Ch. 7

Question 1

How are enzyme rates modified (2 methods)?

Answer 1

1. Noncovalent binding of pathway intermediates to stimulate or inhibit
   enzyme activity
2. Covalent modification by attachment of chemical groups

Question 2

What are the 3 common patterns of regulation?

Answer 2

1. Feedback inhibition
2. Positive regulation
3. Irreversible reactions

Question 3

For biosynthetic pathways, the _____ is usually a negative allosteric effector for a branch point enzyme.

Answer 3

end product

Question 4

How does feedback inhibition work?

Answer 4

Acts to maintain a steady state in which the end product is utilized as rapidly at it is synthesized

Question 5

What are the 3 patterns of feedback inhibition?

Answer 5

1. Simple
2. Cumulative
3. Concerted

Question 6

What is simple feedback inhibition? In what type of biosynthetic pathway is it encountered?

Answer 6

• Regulation by a single end-product
• Encountered in linear biosynthetic pathways and sometimes branched biosynthetic pathways

Question 7

What is concerted inhibition?

Answer 7

Both end products must bind to the regulatory enzyme simultaneously to achieve any inhibition

Question 8

What is cumulative inhibition?

Answer 8

The enzyme is not completely inhibited by any single end product

Question 9

Is cumulative inhibition additive? Explain.

Answer 9

Cumulative but not additive
- One end product might inhibit the enzyme by 25% and a second might inhibit the enzyme by 45% –> both together might inhibit the enzyme 60% (not 70%)

Question 10

Why is it important that a single end product not shut down a common enzyme of a branch point?

Answer 10

Because shutting down a common enzyme of a branch point would prevent the synthesis of all end products

Question 11

When would branched pathways be regulated by simple feedback inhibition?

Answer 11

When a reaction shared by the various branches uses enzymes that have the same catalytic sites but different effector sites (isoenzymes)
- Isoenzymes catalyze the same reaction but are responsive to different end products

Question 12

What is positive regulation?

Answer 12

Regulation by an intermediate in a second pathway (GKL-2)
- One pathway produces an intermediate (G) –> combines with K in a second pathway –> produce L

Question 13

What happens in “precursor activation” (another pattern of positive regulation)?

Answer 13

Precursor intermediate stimulates a regulatory enzyme “downstream” in the same pathway
- Ensures that the rate of the downstream reactions matches that of the upstream reactions

Question 14

What are the 2 generalizations regarding irreversible reactions at branch points?

Answer 14

1. The reactions catalyzed by regulatory enzymes are usually at metabolic branch points
2. Regulatory enzymes often catalyze reactions that are physiologically irreversible

Question 15

What happens when the effector binds to the allosteric site?

Answer 15

The protein undergoes a conformational change, and this changes its kinetic constants

Question 16

What are most regulatory enzymes regulated by?

Answer 16

Conformational changes

Question 17

What does multimeric mean?

Answer 17

Having multiple subunits

Question 18

What do multimeric regulatory enzymes consist of? What does each component do?

Answer 18

1. Regulatory subunit
   - Bind effectors –> conformational change –> inhibit or activate catalytic subunit
2. One or more catalytic subunits
   - Bear the active sites and bind the substrate

Question 19

How does covalent modification occur?

Answer 19

By the reversible attachment of chemical groups to activate or inhibit the protein

Question 20

What groups can be added in covalent modification?

Answer 20

• Acetyl groups
• Phosphate groups
• Methyl groups
• Adenyl groups
• Uridyl groups