2.5 Regulation of Enzyme Activity [HY]

Question 1

Feedback Regulation

Answer 1

Enzymes are subject to regulation by products further down a given metabolic pathway

Question 2

Feed-forward regulation

Answer 2

enzymes may be regulated by intermediates that precede the enzyme in the pathway

Question 3

Negative feedback

Answer 3

• Feedback inhibition
• helps maintain homeostasis: once have enough product, turns it off

Question 4

Competitive Inhibition

Answer 4

• involves occupancy of the active site
• Substrates cannot access enzymatic binding sites if there is an inhibitor in the way
• Raises Km b/c more needed to reach half Vmax

Question 5

How to counteract competitive inhibition?

Answer 5

• can be overcome by adding more substrate so that the substrate-to-inhibitor ratio is higher
• If more molecules of substrate are available than molecules of inhibitor, then the enzyme will be more likely to bind substrate than inhibitor (assuming the enzyme has equal affinity for both molecules).

Question 6

Does adding competitive inhibitor alter vmax?

Answer 6

Adding a competitive inhibitor does not alter the value of vmax because if enough substrate is added, it will outcompete the inhibitor and be able to run the reaction at maximum velocity

Question 7

Noncompetitive Inhibition

Answer 7

• bind to an allosteric site instead of the active site, which induces a change in enzyme conformation
• Once the enzyme’s conformation is altered, no amount of extra substrate will be conducive to forming an enzyme–substrate complex
• the two molecules do not compete for the same site, inhibition is considered
  noncompetitive

Question 8

Allosteric Sites

Answer 8

• non-catalytic regions of the enzyme that bind regulators
• Regulators regulate the availability of the active site

Question 9

Do noncompetitive inhibitors affect vmax?

Answer 9

• Yes, decreases the measured value of vmax because there is less enzyme available to react

Question 10

Do noncompetitive inhibitors affect Km?

Answer 10

• No, because any copies of the
  enzyme that are still active maintain the same affinity for their substrate

Question 11

Mixed Inhibition

Answer 11

• results when an inhibitor can bind to either the enzyme or
  the enzyme–substrate complex, but has different affinity for each
• If the inhibitor had the same affinity for both, it would be a noncompetitive
  inhibitor.
• Bind at allosteric site

Question 12

Do mixed inhibitors affect Km?

Answer 12

• alters the experimental value of Km depending on the preference of the inhibitor for the enzyme vs. the enzyme–substrate complex
• If prefers enzyme, increases Km
• If prefers enzyme-substrate complex, lowers Km

Question 13

Do mixed inhibitors affect vmax?

Answer 13

• yes, vmax is decreased

Question 14

Uncompetitive Inhibition

Answer 14

• bind only to the enzyme–substrate complex and essentially lock the substrate in the enzyme, preventing its release
• Essentially increasing affinity between the enzyme and substrate
• it’s the formation of the enzyme–substrate complex that creates a conformational
  change that allows the uncompetitive inhibitor to bind

Question 15

Do uncompetitive inhibitors affect vmax?

Answer 15

• yes, lower vmax

Question 16

Do uncompetitive inhibitors affect Km?

Answer 16

• yes, lower Km

Question 17

Irreversible Inhibition

Answer 17

• the active site is made unavailable for a prolonged period of time, or the enzyme is permanently altered.
• this type of inhibition is not easily overcome or reversed

Question 18

Allosteric Enzymes

Answer 18

• Have multiple binding sites
• alternate between an active and an inactive form: The inactive form cannot carry out the enzymatic reaction

Question 19

Allosteric activator

Answer 19

• bind to the allosteric site
• causes a conformational shift in the protein
• makes the active site more available for binding to the substrate

Question 20

Allosteric inhibitor

Answer 20

• bind to the allosteric site
• causes a conformational shift in the protein
• will make the active site less available

Question 21

How can you deactivate or activate enzymes?

Answer 21

• activated or deactivated by phosphorylation or dephosphorylation
• cannot predict whether phosphorylation or dephosphorylation will activate an enzyme without experimental determination

Question 22

Glycosylation

Answer 22

• the covalent attachment of sugar moieties, another covalent enzyme modification
• Glycosylation can tag an enzyme for transport within the cell, or can modify protein activity and selectivity

Question 23

Zymogens

Answer 23

• contain a catalytic (active) domain and regulatory domain
• The regulatory domain must be either removed or altered to expose the active site.
• Most zymogens have the suffix –ogen