burk plot

Question 16

How can Km and Vmax be determined on a Lineweaver-Burk plot?

Answer 16

X axis intercept = 1 / Vmax
Y axis intercept = -1 / Km
Slope = Km / Vmax

Question 17

What are enzymes inhibitors?

Answer 17

Molecules that slow down or prevent an enzyme reaction.

Question 18

What are the different types of enzyme inhibitors?

Answer 18

• Irreversible: bind very tightly, generally form covalent bonds (e.g. Nerve gases such as sarin).
• Reversible: non-covalent, can freely dissociate.
  
  • competitive: binds at active site
  • non-competitive: binds at another site on the enzyme

Question 19

How can one distinguish between competitive and non-competitive inhibition?

Answer 19

Competitive inhibitor:

• no effect on Vmax (as increasing [substrate] overcomes inhibition)
• decreases Km (as reduces the proportion of enzyme molecules bound to substrate)

Non-competitive inhibitor:

• decreases Vmax (as decreases turnover number of enzyme by binding to alternative site)
• no effect on Km