Thermodynamics of Metabolism

Question 1

What must be true for reactions to be easily reversed by changing the ratio of products to reactants?

Answer 1

The reaction must be a near-equilibrium reaction

Question 2

What will happen when the concentration of reactants in a reaction is greater than the equilibrium concentration of reactants?

Answer 2

The reaction will move in the forward direction until equilibrium is attained.

Question 3

What are the rates of near-equilibrium reactions regulated by?

Answer 3

The relative concentration of reactants to products

Question 4

What types of reactions are most metabolic reactions?

Answer 4

Near-equilibrium reactions

Question 5

What is the implication for highly exergonic metabolic reaction (delta G is much less than 0)?

Answer 5

The reaction is irreversible and goes to completion.

Question 6

What effect does a highly exergonic reaction have on a pathway that it is a part of?

Answer 6

Confers directionality on the pathway (makes the entire pathway irreversible).

Question 7

What does every metabolic pathway have that commits a pathway to completion?

Answer 7

A committed step (usually near the beginning of the pathway

Question 8

What is required for the reverse reaction of a highly exergonic pathway?

Answer 8

A different pathway for at least one of the reaction steps (or the existence of independent interconversion routes).

Question 9

What does the existence of independent interconversion routes for a forward reaction and its reverse reaction allow?

Answer 9

Independent control of the two processes

Question 10

What type of control would be impossible without independent interconversion routes for a forward reaction pathway and its reverse reaction pathway?

Answer 10

Being able to shut off one reaction while turning on the other, depending on whether product or reactant is required.

Question 11

What remains constant in a pathway?

Answer 11

The concentration of metabolites

Question 12

What step in a reaction determines flux (the rate of flow)?

Answer 12

The RDS (the slowest step or steps)

Question 13

What is true of the RDS of a reaction pathway?

Answer 13

Its delta G is negative and large, so it operates far from equilibrium.

Question 14

Why is the delta G of the RDS large and negative?

Answer 14

Because the RDS is slow, its product is used in the next reaction before it can equilibrate back to the reactants of the previous step.

Question 15

What is true of all near-equilibrium reactions that are downstream from an RDS?

Answer 15

They all have the same flux

Question 16

How can flux control points be identified in practice?

Answer 16

They have a large delta G and are relatively insensitive to concentration variations

Question 17

What do reaction steps with large negative delta G permit?

Answer 17

The establishment of steady flux for a pathway.

Question 18

What are four mechanisms that cells use to control flux through rate determining steps?

Answer 18

1) Allosteric control
2) Covalent modification
3) Substrate cycles
4) Genetic control

Question 19

What type of flux control mechanism controls the concentration of enzyme through protein biosynthesis?

Answer 19

Genetic control

Question 20

What type of flux control mechanism works by phosphorylating/dephosphorylating specific enzyme sites?

Answer 20

Covalent modification

Question 21

What type of flux control mechanism can increase flux by increasing the forward reaction and slowing the reverse?

Answer 21

Substrate cycles (reactions catalyzed by two different enzymes, one forward and one reverse)

Question 22

Which is more sensitive to allosteric effectors, single, unopposed non-equilibrium reactions or substrate cycles (reactions whose forward and reverse reactions are catalyzed by different enzymes)?

Answer 22

Substrate cycles

Question 23

Which control mechanism for flux involves use of substrate, products, co-enzymes, or negative feedback?

Answer 23

Allosteric control