MBC: Cell metabolism: Krebs cycle

Question 1

NET products of TCA cycle (Krebs) for every molecule of PYRUVATE?

Answer 1

2 CO2
3 NADH
1 FADH
1 GTP

Question 2

Aerobic or anaerobic?

Answer 2

only takes place under aerobic conditions

this is due to the fact that the NADH and FADH are only re-oxidised to NAD and FAD during oxidative phosphorylation which takes place in the presence of oxygen only

Question 3

how can amino acids also enter the TCA cycle?

Answer 3

amino group of the acid is removed (excreted as urea)

the carbon skeleton is either funnelled into the production of glucose or fed into the Krebs cycle

Question 4

Explain transamination reactions? Type of reaction?

Answer 4

an amine group is transferred from one amino acid to a keto acid forming a new pair of amino and keto acids

Group transfer reaction

Question 5

How does NADH/ its high energy electrons cross the cytosol into the matrix of the mitochondria?

Answer 5

The glycerol phosphate shuttle- skeletal, muscle and brain

and

The malate-aspartate shuttle- liver, kidney and heart

Question 6

how are the high energy electrons transferred across the mitochondrial membrane? enzymes involved?

Answer 6

Via a shuttle:
i) cytosolic G3P dehydrogenase transfers electrons from NADH to Dihydroxyacetone phosphate (DHAP) to produce G3P

DHAP+ 2H–> G3P

Cytosolic G3P dehydrogenase enzyme

ii) A membrane bound form of the same enzyme transfers the electrons from G3P to FAD to produce FADH. These then gets passed to CoQ, part of the electron transport chain.

Question 7

Enzymes involved in the Malate-aspartate shuttle? What type of reaction is the shuttle?

Answer 7

Aspartate transaminase AT

malate dehydrogenase MDH

REDOX

Question 8

Reactions of the malate-aspartate shuttle?

Answer 8

cytoplasm:

i) aspartate–> oxaloacetate
AT
a-keto glutarate–> glutamate

ii)oxaloacetate–> malate
MDH
NADH–>NAD (during ii)

malate is then shuttled into the mitochondria:

i) malate–>oxaloacetate
MDH
NAD–>NADH (during ii)

ii)oxaloacetate–> aspartate
AT
glutamate–> a-ketoglutarate

aspartate then shuttled back out of the mitochondria via the anti portals (channels)

Question 9

transamination reaction in the malate-aspartate shuttle?

Answer 9

glutamate+oxaloacetate–> a-ketoglutarate+aspartate

Question 10

How many overall molecules of ATP formed from Krebs cycle?

Answer 10

if:
3 molecules ATP formed for every NADH re-oxidised

2 molecules ATP formed for every FADH re-oxidised

AND

products of Krebs cycle gives:

3 NADH
1FADH
1GTP

THEN:
3xNADH + 1xFADH + 1xGTP = 12ATP for every acetyl CoA molecule oxidised

Question 11

What happens in cancer with TCA cycle defects?

Answer 11

mutations in the TCA genes decreases TCA activity.
Instead, aerobic glycolysis is enhanced (where lactate is generated from glucose even when under sufficient oxygen supply)