Enzymes

Question 1

What is the transition state?

Answer 1

High energy intermediate that lies between S and P

Question 2

What is the activation energy?

Answer 2

Minimum energy S must have to allow reaction

Question 3

What is the active site?

Answer 3

The place where substrates bind and where the chemical reaction occurs. Formed by amino acids from different parts of the primary sequence. They are clefts or crevices that have a complementary shape to the substrate

Question 4

What is Vmax?

Answer 4

Maximal rate when all enzyme active sites are saturated with substrate

Question 5

What is Km?

Answer 5

Substrate concentration that gives half the maximal velocity (units of concentration)

Question 6

What does low km give rise to?

Answer 6

High affinity for substrate

Question 7

What does high km give rise to?

Answer 7

Low affinity for substrate

Question 8

What are Vmax values measured in?

Answer 8

Measured In amounts per unit time

1 unit is the amount of enzyme that converts one micromole of product per minute under standard conditions

Question 9

What are competitive inhibitors?

Answer 9

These bind to the active site of an enzyme

Competes with substrate

Affects Km not Vmax

Question 10

What are non competitive inhibitors?

Answer 10

Binds at another site on the enzyme, not active site

Affects Vmax not Km

Question 11

How does temperature increase the rate of reaction?

Answer 11

Increases number of molecules with activation energy

Question 12

How does concentration increase the rate of reaction?

Answer 12

Increases chance of molecular collisions

Question 13

Other than lowering activation energy, how else do enzymes increase the rate of reaction?

Answer 13

Provides a platform for molecules to react so increases the chance of collisions.

Question 14

What are some important features of enzymes?

Answer 14

Highly specific, unchanged after reaction, do not affect equilibrium, increase rate of reaction

Question 15

What is the induced fit hypothesis?

Answer 15

Binding of substrates to an enzyme can induce changes in the conformation - the active site only forms a complementary shape after binding of the substrate

Question 16

How are substrates bound to active sites?

Answer 16

By multiple weak bonds

Question 17

When reaction rate is shown as a function of substrate concentration, what is usually the shape of the graph?

Answer 17

Rectangular hyperbola (the reaction reaches a maximum velocity)

Question 18

What is the Mchaelis-Menten equation and what does it predict?

Answer 18

V0 = Vmax[S] / Km + [S]

V0 - initial velocity

Predicts that a plot of V0 vs substrate concentration will be a rectangular hyperbola

Question 19

For example, if Hexokinase has a Km of 0.1 mM and Glucokinase has a Km of 5 mM, which has a higher affinity for it’s substrate (glucose) ?

Answer 19

Hexokinase - hence why HK is always active whereas GK only becomes active when glucose levels peak after feeding

Question 20

What are reversible inhibitors?

Answer 20

Non-covalent, can freely dissociate (competitive and non competitive)

Question 21

What is one unit in enzyme kinetics?

Answer 21

1 unit = the amount of enzyme that converts 1 micro mol of product per minute under standard conditions

Question 22

The Michaelis-Menten equation can be rearranged to give a linear plot in the from y = mx + c. What is this?

Answer 22

y = mx + c

1/V0 = (Km/Vmax * 1/[S]) + 1/Vmax

Therefore intercept = 1/V0
Gradient = Km/Vmax

Question 23

What are irreversible inhibitors?

Answer 23

Bind very tightly, generally form covalent bonds

Question 24

What is an example of an irreversible inhibitor?

Answer 24

Nerve gases such as sarin (interferes with degradation of acetylcholine at neuromuscular junctions - inhibitor of acetylcholinesterase)

Question 25

Which type of inhibitor affects Km?

Answer 25

Competitive

Question 26

Which type of inhibitor affects Vmax?

Answer 26

Non-competitive

Question 27

What overcomes the effect of competitive inhibitors?

Answer 27

Adding enough substrate

Question 28

Why does Km increase with competitive inhibition

Answer 28

Because the inhibitor competes with the substrate for the active site so Km increases

Question 29

Assuming the Km values of ethanol and methanol are 1 mm and 10 mM respectively, explain why giving ethanol to a patient with methanol poisoning would be beneficial.

Answer 29

Ethanol has a lower Km value than methanol so the enzyme has a higher affinity to ethanol than to methanol. Therefore, more ethanol will bind to the enzyme than methanol.

Question 30

What are zymogens?

Answer 30

Inactive precursors of enzymes