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

Question 1

where does TCA cycle occur

what is the nature of it?

Answer 1

in mitochondria of all aerobic cells

irreversible

Question 2

what is the net reaction of the TCA cycle

Answer 2

Question 3

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

Answer 3

10 ATP:

3 NADH -> 7.5 ATP

FADH2 -> 1.5 ATP

Question 4

is there any net removal or addition of TCA intermediates?

Answer 4

no

Question 5

which reactions of the TCA cycle are irreversible?

Answer 5

STEP 1: citrate synthase reaction

Question 6

what inhibits the citrate synthase reaction of the TCA cycle?

Answer 6

STEP 1

ATP, NADH - signs of high energy - inhibit it

also inhibited by high levels of citrate, = product inhibition

Question 7

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

Answer 7

aconitase converts citrate to isocitrate

this is reversible

citrate <-> cis-aconitate <-> isocitrate

Question 8

what is the rate limiting step of the TCA cycle?

what regulates it?

Answer 8

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

Question 9

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

Answer 9

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

Question 10

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

Answer 10

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

Question 11

what reaction in the TCA cycle produces energy as GTP?

Answer 11

succinyl CoA synthase aka thiokinase takes succinyl CoA, makes succinate

takes GDP + Pi, releases GTP + CoASH

Question 12

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

what enzymes do this?

Answer 12

convert succinate to OAA

succinate DH, fumarase, malate DH

Question 13

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

Answer 13

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

Question 14

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

detail where the ATP comes from

Answer 14

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

Question 15

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

Answer 15

amphibolic

Question 16

what regulates the TCA cycle?

Answer 16

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)

Question 17

what are the anabolic abilites of the TCA cycle?

Answer 17

1) Citrate -> fatty acid synthesis
2) Succinyl CoA -> heme synthesis
3) OAA and Malate -> glucose synthesis
4) OAA -> glucose, aspartate, asparagine, pyrimidines synthesis

Question 18

what are the anaplerotic rxns of the TCA cycle?

Answer 18

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

Question 19

what are the analproetic reactions of the TCA cycle?

Answer 19

1) Pyruvate Carboxylase converts pyruvate -> OAA
2) many amino acids -> TCA cycle intermediates, or -> OAA directly
3) PEP carboxykinase converts PEP to OAA

Question 20

what stimulates the analproetic pyruvate carboxylase reaction?

Answer 20

Acetyl CoA

occurs if acetyl CoA cannot enter the TCA cycle as OAA

Question 21

what TCA cycle intermediate can glutamate become?

Answer 21

a-KG

Question 22

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

Answer 22

succinyl CoA

Question 23

what tca cycle intermediate can phenylalanine or tyrosine make?

Answer 23

fumarate

Question 24

what tca cycle intermediate can aspartate make?

Answer 24

OAA

Question 25

what are ATP and NADH effect on the TCA cycle

Answer 25

decrease citrate synthase, ICDH, a-KGDH

Question 26

is acetyl CoA a limiting reactant in TCA cycle

Answer 26

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