L17- TCA Cycle

Question 1

Which enzyme catalyzes the oxidation of pyruvate to acetyl-CoA?

Answer 1

Pyruvate dehydrogenase.

Question 2

Which three enzymes constitute the pyruvate dehydrogenase complex?

Answer 2

Pyruvate decarboxylase (E1), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3).

Question 3

List the coenzymes of pyruvate dehydrogenase.

Answer 3

Coenzyme A/acetyl Coenzyme A (CoASH), thiamine pyrophosphate (TPP), lipoate, FAD and NAD+.

Question 4

Which pyruvate dehydrogenase coenzyme accepts an acyl group from lipoate, forming acetyl-CoA?

Answer 4

CoASH (coenzyme A).

Question 5

Coenzyme A serves as a carrier of acyl groups in many metabolic reactions. What high energy bond is formed between CoASH and acyl groups?

Answer 5

Thiol ester bond.

Question 6

From which vitamin is the coenzyme thiamine pyrophosphate derived?

Answer 6

Vitamin B1 (thiamine).

Question 7

Name two medical conditions associated with thiamine deficiency.

Answer 7

Beri-beri and Wernicke-Korsakoff syndrome.

Question 8

What is the function of the pyruvate dehydrogenase coenzyme thiamine pyrophosphate (TPP)?

Answer 8

TPP binds to pyruvate, leading to its decarboxylation. The remaining 2-carbon hydroxyethyl fragment is then transfered to lipoate, another coenzyme of pyruvate dehydrogenase.

Question 9

Thiamine pyrophosphate (TPP) is bound to which enzyme in the pyruvate dehydrogenase complex?

Answer 9

Pyruvate decarboxylase (E1).

Question 10

Which structural group on coenzyme A forms a thiol ester linkage with acyl groups?

Answer 10

The sulfhydryl group.

Question 11

The coenzyme lipoate is bound to which enzyme in the pyruvate dehydrogenase complex?

Answer 11

Dihydrolipoyl transacetylase (E2).

Question 12

Which enzyme in the pyruvate dehydrogenase complex catalyzes the rapid decarboxylation of pyruvate?

Answer 12

Pyruvate decarboxylase (E1).

Question 13

Which component of the pyruvate dehydrogenase complex is a target for trivalent arsenic poisoning?

Answer 13

The lipoate coenzyme.

Question 14

What kind of bond covalently links lipoate to the enzyme dihydrolipoyl transacetylase (E2)?

Answer 14

An amide bond.

Question 15

The coenzyme flavin adenine dinucleotide (FAD) is derived from which vitamin?

Answer 15

Vitamin B2 (riboflavin).

Question 16

What is the function of the FAD coenzyme bound to the dihydrolipoyl dehydrogenase enzyme (E3) of the pyruvate dehydrogenase complex?

Answer 16

FAD accepts electrons from the reduced form of lipoate, regenerating oxidized lipoate and forming FADH2. Note that FAD/FADH2 is covalently bonded to E3.

Question 17

What is the function of the coenzyme NAD+ with regards to the series of reactions catalyzed by the pyruvate dehydrogenase complex?

Answer 17

NAD+ accepts electrons from the FADH2 that is bound to E3, regenerating FAD and producing NADH.

Question 18

What inhibits each enzyme (E1, E2, and E3) of the pyruvate dehydrogenase complex (PDH)?

Answer 18

Acetyl-CoA inhibits PDH at the E2 component, and NADH inhibits PDH at the E3 component. The E1 component may also be inhibited by phosphorylation on a specific serine residue.

Question 19

How do elevated concentrations of pyruvate regulate the activity of the pyruvate dehydrogenase complex (PDH)?

Answer 19

Pyruvate inhibits the kinase that inactivates the E1 component by phosphorylation. Therefore, pyruvate helps to keep PDH in its active form.

Question 20

What regulatory effect does insulin exert on the pyruvate dehydrogenase complex (PDH)?

Answer 20

Insulin activates a phosphatase that can remove the phosphate group on E1. Therefore, insulin helps to activate PDH.

