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Biochemistry: 2nd yr > Quiz 6 - TCA cycle > Flashcards

Quiz 6 - TCA cycle Flashcards

1
Q

What are the 4 fates of pyruvate?

A

Transamination: Alanine

Carboxylation: Oxaloacetate

Oxidative decarboxylation: Acetyl-CoA

Reduction: Lactate

<em>Glucose becomes pyruvate through glycolysis</em>

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2
Q

What are other sources of acetyl-CoA?

A
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3
Q

What are the 3 fates of acetyl CoA?

A

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>

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4
Q

Know the diagram

A
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5
Q

What is made in TCA cycle?

What is degraded?

A

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

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6
Q

Where does Krebs cycle occur?

A

Mitochondria

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7
Q

What is primary function of Krebs cycle?

A

The oxidation of Acetyl-CoA to Co2.

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

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8
Q

What is the overall result of TCA cycle?

What is not required in this pathway?

A

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

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9
Q

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

What does this represent?

A

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.

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10
Q

What is the TCA cycle central to the oxidation of?

Is there any hormonal control of the cycle?

A

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

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11
Q

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

A

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

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12
Q

List the key enzymes of TCA cycle and their function

A
  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
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13
Q

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

A

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>

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14
Q

Know the diagram

A
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15
Q

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

A

1. Cleavage between Cs alpha and beta to a carbonyl

2. An alpha cleavage of an alpha hydroxyketone

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16
Q

What is a better way to cleave acetate?

A

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.

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17
Q

Pyruvate is oxidatively decarboxylated to form what substance?

What does pyruvate use to create this substance?

A

Acetyl-CoA

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

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18
Q

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

A

1. For entry into TCA if ATP is needed

2. For fatty acid synthesis if sufficient ATP is present

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19
Q

What is pyruvate dehydrogenase inhibited by?

Is the pyruvate dehydrogenase reaction irreversible?

What other mitochndrial enzyme uses pyruvate?

A

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

20
Q

What are the cofactors and enzymes used by pyruvate dehydrogenase?

A

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)

21
Q

What activates Pyruvate dehydrogenase in liver?

A

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

22
Q

Who are most likely to have thiamine deficiency?

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

A

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

23
Q

What does insufficient thiamine lead to?

A

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>

24
Q

What other enzyme complexes similar to pyruvate dehydrogenase use thiamine?

A

1. a-ketoglutarate dehydrogenase(citric acid cycle)

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

25
Q

Coenzyme A structure

A
26
Q

Polypeptide chain of E2(dihydrolipoyl transacetylase)

A
27
Q

Know the cycle of pyruvate to acetyl-CoA

A
28
Q

What are allosteric inhibitors of Citrate synthase?

Does CS have large negative delta G?

A

NADH and succinyl CoA

Yes it has large negative delta G.

29
Q

What does enzyme Aconitase do?

What is the stereochemistry of the reaction?

What kind of cluster does aconitase use?

A

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.

30
Q

Oxidative decarboxylation of what substance occurs under Isocitrate dehydrogenase?

A

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

31
Q

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?

A

The 5 coenzymes are:

TPP, CoASH, Lipoic acid, NAD+, FAD

Second oxidative decarboxylation

The enzyme is similar to pyruvate dehydrogenase

32
Q

Is Succinyl-CoA synthatase reaction a substrate level phosphorylation?

What is made? What is synthesis driven by? What does it involve?

A

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

33
Q

What does succinate dehydrogenase involve?

What is the mechanism?

The enzyme is part of which pathway and where?

What happens to the electrons?

A

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.

34
Q

What does fumarase do?

A

Hydration across double bond

35
Q

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?

A

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.

36
Q

Know the diagram

A
37
Q

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

A

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

38
Q

Know the table

A
39
Q

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

A

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

40
Q

What are the intermediates of biosynthesis for the TCA cycle?

A

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

41
Q

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?

A

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.

42
Q

Know anaploretic reactions 1,2,4

A
43
Q

Know the diagram

A

Know the diagram

44
Q

Know this diagram for sure!

A
45
Q

Know diagram

A
46
Q

What is the regulation of TCA cycle?

A

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

Biochemistry: 2nd yr (3 decks)

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