Session 3: Enzymes

Question 1

What is Km? What are its units?

Answer 1

The substrate concentration that gives 1/2 of the maximal rate of reaction
Units are in concentration

Question 2

What is Vmax? What are its units?

Answer 2

The maximal rate of reaction. The theoretical rate of reaction where all enzymes are saturated with substrate.
Units are mol/min

Question 3

What does Km value suggest? What does a low Km value mean?

Answer 3

Enzyme affinity for the substrate. A low Km suggest high enzyme affinity for substrate.

Question 4

How does an increase in enzyme concentration affect the Vmax of a reaction? Why?

Answer 4

Increases Vmax because Vmax is dependent on the amount of enzymes present. Increase in enzyme concentration means that more enzymes will bind to substrate and produce more product => increases Vmax

Question 5

On a Lineweaver-Burk plot, what do the y-intercept and the x-intercept show?

Answer 5

y-intercept = Vmax, the lower the y-intercept = higher Vmax
x-intercept = Km

Question 6

What does a low y-intercept of a Lineweaver-Burk plot suggest?

Answer 6

Lower the y-intercept = higher Vmax = higher enzyme concentration
If the y-intercept is high, the enzyme concentration is low

Question 7

Does changing the concentration of enzyme affect Km? Why?

Answer 7

No, because Km involves the substrate concentration, not enzyme concentration

Question 8

How does a competitive inhibitor affect Vmax? Why?

Answer 8

It does not affect Vmax because the competitive inhibitor does not compete well in high substrate concentration (but they do in low substrate concentration)

Question 9

How does a competitive inhibitor affect Km? Why?

Answer 9

It increases Km because when the competitive inhibitor binds the enzyme, it becomes inactive, inactive enzymes have NO affinity for substrate and no activity either

Question 10

How do competitive inhibitors work?

Answer 10

When the competitive inhibitor binds the enzyme, it becomes inactive. Inactive enzymes have NO affinity for substrate and no activity either. So they reduce the proportion of enzyme molecules bound to a substrate thus reducing rate of reaction.

Question 11

Where do competitive inhibitors bind to the enzyme molecules?

Answer 11

At active site

Question 12

Where do non-competitive inhibitors bind to the enzyme molecules?

Answer 12

Binds away from active site - at a different point

Question 13

How do non-competitive inhibitors work?

Answer 13

Affect the overall conformation of the enzyme by binding to the enzyme at a different point (not competing with the substrate for the active site), this causes the shape of the active site to change and the substrate is unable to bind properly because they no longer share specificity

Question 14

How does increasing substrate concentration reduce the effect of competitive inhibitors?

Answer 14

Diminish the competition for the substrate to properly bind to the active site and causes an increased percentage of the enzyme active

Question 15

Why does increasing substrate concentration have no effect on non-competitive inhibitors?

Answer 15

As the inhibitor is not in direct competition with the substrate for the active site, increasing substrate levels cannot mitigate the inhibitor’s effect

Question 16

Why do non-competitive inhibitors not affect Km but decrease Vmax?

Answer 16

The inhibitor molecules do not compete with the substrates for active sites on the enzyme surface, but some other site on the enzyme. This results in a decrease in the enzyme molecules that can catalyse the reaction = Vmax is lowered.

Km doesn’t change because enzyme-substrate affinity remains the same, the active site is not competed for by the inhibitor