Lecture 32– Energy II: Acetyl CoA, Mitochondria, Oxygen

Question 1

TCA (tricarboxylic acid cycle)/Krebs/Citric acid cycle (8)

Answer 1

Occurs in the mitochondrial matrix.
Aerobic process - requires oxygen.
Amount of oxaloacetate remains the same - continually recycled.

Two cycles = One glucose molecule.
Every turn of the cycle three NADH, one FADH2, one GTP.
ATP not produced in TCA cycle.

Purpose of TCA cycle is to make NADH and FADH2 which are used in ETC, providing electrons for oxidative phosphorylation.

Enzymes are clinically important.

Question 2

Regulate entry into TCA cycle - Pyruvate —> ACoA (4)

Answer 2

Pyruvate —> ACoA : Irreversible reaction
Enzyme: Pyruvate dehydrogenase
Inhibited: NADH, ATP, ACoA (products of reaction)
Stimulated: ADP, Pyruvate

Question 3

Regulate entry into TCA cycle - NADH and ACoA (2)

Answer 3

Build up of NADH and ACoA inform enzyme that energy needs of cell are being meet or that FA have been broken down producing NADH and ACoA - has the effect of sparing glucose.

Question 4

Muscle and Ca2+ (4)

Answer 4

Muscle pyruvate dehydrogenase is activated by phophatases (removes phosphorylation).
This phophatase is stimulated by Ca2+.
Allows muscle to link contraction for a process to produce ATP.
Increased Ca2+ = Cause contraction in process = More ATP.

Question 5

Liver and Ca2+ (4)

Answer 5

Liver adrenaline increases Ca2+ (activation of a-adrenergic receptors) and IP3.
In liver/adipose tissue, insulin (signifies fed state) stimulates the phosphatase which funnels glucose to FA synthesis (for storage).

Question 6

Regulate entry into TCA cycle - ACoA —> Citrate (3)

Answer 6

ACoA —> Citrate
Enzyme: Citrate synthase
Inhibited: Citrate
When there is enough ATP the ACoA will be directed to other pathways (e.g. FA synthesis to be stored).

Question 7

Regulate entry into TCA cycle - Isocitrate —-> a-ketogluterate (3)

Answer 7

Isocitrate —-> a-ketogluterate
Enzyme: isocitrate dehydrogenase
Inhibited: NADH, ATP
Stimulated: ADP

Question 8

Regulate entry into TCA cycle - a-ketogluterate —> Succinyl CoA (2)

Answer 8

a-ketogluterate —> Succinyl CoA
Enzyme: a-ketogluterate dehydrogenase
Inhibited: NADH, ATP, Succinyl CoA

Question 9

Regulate entry into TCA cycle - Citrate build-up (3)

Answer 9

Two control points
Inhibition of isocitrate dehydrogenase and a-ketogluterate dehydrogenase leads to build up of citrate.
Citrate then transported out of mitochondria = inhibits PFK = Stops glycolysis = Acts as a source for acetyl CoA for FA synthesis.

Question 10

Beriberi (5)

Answer 10

Deficiency in Vitamine B1 = Thiamine.
Common in far east where rice is staple.
Characterised by cardiac and neurological symptoms.
Thiamine is a prosthetic group (required to make these enzymes) for pyruvate and a-ketogluterate dehydrogenase.
Important in regulating TCA cycle - deficient in someone with Beriberi - inability to generate sufficient ATP.

Question 11

Fate of NADH and FADH2 (9)

Answer 11

Used in ETC (electron transport chain).
Removal of hydrogen atoms from oxidisable substrates.
Hydrogen atoms enter ETC and is split to give an electron and proton.
Electrons passed through enzymes (cytochromes left to right/ high to low energy), until it finally reacts with oxygen.
Proton pumped across inner mitochondrial membrane into inter membrane space- generating a proton (pH) gradient.
This gradient produces ATP.
Electron transport is coupled to ATP synthesis. 3 protons = 1 ATP, but one proton is required to transport ATP out of matrix so 4 protons = 1 ATP.
NADH = 3 ATP (produced) = 10 H+ (pumped out)
FADH2 = 2 ATP (produced) = 6 H+ (pumped out)

NADH —> NAD+ + H+ + 2e- (Oxidation)

Question 12

What regulates ETC? (2)

Answer 12

Tightly coupled to the demands of the cell for energy (phosphorylation ADP –> ATP).
Exceptions - Regulated uncoupling (in tissue where uncoupling protein is expressed) leads to generation of heat.

Question 13

ATP Synthesis (4)

Answer 13

ATP Synthase is a transmembrane protein which acts as a motor.
H+ pumped out into IMM, creating a pH gradient transmembrane potential (proton motive force).
The H+ will be able to drop back down their H+/pH gradient through ATP synthase, generating ATP from ADP as it moves through.

Question 14

Mitochondria and heat generation in new born (5)

Answer 14

Proton movement across IMM may not be coupled with ATP synthesis.
Neonates can not shiver (which is a good way of generating heat), so they lose heat from their surface. They possess brown fat (predominantly around neck and shoulders).
Brown fat contains a large amount of mitochondria (giving it’s colour).
Infant and adults have different mitochondria. One difference is that they contain a protein called uncoupling protein (uncouples the proton gradient with ATP synthesis).
Alternative route by which H+ can move down the conc gradient and it doesn’t generate ATP, it generates heat.
It is assumed that infants mitchondria (in brown tissue) undergo the uncoupling protein route to generate heat.

Question 15

OXPHOS disease (5)

Answer 15

Common degenerative disease.
Caused by mutations in genes encoding proteins of the ETC.
Symptoms: Dementia, Epilepsy, Fatigue.
Symptoms may be evident near birth to early adulthood.
Metabolic consequence - congenital lactic acidosis (affect mitochondria in children, causes build up of lactic acid).