Quiz 6 - TCA cycle

Question 1

What are the 4 fates of pyruvate?

Answer 1

Transamination: Alanine

Carboxylation: Oxaloacetate

Oxidative decarboxylation: Acetyl-CoA

Reduction: Lactate

_{<em>Glucose becomes pyruvate through glycolysis</em>}

Question 2

What are other sources of acetyl-CoA?

Answer 2

Question 3

What are the 3 fates of acetyl CoA?

Answer 3

Acetyl-CoA enters the:

Tricarboxylic acid cycle

Ketone bodies

Sterol-fatty acids

_{<em>P<span>Y</span>ruvate, amino acids and fatty acids all create acetyl-CoA</em>}

Question 4

Know the diagram

Answer 4

Question 5

What is made in TCA cycle?

What is degraded?

Answer 5

Some ATP is made and more NADH is made.

NADH goes on to make more ATP through ETC and oxidative phosphorylation.

Pyruvate(acetate) from glycolysis is degraded to CO2

Question 6

Where does Krebs cycle occur?

Answer 6

Mitochondria

Question 7

What is primary function of Krebs cycle?

Answer 7

The oxidation of Acetyl-CoA to Co2.

The energy released from this oxidation is saved as NADH, FADH2 and GTP

Question 8

What is the overall result of TCA cycle?

What is not required in this pathway?

Answer 8

Acetyl-CoA = 2CO2

3NAD + FAD + GDP +Pi = 3NADH + FADH2

Oxygen is not required in the cycle, the pathway will not occur anaerobically because NADH and FADH2 will accumulte if oxygen is not available for the ETC

Question 9

What is the only fate of acetyl CoA in TCA cycle?

What does this represent?

Answer 9

The oxidation of acetyl CoA to CO2.

The citric acid cycle does not represent a pathway by which there can be a net synthesis of glucose from acetyl-CoA or the net conversion of acetyl-CoA to citrate, malate or any other intermediate in the cycle.

Question 10

What is the TCA cycle central to the oxidation of?

Is there any hormonal control of the cycle?

Answer 10

The cycle is central to the oxidation of any fuel that yields acetyl-CoA including:

glucose

fatty acids

ketone bodies

ketogenic amino acids

alcohol

There is no hormonal control of the cycle as control is exerted by energy status of the cell. TCA activity is necessary irrespective of fasting or fed stage

Question 11

Where are all TCA enzymes located? Which enzyme is not present in this location?

Answer 11

All TCA enzymes are located in the matrix of mitochondria except succinyl dehyrogenase which is found in the inner membrane

Question 12

List the key enzymes of TCA cycle and their function

Answer 12

1. Isocitrate dehydrogenase: major control enzyme, inhibited by NADH and ATP, activated by ADP
2. a-ketoglutarate dehydrogenase: similar to pyruvate dehyrogenase complex. Requires thiamine, lipoic acid, CoA, FAD and NAD. Lack of thimaine slows oxidation in TCA cycle
3. Succinyl-CoA synthetase(succinate thiokinase): catalyzes substrate level phosphorylation of GDP to GTP
4. Succinate dehydrogenase: inner mitochondrial membrane, functions as complex 2 of ETC.
5. Citrate synthase: condenses incoming acetyl-CoA with oxaloacetate to form citrate

Question 13

What other functions do the intermediates citrate, succinyl-CoA and malate have other than in TCA cycle?

Answer 13

Citrate leaves mitochondria (citrate shuttle) to deliver acetyl CoA into cytoplasm for fatty acid synthesis.

Succinyl-CoA is high energy substitute used for heme synthesis and to activate ketone bodies in extrahepatic tissues

Malate leaves mitochondria(malate shuttle) for gluconeogenesis

_{<em>As intermediates are drawn out of the TCA cycle, the cycle slows and they must be replaced to ensure sufficient energy for the cell</em>}

Question 14

Know the diagram

Answer 14

Question 15

What two normal ways to cleave C-C bonds and oxidize don’t work for CO2 when oxidizing acetate to CO2?

