biochemistyr exam 3 sidequest TCA

The Tricarboxylic Acid Cycle

If one were to introduce a pulse of acetyl CoA with the two carbons in acetyl CoA labeled with a radioactive tag, followed by unlabeled acetyl CoA, what would be the ultimate fate of the two carbons?

Both would be lost as CO₂

Both would remain incorporated in the structures that go through the cycle

One carbon would be lost as CO₂, while the other would remain incorporated

One carbon would be lost as CO₂, while the other would remain incorporated

How many oxidation-reduction steps are found in one turn of the cycle?

One

Two

Three

Four

Four

If one NADH generates 3 ATPs through oxidative phosphorylation and 1 FADH₂ generates 2 ATPs through oxidative phosphorylation, how many ATPs can be generated from the products of one cycle through the TCA pathway?

Four

Eleven

Twelve

Fifteen

Twelve - Twelve ATPs can be generated: 9 from the 3 NADHs, 2 from the FADH₂, and 1 directly generated by the combination of succinyl-CoA synthetase and nucleoside diphosphate kinase (reaction 5).

How many carbon atoms are present at the start of the citric acid cycle?

2

4

6

4

How are citrate and isocitrate related?

Conformational isomers

Constitutional isomers

Stereoisomers

Constitutional isomers

How many steps in the cycle involve the loss of a carbon atom?

1

2

3

4

2

Identify the oxidizing agents in the cycle from the list below.

(Select all that apply.)

CoA

NAD⁺

P_i

FAD

NAD^{+ ,} FAD

Aconitase is a key enzyme in the TCA cycle that converts citrate to isocitrate. In doing so, it utilizes an iron-sulfur cluster of a type found in many enzymes. If this enzyme-substrate system is studied in isolation, the ΔGº′ is only +6.7 kJ/mol with an equilibrium mixture of about 90% citrate, 4% cis-aconitate, and 6% isocitrate. Why doesn’t the TCA cycle stop here?

The ΔGº′ is positive, so the reaction will proceed spontaneously

The product, isocitrate, is being constantly removed by the enzyme isocitrate dehydrogenase

Citrate will build up and eventually push enough isocitrate through the system

The product, isocitrate, is being constantly removed by the enzyme isocitrate dehydrogenase

Aconitase is a key enzyme in the TCA cycle that converts citrate to isocitrate. In doing so, it utilizes an iron-sulfur cluster of a type found in many enzymes. When the fourth iron is added, what are the six atoms to which this iron is coordinated?

Three sulfurs, one iron, and two waters

Three sulfurs, one oxygen from water, one carboxyl oxygen, and one iron

Three sulfurs, one oxygen from water, one carboxyl oxygen, and one hydroxyl oxygen from C₃ of citrate

Three sulfurs, two irons, and one water molecule

Three sulfurs, one oxygen from water, one carboxyl oxygen, and one hydroxyl oxygen from C₃ of citrate

Watch Part 1 of the movie. At what stage is the "uniqueness" of the label in the labeled carbonyl carbon lost?

Isocitrate to 'alpha'-ketoglutarate

Succinyl-CoA to succinate

Succinate to fumarate

Fumarate to malate

Succinyl-CoA to succinate "Scrambling" occurs because the product, succinate (and the following product, fumarate), is achiral so carbons 1 and 4 of succinate become impossible to distinguish.

Why is the label on the labeled carbonyl from the first round lost during the second round?

The label itself is unstable

The label is siphoned off before starting the second cycle

The label now appears in two carboxyl groups that will be lost as CO₂ during the second cycle

The label now appears in two carboxyl groups that will be lost as CO₂ during the second cycle - The labels are now on carboxyl groups that are lost at the isocitrate to alpha-ketoglutarate and alpha-ketoglutarate to succinyl-CoA steps.

Watch Part 2 of the movie. At what stage is the "uniqueness" of the label in the labeled methyl group lost?

Isocitrate to 'alpha'-ketoglutarate

Succinyl-CoA to succinate

Succinate to fumarate

Fumarate to malate

Succinyl-CoA to succinate "Scrambling" occurs because the product, succinate (and the following product, fumarate), is achiral so carbons 2 and 3 of succinate become impossible to distinguish.

Why is the label from the methyl group not lost after the first round, only diluted?

The ¹⁴C isotope is very stable

The label was placed in a position that is never lost during the cycle, only diluted by new material

The enzymes have a very strong affinity for ¹⁴C labeled material

The label was placed in a position that is never lost during the cycle, only diluted by new material- The 2 and 3 positions of succinate and fumarate are located in the core and continue to cycle each time, never being put in a position to be lost as CO₂.