Lecture 9: Citrate Cycle Flashcards
Function of citrate cucle
convert nrg from oxidation of acetyl-CoA into
NADH
FADH2
GTP
citrate cycle intermediates
8
continualling replenished to maintain smoothrunning energy conversion process
How is flux through citrate cycle monitored?
by resetting level of available substrate after each turn of the cycle (keep adding in substrate)
keep adding substrate to keep it running
2 things citrate cycle does
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
Overview of citrate cycle (what goes in, what comes out)
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
where are the enzymes of the citrate cycle found
inside mitochondrial matrix
“currency exchange” for redox nrg and ATP synthase from NADH
- 5ATP/NADH
7. 5 between the three
currency exchange rate for oxidation FADH2?
1.5 ATP/FADH2
How many ATP from citrate cycle?
10!
7.5 for NADH, 1.5 for FADH, 1 for GTP
why is it called the citrate cycle?
citrate is the first product of the pathway
What does the citrate cycle accomplish for the cell
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)
overall net reaction?
Acetyl-CoA + 3NAD++FAD+GDP+Pi+2H2O–> CoA + 2CO2 + 3NADH +3H+ + FADH2+GTP
standard free nrg change=-57.3kJ/mol
regulated enzymes in citrate cycle
pyruvate dehydrogenase
citrate synthase
isocitrate dehydrogenase
alpha-ketoglurarate dehydrogenase
would the citrate cycle be activated or inhibitied by high energy charge in the cell
inhibited
There are 6 CO2 molecs in the complete oxidation of glucose. Where do they come from?
4 from citrate cycle
2 from pyruvate dehydrogenase reaction
review slide 13!!!
review slide 13!!!!!
reaction 1
condensation of oxaloacetate and acetul Co-A by citrate synthase to make citrate
standard free nrg change -31.4 kJ/mol
irreversible
reaction 2
citrate isomerization by aconitase to make isocitrate
standard free nrg change +6.3 kJ/mol
reversible, depends on conc
fluorocitrate
targets and inhibits aconitase
its a poison
it stops the citrate cycle=death
produced by lots of plants in Australia
Reaction 3
oxidative decarboxylation of isocitrate by isocitrate dehydrogenase to form alpha-ketoglutarate
standard free nrg change -8.4 kJ/mol, reversible
Makes NADH!!!!!
reaction 4
oxidative decarboylation of alpha-ketoglutarate by alpha-ketoglutarate dehydrogenase to make succinyl-CoA
standard free nrg change -30.1 kJ/mol: IRREVERSIBLE
Makes NADH!!!!
Reaction 5
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
GDP–>GTP
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
What do we call the GTP in reaction 5?
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)
Reaction 6
Oxidation of succinate by succinate dehydrogenase to fumarate
make FADH2!!!!
standard free nrg change is 0, reversible
reaction 7
hydration of fumarase to form malate
standard free nrg change -3.8kJ/mol, close to zero
reaction 8
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
the net reaction of glycolysis, pyruvate dehydrogenase reaction, and the citrate cycle yeilds what?
6CO2 10NADH 6H+ 2FADH2 4ATP
why 4ATP??
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
radioactive acetyl CoA experiments
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
regulation of citrate cycle
pyruvate dehydrogenase
pyruvate carboxylase
pyruvate dehydrogenase regulation
activated by CoA, NAD+, ADP
inhibited by NADH, AcetylCoA, ATP
pyruvate carboxylase
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
isocitrate dehydrogenase
stimulated by ADP
inhibited by NADH and ATP
alpha-ketoglutarate dehydrogenase
stimulated by AMP
inhibited by NADH, succinyl-CoA, ATP
