Lecture 9: Citrate Cycle

Question 1

Function of citrate cucle

Answer 1

convert nrg from oxidation of acetyl-CoA into
NADH
FADH2
GTP

Question 2

citrate cycle intermediates

Answer 2

8

continualling replenished to maintain smoothrunning energy conversion process

Question 3

How is flux through citrate cycle monitored?

Answer 3

by resetting level of available substrate after each turn of the cycle (keep adding in substrate)

keep adding substrate to keep it running

Question 4

2 things citrate cycle does

Answer 4

the “hub” of cellular metabolism–at the center

1) links oxidation of metabolic fuels to ATP synthesis
2) provides shared metabolites for lots of other metabolic pathways

Question 5

Overview of citrate cycle (what goes in, what comes out)

Answer 5

8 reactions
oxidize acetyl-CoA
generate 2 CO2
3NADH, one FADH2
1GTP (step five) thats converted to ATP
reduce 3NAD and 1FAD
REMEMBER: Citrate cycle happens once for each 3 carbon molec, so twice for one glucose

Question 6

where are the enzymes of the citrate cycle found

Answer 6

inside mitochondrial matrix

Question 7

“currency exchange” for redox nrg and ATP synthase from NADH

Answer 7

2. 5ATP/NADH

7. 5 between the three

Question 8

currency exchange rate for oxidation FADH2?

Answer 8

1.5 ATP/FADH2

Question 9

How many ATP from citrate cycle?

Answer 9

10!

7.5 for NADH, 1.5 for FADH, 1 for GTP

Question 10

why is it called the citrate cycle?

Answer 10

citrate is the first product of the pathway

Question 11

What does the citrate cycle accomplish for the cell

Answer 11

1) transfer 8 e- from acetyl-CoA to coenzymes NAD+ and FAD to make 3NADH and 1 FADH2. (later oxidized by ETS to make ATP by oxidative phosphorylayion)
2) generate 2CO2 as waste. use substrate level phosphorylation to make 1GTP, which is converted to ATP
3) supplies metabolic intermediates for amino acid and porphyrin synth (to make heme)

Question 12

overall net reaction?

Answer 12

Acetyl-CoA + 3NAD++FAD+GDP+Pi+2H2O–> CoA + 2CO2 + 3NADH +3H+ + FADH2+GTP

standard free nrg change=-57.3kJ/mol

Question 13

regulated enzymes in citrate cycle

Answer 13

pyruvate dehydrogenase
citrate synthase
isocitrate dehydrogenase
alpha-ketoglurarate dehydrogenase

Question 14

would the citrate cycle be activated or inhibitied by high energy charge in the cell

Answer 14

inhibited

Question 15

There are 6 CO2 molecs in the complete oxidation of glucose. Where do they come from?

Answer 15

4 from citrate cycle

2 from pyruvate dehydrogenase reaction

Question 16

review slide 13!!!

Answer 16

review slide 13!!!!!

Question 17

reaction 1

Answer 17

condensation of oxaloacetate and acetul Co-A by citrate synthase to make citrate

standard free nrg change -31.4 kJ/mol
irreversible

Question 18

reaction 2

Answer 18

citrate isomerization by aconitase to make isocitrate
standard free nrg change +6.3 kJ/mol
reversible, depends on conc

Question 19

fluorocitrate

Answer 19

targets and inhibits aconitase
its a poison
it stops the citrate cycle=death
produced by lots of plants in Australia

Question 20

Reaction 3

Answer 20

oxidative decarboxylation of isocitrate by isocitrate dehydrogenase to form alpha-ketoglutarate
standard free nrg change -8.4 kJ/mol, reversible

Makes NADH!!!!!

Question 21

reaction 4

Answer 21

oxidative decarboylation of alpha-ketoglutarate by alpha-ketoglutarate dehydrogenase to make succinyl-CoA

standard free nrg change -30.1 kJ/mol: IRREVERSIBLE
Makes NADH!!!!

Question 22

Reaction 5

Answer 22

convert succiny;-CoA to succinate by succinyl-CoA synthase

substrate level phosphorylation generates ATP (from GTP made in this step)

standard free nrg change -3.3 kJ/mol; reversible

Question 23

GDP–>GTP

Answer 23

first form molec where phosphate (Pi) is attached to succcinyl group
then phosphoryl group transferred into another side chain in the enzyme
then its transferred into the GDP to make GTP

Question 24

What do we call the GTP in reaction 5?

Answer 24

ATP equivalent

the reaction that interconverts ATP and GTP does a readily reversible phosphoryl transfer reaction (standard free nrg change is pretty much 0, so its all about concs)

Question 25

Reaction 6

Answer 25

Oxidation of succinate by succinate dehydrogenase to fumarate

make FADH2!!!!
standard free nrg change is 0, reversible

Question 26

reaction 7

Answer 26

hydration of fumarase to form malate

standard free nrg change -3.8kJ/mol, close to zero

Question 27

reaction 8

Answer 27

Oxidation of malate by malate dehydrogenase makes oxaloacetate

NADH formed
standard free nrg change +29.7kJ/mol–> VERY UNFAVORABLE

EXCEPTION!!!: the reaction proceeds despite being unfavorable because oxaloacetate is in VERY low concs in actual conditions. actual conditions can overcome standard free energy change

Question 28

the net reaction of glycolysis, pyruvate dehydrogenase reaction, and the citrate cycle yeilds what?

Answer 28

6CO2
10NADH
6H+
2FADH2
4ATP

Question 29

why 4ATP??

Answer 29

2 from glycolysis
2 from citrate cycle (from GTP, the ATP equivilant)

1 from each round of glycolysis, 1 from each round of citrate cycle

Question 30

radioactive acetyl CoA experiments

Answer 30

first carbon is released as CO2 at SECOND turn of cycle
second carbon is released at the FOURTH turn of the cycle

2 carbons enter before the first step, and they are metabolites that remain for FOUR rounds of the cycle

Question 31

regulation of citrate cycle

Answer 31

pyruvate dehydrogenase

pyruvate carboxylase

Question 32

pyruvate dehydrogenase regulation

Answer 32

activated by CoA, NAD+, ADP

inhibited by NADH, AcetylCoA, ATP

Question 33

pyruvate carboxylase

Answer 33

stimulated by acetyl-CoA to mainatin OAA for citrate synthesis

balances input of OAA with acetyl-CoA

needed when fat stores are main source of metabolic nrg b/c it provides OAA to keep citrate cycle going

ATP used here not counted in book keeping

Question 34

isocitrate dehydrogenase

Answer 34

stimulated by ADP

inhibited by NADH and ATP

Question 35

alpha-ketoglutarate dehydrogenase

Answer 35

stimulated by AMP

inhibited by NADH, succinyl-CoA, ATP