TCA Cycle: Fumarase and Malate Dehydrogenase

Question 1

What reaction is catalyzed by fumarase?

Answer 1

Hydration of the double bond of fumarate to form malate.

Question 2

What transition state does the hydration of fumarate to maleate proceed by? Why?

Answer 2

Carbanion, because the addition of -OH occurs before the addition of H+

Question 3

Which enzyme catalyzes the hydration of fumarate to malate?

Answer 3

fumarase

Question 4

Which enzyme catalyzes the regeneration of oxaloacetate?

Answer 4

malate dehydrogenase

Question 5

What is the final enzyme of the TCA Cycle?

Answer 5

malate dehydrogenase

Question 6

What reaction does malate dehydrogenase catalyze?

Answer 6

regeneration of oxaloacetate from malate

Question 7

What oxidizing agent is necessary for malate dehydrogenase to oxidize the -OH group of malate?

Answer 7

NAD+

Question 8

Is the oxidation of malate to oxaloacetate endergonic or exergonic?

Answer 8

endergonic; this means that the concentration of malate is much greater than the concentration of oxaloacetate in vivo and under equilibrium conditions.

Question 9

What allows the endergonic reaction of malate to oxaloacetate catalyzed by malate dehydrogenase to proceed?

Answer 9

It is coupled to a very exergonic reaction (the reaction catalyzed by citrate synthase that also begins the cycle).

Question 10

What makes the reaction catalyzed by citrate synthase so exergonic?

Answer 10

Cleavage of the high energy thioester bond of citryl-Co-A