Answer 15

1. Cleavage between Cs alpha and beta to a carbonyl

2. An alpha cleavage of an alpha hydroxyketone

Question 16

What is a better way to cleave acetate?

Answer 16

It is better to condense acetate with oxaloacetate and carry out a Beta cleavage. TCA combines this with oxidation to form CO2, regenerate oxaloacetate and capture all the energy as NADH and ATP.

Question 17

Pyruvate is oxidatively decarboxylated to form what substance?

What does pyruvate use to create this substance?

Answer 17

Acetyl-CoA

Pyruvate uses TPP, CoASH, lipoic acid, FAD and NAD+

Question 18

Describe 2 reasons why pyruvate from aerobic glycolysis enters mitochondria where it is converted to acetyl-CoA?

Answer 18

1. For entry into TCA if ATP is needed

2. For fatty acid synthesis if sufficient ATP is present

Question 19

What is pyruvate dehydrogenase inhibited by?

Is the pyruvate dehydrogenase reaction irreversible?

What other mitochndrial enzyme uses pyruvate?

Answer 19

PDH is inhibited by acetyl-CoA.

The PDH reaction is irreversible and cannot be used to convert acetyl-CoA to pyruvate or glucose

The other mitochondrial enzyme is pyruvate carboxylase

Question 20

What are the cofactors and enzymes used by pyruvate dehydrogenase?

Answer 20

Thiamine pyrophosphate(TPP) from vitamin thiamine

Lipoic acid

Coenzyme A (CoA) from pantothenate

FAD(H2) from riboflavin

NAD(H) from niacin(some synthesized from tryptophan)

Question 21

What activates Pyruvate dehydrogenase in liver?

Answer 21

Insulin. In the brain and nerves PDH is not responsive to hormones.

Question 22

Who are most likely to have thiamine deficiency?

What are some symptoms when alcohol interferes with thiamine absorption in the intestine?

Answer 22

Alcoholics and they may develop Wernicke peripheral neuropathy and Korsakoff psychosis.

Symptoms of alcohol interfering with thiamine absorption include:

Ataxia

Ophthalmoplegia, nystagmus

Memory loss and cofabulation

Cerebral hemorrhage

Question 23

What does insufficient thiamine lead to?

Answer 23

Insufficient thiamine impairs glucose oxidation, causing highly aerobic tissues such as brain and cardiac muscle to fail first.

_{<em>Congestive heart failure may be a complication(wet beri-beri owing to inadequate ATP and accumulation of ketoacids in the cardiac muscle.</em>}

_{<em>Branched chain amino acids are sources of energy in brain and muscle</em>}

Question 24

What other enzyme complexes similar to pyruvate dehydrogenase use thiamine?

Answer 24

1. a-ketoglutarate dehydrogenase(citric acid cycle)

2. Branched-chain ketoacid dehydrogenase(metabolism of branched-chain amino acids)

Question 25

Coenzyme A structure

Answer 25

Question 26

Polypeptide chain of E2(dihydrolipoyl transacetylase)

Answer 26

Question 27

Know the cycle of pyruvate to acetyl-CoA

Answer 27

Question 28

What are allosteric inhibitors of Citrate synthase? Does CS have large negative delta G?

Answer 28

**NADH and succinyl CoA** Yes it has **large negative delta G.**

Question 29

What does enzyme Aconitase do? What is the stereochemistry of the reaction? What kind of cluster does aconitase use?

Answer 29

**Isomerization of citrate to isocitrate** which has secondary OH which can be oxidized because citrate is a poor substrate for oxidization. The stereochemistry of the reaction is that **aconitase removes pro R H of the pro arm of citrate** Aconitase uses **Iron-sulfur cluster**.

Question 30

Oxidative decarboxylation of what substance occurs under Isocitrate dehydrogenase?

Answer 30

Oxidative decarboxylation of **isocitrate** to yield alpha ketoglutarate Involves NAD+ chemistry(Hydral removal)followed by decarboxylation. IDH is a link to ETC because it makes NADH

Question 31

What are the 5 coenzymes used by Alpha-ketoglutarate dehydrogenase? Is this the 1st or 2nd oxidative carboxylation? Which enzyme is Alpha-ketoglutarate similar to?

