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Metabolism > TCA cycle > Flashcards

TCA cycle Flashcards

1
Q

Where does the Krebs cycle not take place?

A

mature red blood cells, as they lack mitochondria

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2
Q

Citric Acid Cycle definition

A

A series of chemical reactions used by all aerobic organisms to release stored energy through oxidation of acetyl-CoA derived from carbohydrates, fats and proteins into ATP and carbon dioxide

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3
Q

What else does the TCA cycle produce?

A

precursors of certain amino acids

NADH and FADH2

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4
Q

What is FAD+ an example of?

A

cofactor

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5
Q

Cofactor definition

A

a substance whose presence is essential for the activity of an enzyme

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6
Q

Features of FAD+

A

Bound solidly onto the enzymes they’re involved with

less negative redox potential than NAD

can’t diffuse through membrane, thus reactions must be attached to membrane

lower reducing potential, so feeds electrons into ETC at ubiquinone at complex II

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7
Q

Features of NAD+

A

higher redox potential

not bound onto enzyme

type of coenzyme

NADH product diffuse to complex I in ETC

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8
Q

What process occurs before TCA ?

A

pyruvate decarboxylation

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9
Q

Stages of pyruvate decarboxylation

A
  1. Pyruvate transported into mitochondrion via specific pyruvate-H+ symport
  2. Cofactor TPP decarboxylares pyruvate, forming CO2 and a 2C compound, catalysed by pyruvate decarboxylase
  3. CoA added which is catalysed by dihydrolipoyl transactylase, forming Acetyl CoA
  4. Formation of NADH, catalysed by dihydrolipoyl dehydrogenase. Two electrons are transferred first to a FAD then to NAD
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10
Q

Brief structure of pyruvate dehydrogenase complex

A

three subunits: E1, E2 and E3

Lipoamide arm bound to E2 which guides the substrate from one subunit to the next, which ensures that the pyruvate decarboxylation occurs in the right direction

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11
Q

Why pyruvate dehydrogenase is so important?

A

irreversible reactions, thus become committed to Krebs

beyond PDH glucose cannot be resynthesised

it is inhibited by ATP and stimulated by ADP, thus acts as a key regulator of TCA

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12
Q

What inhibits pyruvate dehydrogenase + importance?

A

ATP- prevents excess breakdown of unnecessary glucose- inactivated by phosphorylation of a kinase stimulated by high ATP levels

NADH-

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13
Q

How are levels of pyruvate dehydrogenase upregulated?

A

magnesium, calcium and insulin stimulate PDH phosphatase, dephosphorylating the enzyme, thus activating it

all markers or energetic contractions

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14
Q

What causes poisoning when arsenic is ingested?

A

arsenite forms stable complex with thiol group of lipoic acid, which is found attached to the E2 group in the pyruvate dehydrogenase complex

leads to the inhibition of pyruvate dehydrogenase, which causes an increase in pyruvate and lactate

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15
Q

1st reaction in TCA + Enzyme + type of reaction

A

oxaloacetate (4) + acetylcoA (2) –> citrate

water –> CoA + H+

citrate synthase

condensation

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16
Q

2nd reaction in TCA + Enzyme

A

citrate —> isocitrate

17
Q

3rd reaction in TCA + Enzyme + type of reaction

A

isocitrate –> alpha- ketoglutarate

NAD+ –> NADH + H+ + CO2

isocitrate dehydrogenase

process of oxidative decarboxylation

18
Q

4th reaction in TCA + enzyme + type of reaction

A

alpha-ketoglutarate –> succinyl CoA

NAD+ + CoA –> NADH + CO2 + H+

alpha ketoglutarate dehydrogenase

oxidative decarboxylation

19
Q

What type of enzyme is alpha ketoglutarate dehydrogenase?

A

lipoic acid coenzyme and thiamine pyrophosphate TPP coenzyme

20
Q

5th reaction in TCA + enzyme + type of reaction

A

Succinyl CoA–> succinate + CoA

Pi + GDP –> GTP

succinyl-CoA synthase
thioesterase deacylation

21
Q

GTP function

A

phosphoryl donor in protein synthesis, gluconeogenesis and signal transductions

translocation of proteins into the mitochondrial matrix

conversion into ATP

22
Q

how is GTP converted into ATP?

A

GTP + ADP —> ATP + GDP

nucleoside diphosphokinase

23
Q

6th reaction in TCA + Enzyme + type of reaction

A

succinate —> fumarate

FAD —> FADH2

succinate dehydrogenase

oxidation

24
Q

Key features of succinate dehydrogenase

A

embedded in mitochondrial membrane

directly linked to ETC

only enzyme common to both TCA and ETC

25
Q

Why is reduced FAD produced, not NADH?

A

FAD is the hydrogen acceptor because the free energy is insufficient to reduce NAD+

26
Q

7th reaction in TCA + Enzyme

A

fumarate to malate

27
Q

8th reaction in TCA + Enzyme + type of reaction

A

malate –> oxaloacetate

NAD+ –> NADH + H+

malate dehydrogenase

28
Q

advantages of a cycle

A
  1. catalytically small amounts of cycle intermediates required to oxidise large amounts of acetyl CoA
  2. only small amount of oxaloacetate needed, as it is regenerated
  3. block the cycle at succinate dehydrogenase and it becomes an open pathway, less efficient- now for each acetyl CoA oxidised, 1 molecule of oxaloacetate is needed
29
Q

disadvantage of a cycle

A

the removal of cycle intermediates for biosynthesis can deplete levels of cycle intermediates

need anaplerotic reactions to maintain concentrations

30
Q

Examples where anapleurotic reactions replenish TCA cycle intermediates

A

aspartate –> oxaloacetate

amino acids/purine nucleotides–> fumarate

amino acids –> glutamate –> a ketoglutarate

odd numbered FAs/valine isoleucine –> propionyl CoA –> succinyl CoA

31
Q

Control of TCA cycle

A

most control of energy metabolism is not related to substrate availability instead a response for the demand for ATP

32
Q

Other functions of TCA cycle

A

routes of disposal for amino acids and odd chain fatty acids- 3 carbon units that occur during beta oxidation

intermediates can be used as starting points for biosynthesis

33
Q

intermediates and their potential products

A

citrate - fatty acids

alpha ketoglutarate- glutamate

succinyl CoA - porphyrins, haeme

oxaloacetate- aspartate, pyrimidines, glucose

34
Q

How much ATP is produced per molecule of TCA product?

A

NADH- 2.5

FADH2- 1.5

GTP - 1

35
Q

different sources of acetyl CoA

A

fatty acid beta oxidation

ketone body oxidation

amino acid degradation

sugars glycolysed into pyruvate

36
Q

What can mutations in isocitrate dehydrogenase cause?

A

found in 60-90% of secondary gliomas

12-18% leukemias

lead to the production of an oncometabolite

37
Q

What is beriberi?

A

thiamine deficiency, which forms TPP which is the prosthetic group of pyruvate dehydrogenase, alpha ketoglutarate dehydrogenase and transketolase

symptoms: sheep-like gait, weakness

mostly in asian populations, as rice has a low thiamine content and alcoholics

38
Q

Albert Szent- Gyorgyi’s experiment

A

minced tissue and showed that it took up oxygen rapidly

oxygen uptake was greatly increased when carbohydrates or other C3 and C4 products were added- more TCA

39
Q

What did Krebs and Johnson show?

A

Citrate was not only oxidised rapidly but it was also reformed, thus suggesting a cycle

Metabolism (13 decks)

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