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ESA 2 - MEH > Session 2: Carbohydrates > Flashcards

Session 2: Carbohydrates Flashcards

1
Q

What happens to pyruvate before entering the TCA cycle?

A

Pyruvate dehydrogenase oxidises pyruvate (using NAD+ as oxidising agent) and adds CoA to form acetyl CoA and CO2 (and NADH)

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2
Q

Where does the link reaction take place?

A

Mitochondrial matrix

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3
Q

Name a cofactor that pyruvate dehydrogenase requires.

A

Vitamin B1

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4
Q

Is the link reaction reversible or irreversible?

A

Irreversible because CO2 is released

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5
Q

What activates PDH activity? (5)

A

Low energy substrates activate PDH by dephosphorylation:

  1. Pyruvate
  2. CoA
  3. NAD+
  4. ADP
  5. Insulin
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6
Q

What inhibits PDH activity? (4)

A

High energy substrates and products inhibit activity by phosphorylation:

  1. Acetyl CoA
  2. NADH
  3. ATP
  4. Citrate
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7
Q

What does PDH deficiency lead to?

A

Lactic acidosis

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8
Q

Where does the TCA cycle take place?

A

Mitochondrial matrix

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9
Q

What molecules does the TCA cycle require? (3)

A

NAD+
FAD
Oxaloacetate

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10
Q

What is the function of the TCA cycle? (2)

A
  1. Break the C-C bond in acetate

2. Oxidise C atoms to CO2

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11
Q

Does the TCA cycle function in the absence of oxygen?

A

No

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12
Q

Approximately how many molecules of ATP doe the TCA cycle produce?

A

32 molecules of ATP

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13
Q

Describe the events of the TCA cycle. (6)

A
  1. Acetyl CoA (2C) join oxaloacetate (4C) to form citrate (6C)
  2. Citrate undergoes isomerisation to form Isocitrate
  3. Isocitrate gets oxidised (NAD+ -> NADH) to a-ketoglutarate (5C) by Isocitrate dehydrogenase
  4. A-ketoglutarate gets oxidised to succinyl-CoA (4C) (NAD+ -> NADH) with the addition of CoA by a-ketoglutarate dehydrogenase
  5. Succinyl-CoA releases CoA to form Succinate (4C) - this releases energy in the form of GTP
  6. Succinate undergoes isomerisation and oxidation to form oxaloacetate (along with FADH2 and NADH)
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14
Q

What are the products of one TCA cycle?

A

Multiply by 2 for each glucose because 2 pyruvate molecules:
6 NADH
2 FADH2
2 GTP

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15
Q

Which enzymes and molecules are involved in the regulation of the TCA cycle?

A
  1. Isocitrate dehydrogenase
  2. a-ketoglutarate dehydrogenase
  3. ATP/ADP ratio
  4. NADH/NAD+ ratio
  5. Succinyl-CoA
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16
Q

How does ATP regulate the TCA cycle?

A

High ATP/ADP ratio down-regulates the TCA cycle

17
Q

How does NADH regulate the TCA cycle?

A

High NADH/NAD+ ratio down-regulates the TCA cycle

18
Q

What regulates Isocitrate dehydrogenase?

A
  1. Down-regulated by NADH and ATP

2. Up-regulated by ADP

19
Q

What regulates a-ketoglutarate dehydrogenase?

A

NADH, ATP and succinyl CoA (product inhibition) down-regulates the enzyme activity

20
Q

What is the significance of Succinate?

A

Used in haem synthesis and can be interconverted to amino acids

21
Q

Why does the TCA cycle not function in the absence of O2?

A
  • Oxygen is the final acceptor of electrons in electron transport chain
  • Therefore, in the absence of oxygen, oxidative phosphorylation is not carried out as effectively
  • NAD+ and FAD not regenerated from donation of electrons
  • NADH and FADH therefore builds up and down-regulates the TCA cycle
22
Q

Which stage of carbohydrates metabolism produces most energy?

A

Electron transport chain (stage 4)

23
Q

Where does the electron transport chain and oxidative phosphorylation take place?

A

Inner membrane of the mitochondria

24
Q

What happens in the electron transport chain?

A
  • carrier molecules transfer high-energy electrons from NADH and FADH2 into a series of 4 highly specialised complexes which span the inner mitochondrial membrane
  • oxygen acts as the terminal electron acceptor;
  • 3 of the complexes act as proton translocating complexes (PTCs) which use the free energy from electron transport to move protons from inside the matrix to the inter-membrane space
  • the PTCs help create a proton concentration gradient across the inner mitochondrial membrane
25
Q

How does the electron transport chain release energy?

A

Electrons move easily (feasible process) from less electronegative to more electronegative (oxygen) molecules

26
Q

What is proton motive force (p.m.f.)?

A

Electrochemical potential difference of protons generated by the ETC which acts as a proton pump, using the energy of electrons from electron carriers - separates

27
Q

What is the role of ATP synthase in oxidative phosphorylation?

A

It is a complex through which protons re-enter the matrix, using the energy derived from the proton gradient (p.m.f.) across the inner mitochondrial membrane, driving synthesis of ATP from ADP and Pi

28
Q

How does oxidative phosphorylation differ from substrate level phosphorylation?

A
  1. PTCs and ATP synthase (OP) vs. Soluble enzymes (SLP)
  2. Energy coupling occurs indirectly through p.m.f vs. Coupling occurs directly through formation of high-energy of hydrolysis bond (SLP)
  3. Cannot occur in the absence of O2 (OP) vs. Can occur to limited extent (SLP)
  4. Major process for ATP synthesis (OP) vs. Minor process for ATP synthesis (SLP)
29
Q

How does high [ATP] in mitochondrial matrix regulate ATP synthase?

A
  1. High [ATP] means [ADP] is low - ATP synthase stops due to lack of substrate - this prevents transport of protons back into matrix
  2. [H+] in the intermembrane space increases to a level that prevents more protons being pumped into the space
  3. Electron transport stops due to absence of proton pumping
30
Q

What are the various means of inhibiting oxidative phosphorylation?
Name the substances involved.

A
  1. Inhibition of electron transport - CN-, CO
  2. Uncouplers - dinitrophenol, dinitrocresol, fatty acids, thermogenin (UCP1)
  3. Ox/phos diseases - mutation in mtDNA that leads to defects in PTCs and ATP synthase
31
Q

How do cyanide and carbon monoxide inhibit oxidative phosphorylation?

A

The terminal PTC has a heme group that gets poisoned by CN- and CO (like in Hb) and prevents the acceptance of electrons by O2

32
Q

What is the effect of uncouplers on oxidative phosphorylation?

A
  • increase the permeability of the mitochondrial inner membrane to protons - protons enter mitochondrial matrix without driving ATP synthase
  • ETC and ATP synthesis uncoupled - ET goes on but ATP is not synthesised
  • p.m.f. is dissipated as heat
  • no drive for ATP synthesis
33
Q

In which tissues is UCP1 (thermogenin) expressed?

A

Brown adipose tissue

34
Q

What is the function of UCP1 (thermogenin)?

A
  • It is an uncoupler that allows protons to re-enter the mitochondrial matrix without driving ATP synthesis, which dissipates p.m.f as heat rather than ATP
35
Q

What activates UCP1?

A

Noradrenaline

36
Q

When is UCP1 activated?

A

In response to colder environments - involved in non-shivering thermogenesis

ESA 2 - MEH (4 decks)

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