Answer 31

The **5 coenzymes** are: **TPP, CoASH, Lipoic acid, NAD+, FAD** **Second** oxidative decarboxylation The enzyme is similar to **pyruvate dehydrogenase**

Question 32

Is Succinyl-CoA synthatase reaction a substrate level phosphorylation? What is made? What is synthesis driven by? What does it involve?

Answer 32

Yes succinyl-CoA synthatase reaction is a **substrate level phosphorylation reaction.** A **nucleoside triphosphate** is made. Its synthesis is driven by **CoA ester** and involves a **phosphohistidine**

Question 33

What does succinate dehydrogenase involve? What is the mechanism? The enzyme is part of which pathway and where? What happens to the electrons?

Answer 33

An **oxidation** involving **FAD**. The mechanism involves **hydride removal by FAD and deprotonation** Succinate dehydrogenase is part of the **ETC in inner mitochondrial membrane** The electrons transferred from succinate to FAD to form FADH2 and are passed directly to ubiquinone(UQ) in ETC pathway.

Question 34

What does fumarase do?

Answer 34

Hydration across double bond

Question 35

What oxidation does malate dehydrogenase do? Is the reaction energetically expensive? Malate dehydrogenase reaction and which other reactions form a reaction triad over and over again?

Answer 35

**NAD+ dependent oxidation**. The carbon that gets oxidized is the one that received -OH from previous reaction(fumarase). Yes. Delta G is 30 kj/mol **Succinate dehydrogenase, fumarase and malate dehydrogenase reactions** form reaction triad.

Question 36

Know the diagram

Answer 36

Question 37

For one acetate through the TCA cycle, how many CO2, ATP and reduced coenzymes are produced?

Answer 37

2 CO2, 1 ATP and 4 reduced coenzymes for 1 acetate

Question 38

Know the table

Answer 38

Question 39

What is the fate of carbon in the TCA cycle(Carboxyl C and Methyl C)?

Answer 39

**Carboxyl C** of acetate turns to CO2 only in second turn of cycle following entry of acetate **Methyl C** of acetate survives 2 cycles completely but half of what's left exits the cycle on each turn after that

Question 40

What are the intermediates of biosynthesis for the TCA cycle?

Answer 40

**Alpha ketoglutarate** is transaminated to make glutamate which can be used to make purine nucleotides Arg and Pro **Succinyl-CoA** can be used to make porphyrins **Fumarate and oxaloacetate** can be used to make several amino acids and pyrimidine nucleotides **Mitochondrial citrate** can be exported to be a cytoplasmic source of acetyl CoA and oxaloacetate

Question 41

What are anploretic reactions? What are the enzymes involved in this reaction? Which reaction goes the wrong way? What binds tightly and what binds weakly to the enzyme?

Answer 41

They are **filling up reactions**. **PEP carboxylase** converts PEP to oxaloacetate **Pyruvate carboxylase** converts pyruvate to oxaloacetate **Malic enzyme** converts pyruvate to malate **PEP Carboxykinase** could have been an anaploretic reaction but it goes **the wrong way** because CO2 binds weakly and oxaloacetate binds tightly to the enzyme.

Question 42

Know anaploretic reactions 1,2,4

Answer 42

Question 43

Know the diagram

Answer 43

Know the diagram

Question 44

Know this diagram for sure!

Answer 44

Question 45

Know diagram

Answer 45

Question 46

What is the regulation of TCA cycle?

Answer 46

These **3 reactions are the key sites**. **Citrate synthase** - ATP, NADH and succinyl-CoA inhibit **Isocitrate dehydrogenase**- ATP inhibits, ADP and NAD+ activates **Alpha ketoglutarate dehydrogenase**- NADH and succinyl -CoA inhibit, AMP activates **Pyruvate dehydrogenase:** ATP, NADH, acetyl-CoA inhibit, NAD+, CoA activate