Question 21

X-linked Leigh’s disease is caused by a deficiency of which enzyme?

Answer 21

Pyruvate dehydrogenase.

Question 22

What clinical sign would you expect to observe in a patient with partial deficiency of pyruvate dehydrogenase?

Answer 22

Metabolic acidosis due to the buildup of lactate and alanine from the interconversion of pyruvate.

Question 23

Which TCA cycle enzyme catalyzes the condensation of acetyl-CoA with oxaloacetate to form citrate?

Answer 23

Citrate synthase.

Question 24

Which TCA cycle enzyme catalyzes a substrate-level phosphorylation?

Answer 24

Succinyl-CoA synthetase.

Question 25

In which cellular compartment does the TCA cycle occur?

Answer 25

Mitochondrial matrix.

Question 26

Fluoroacetate is able to inhibit which TCA cycle enzyme?

Answer 26

Aconitase.

Question 27

Which coenzymes are required for proper activity of the enzyme alpha-ketoglutarate dehydrogenase?

Answer 27

Thiamine pyrophosphate (TPP), lipoate, coenzyme A (CoASH), FAD and NAD+.

Question 28

Which TCA cycle enzymes catalyze irreversible reactions?

Answer 28

Citrate synthase and alpha-ketoglutarate dehydrogenase.

Question 29

What is the chemical equation for the reaction catalyzed by succinyl CoA synthetase?

Answer 29

Succinyl-CoA + GDP + Pi -> succinate + GTP.

Question 30

The enzyme succinate dehydrogenase requires which coenzyme(s)?

Answer 30

FAD.

Question 31

What is the function of the enzyme malate dehydrogenase with respect to the TCA cycle?

Answer 31

Malate dehydrogenase catalyzes the reversible oxidation of malate to oxaloacetate. This regenerates oxaloacetate so that the TCA cycle may go through another round of oxidative metabolism.

Question 32

Which TCA cycle intermediate is also a precursor for heme synthesis?

Answer 32

Succinyl-CoA.

Question 33

Which TCA cycle intermediate may be exported to the cytoplasm for synthesis of glucose?

Answer 33

Malate.

Question 34

List three metabolites that can be directly converted to oxaloacetate.

Answer 34

Pyruvate (by action of the enzyme pyruvate carboxylase), aspartate and malate.

Question 35

Define the term “anaplerotic” reaction.

Answer 35

An anaplerotic reaction is a reaction that is not part of the TCA cycle but whose product is a TCA cycle intermediate. It is meant to replenish the TCA cycle intermediates.

Question 36

True or False. Reactions of the TCA cycle are slowed by an increase in the NADH/NAD+ ratio.

Answer 36

True. They are also slowed by an increase in ATP.

Question 37

Which TCA cycle enzyme(s) is/are subject to allosteric regulation?

Answer 37

Citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase.

Question 38

Which TCA cycle enzyme(s) produce NADH?

Answer 38

Isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase and malate dehydrogenase.

Question 39

Which TCA cycle enzyme(s) produce FADH2?

Answer 39

Succinate dehydrogenase.

Question 40

Which TCA cycle enzyme(s) produce GTP via substrate-level phosphorylation?

Answer 40

Succinyl-CoA synthetase.

Question 41

List the allosteric activators and inhibitors of citrate synthase.

Answer 41

Activators: ADP. Inhibitors: NADH, ATP, succinyl-CoA, citrate

Question 42

List the allosteric activators and inhibitors of isocitrate dehydrogenase.

Answer 42

Activators: isocitrate, ADP, AMP, Ca++. Inhibitors: NADH, ATP

Question 43

List the allosteric activators and inhibitors of alpha-ketoglutarate dehydrogenase.

Answer 43

Activators: Ca++. Inhibitors: NADH, succinyl-CoA

Question 44

Which TCA cycle substrates and coenzymes cannot cross the mitochondrial membrane?

Answer 44

Acyl-CoA compounds (such as acetyl-CoA), oxaloacetate, NAD+ and NADH.