METABOLISM SYLLABUS 4: TCA Cycle (aka Krebs Citric Acid Cycle) Flashcards Preview

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Flashcards in METABOLISM SYLLABUS 4: TCA Cycle (aka Krebs Citric Acid Cycle) Deck (26):

where does TCA cycle occur

what is the nature of it?

in mitochondria of all aerobic cells



what is the net reaction of the TCA cycle

A image thumb

how many ATP are yielded per mole of Acetyl CoA that enters the TCA cycle?

10 ATP: 

3 NADH -> 7.5 ATP 

FADH2 -> 1.5 ATP



is there any net removal or addition of TCA intermediates?



which reactions of the TCA cycle are irreversible?

STEP 1: citrate synthase reaction 



what inhibits the citrate synthase reaction of the TCA cycle?


ATP, NADH - signs of high energy - inhibit it 

also inhibited by high levels of citrate, = product inhibition


what is the reversibility of step 2 of the TCA cycle?

aconitase converts citrate to isocitrate 

this is reversible 


citrate cis-aconitate isocitrate


what is the rate limiting step of the TCA cycle?

what regulates it?

STEP 3: isocitrate DH reaction 

isocitrate undergoes oxidative decarboxylation 

Co2 is released 

NAD+ is reduced to NADH 

results in aKg, alpha ketoglutarate 

regulated by energy charge: stimulated by ADP - low energy charge - and inhibited by ATP and NADH - high energy charge


what is the structure of the a-KGDH enzyme? what is its regulation as such?

E1, E2, E3, like in the PDH complex 

E1 enzyme: aKg decarboxylase: has thiamine PP as a cofactor, decarboxylates aKg to Co2 + a 4C succinyl product 

E1 is inhibited by ATP

E2: uses lipoid acid; transfers the 4C succinyl product to CoASH

E2 is inhibited by succinyl CoA  

E3: uses FAD and NAD to regenerate oxidized lipoic acid for E2

E3 is inhibited by NADH 


why is GTP, not ATP, produced during TCA cycle?

b/c occurs in the mito and the mito can make some of its own proteins, and they need GTP for ribosome assemblies and elongation factors and movements of mRNAs 

GTP though is equivalent to ATP, can be converted easily


what reaction in the TCA cycle produces energy as GTP?

succinyl CoA synthase aka thiokinase takes succinyl CoA, makes succinate

takes GDP + Pi, releases GTP + CoASH


what is the purpose of steps 6-8 of the TCA cycle?

what enzymes do this?

convert succinate to OAA 

succinate DH, fumarase, malate DH


what is the equilibrium of the malate DH reaction in vitro versus in vivo?

in vitro: greatly favors OAA -> malate 


in vivo: OAA is continually being removed by citrate synthase rxn; this keeps [OAA] low; pulls rxn in direction toward OAA formation


what is the overall yield from G6P -> 6CO2?

detail where the ATP comes from

32 ATPs per G6P: 

Glucose -> 2 pyruvate + 2 NADH = 2ATP 

2NADH -> mito = 5 ATP 

2 Pyruvate -> 2 Acetyl CoA + 2NADH 

2NADH -> 5 ATP 

2 Acetyl CoA -> 20 ATP 

= 32 total ATP per G6P


is the TCA cycle a catbolic, anabolic, or amphibolic pathway?



what regulates the TCA cycle?

1) respiratory control: regulates oxidation of NADH and FADH2 by mito respiratory chain 

2) energy charge: ATP inhibits citrate synthase, isocitrate DH, aKg DH

3) [OAA]: OAA is pulled out of cycle during gluconeogenesis, reduced to malate when NADH levels are elevated (aka after EtOH consumption)


what are the anabolic abilites of the TCA cycle?

1) Citrate -> fatty acid synthesis 

2) Succinyl CoA -> heme synthesis 

3) OAA and Malate -> glucose synthesis

4) OAA -> glucose, aspartate, asparagine, pyrimidines synthesis 


what are the anaplerotic rxns of the TCA cycle?

reactions that occur if OAA is removed or if intermediates are pulled out such that OAA isn't regenerated and the cycle stops as acetyl CoA accumulates 

these reactions replenish the pulled-out OAA or deficient OAA


what are the analproetic reactions of the TCA cycle?

1) Pyruvate Carboxylase converts pyruvate -> OAA

2) many amino acids -> TCA cycle intermediates, or -> OAA directly

3) PEP carboxykinase converts PEP to OAA 


what stimulates the analproetic pyruvate carboxylase reaction?

Acetyl CoA 

occurs if acetyl CoA cannot enter the TCA cycle as OAA 


what TCA cycle intermediate can glutamate become?



what tca cycle intermediate can valine , isoleucine, methionine, threonine make?

succinyl CoA


what tca cycle intermediate can phenylalanine or tyrosine make?



what tca cycle intermediate can aspartate make?



what are ATP and NADH effect on the TCA cycle

decrease citrate synthase, ICDH, a-KGDH


is acetyl CoA a limiting reactant in TCA cycle

not usually b/c also produced from oxidation of fatty acids, several amino acids, ethanol

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