Download The citric acid cycle • Also known as the Kreb`s cycle

The citric acid cycle
• Also known as the Kreb’s cycle
-named after Hans Krebs, whose work on this cycle earned him a Nobel prize in 1953
• Or TCA cycle (tricarboxylic acid) cycle (not The Citric Acid cycle)
The citric acid cycle requires _____________ conditions
• O2 serves as the final electron acceptor as pyruvate is completely oxidized to CO2 and H2O
If ____________ conditions exist, we get < 10% of the energy generated under aerobic conditions
• Can you determine what the exact amount of energy is under these conditions??
Results:
1. _____________________
Oxidation of Acetyl-CoA to CO2 produces energy
Energy is generated as ATP (GTP)
Energy is also found in the reducing power of NADH and FADH2
2.
______________________________
Carbohydrates, lipids, amino acids, nucleotides, porphyrins
Intermediates of TCA cycle are starting points for biosynthetic reactions (exit points)
Other pathways feed into the cycle (entrance points)
TCA is a cyclic pathway
This is different from glycolysis (which is linear).
Other differences:
Glycolysis occurs in the cytoplasm
Glycolysis does not require oxygen (can occur under anaerobic and aerobic conditions),
while TCA requires oxygen (only occurs under aerobic conditions)
Chemistry C483
Fall 2009
Prof Jill Paterson
31-1
Chemistry C483
Fall 2009
Prof Jill Paterson
31-2
Let’s look at the citric acid cycle in detail.
As we do, note:
1. the flow of C
2. the movement of electrons
3. the production of energy
simplify further:
Net: oxidation of acetyl CoA by the release of electrons
if shown cycle, know:
1. type of reaction occurring
2. type of enzyme catalyzing the reaction
3. be able to follow the C atoms
4. the steps where energy is produced (and if it is ATP, NADH, FADH2)
5. the steps where CO2 is released
6. the committed step
7. regulatory steps
8. structure of citrate (tricarboxylic acid)
Step 1: Citrate synthase
1.
Citrate formed (condensation of oxaloacetate
with acetyl CoA)
2.
Aldol condensation (acetyl to keto double bond)
3.
Only reaction where C-C bond is formed
Rxn
ATP
NADH
FADH2
Total
Chemistry C483
Fall 2009
Prof Jill Paterson
31-3
4. ∆G = -31.5 kJ/mol (but we do not create ATP!! This favorable energy ensures the cycle
moves in 1 direction. Not none of these steps are listed as reversible)
5. Synthase catalyzes this reaction (catalyzes for addition to a double bond, or elimination
to form a double bond. Does not require ATP)
Step 2: aconitase
1.
Isomerization reaction: tertiary alcohol to secondary alcohol
2.
A lyase reaction occurs to allow for isomerization (next slide).
Overall, nonhydrolytic cleavage.
3.
This rearrangement occurs to allow for further oxidation of
the molecule.
Rxn
ATP
NADH
FADH2
Total
Step 3: Isocitrate dehydrogenase
• First of four oxidation/reduction steps
First of four oxidative decarboxylation
steps, coupled to NAD+ reduction
• Irreversible
• Also a non-hydrolytic cleavage reaction
• Transfer of a hydride to NAD+
• First reaction where we lose electrons
Chemistry C483
Fall 2009
Prof Jill Paterson
31-4
Rxn
ATP
NADH
FADH2
Total
Step 4: α-ketoglutarate dehydrogenase complex
• Second oxidative decarboxylation reaction, coupled to
NAD+
• Also a non-hydrolytic cleavage
• Mechanism is identical to pyruvate dehydrogenase,
except succinyl group is activated, not acetyl (SuccinylCoA thioester is a HIGH energy bond)
• Purpose of this step is to generate the high energy
succinyl CoA
Rxn
ATP
NADH
FADH2
Total
Step 5: Succinyl-CoA synthetase
• Energy of succinyl CoA is transferred
(conserved) to GTP
• SUBSTRATE LEVEL PHOSPHORYLATION:
group transfer reaction
• ONLY step where ATP is directly formed
• All other ATP is produced by oxidative
phosphorylation
Oxid. Phosphor. is the oxidation
of reduced cofactors (NADH,
FADH2), to form ATP from ADP +
Pi (see future notes)
Chemistry C483
Fall 2009
Prof Jill Paterson
31-5
Rxn
ATP
NADH
FADH2
Total
Step 6: succinate dehydrogenase complex
• This enzyme is located in the inner mitochondrial
membrane (all other reactions are in the mitochondrial
matrix)
Also known as Complex II, which feeds e- directly
into the electron transport chain (we will see in a
bit)
• Oxidation/reduction with formation of double bond
• Formation of FADH2 (note we get NADH with hydroxyl
and carbonyl groups, FADH2 with other reactions)
• Enzyme has a Fe-S cluster (remember why??)
• Only the trans isomer is formed
Rxn
ATP
NADH
FADH2
Total
Chemistry C483
Fall 2009
Prof Jill Paterson
31-6
Step 7: Fumarase
• Trans addition of water
• Only the L-isoform forms
Rxn
ATP
NADH
FADH2
Total
Step 8: Malate dehydrogenase
• Oxidation of L-malate
• Regenerates oxaloacetate
• Reaction is at equilibrium (so reversible)
Results of TCA cycle
1.
Oxidation of 1 acetyl-CoA to 2 CO2
(release CO2 at steps 3 & 4)
2.
3 NAD+ are reduced to NADH with dehydrogenase reactions
(steps 3, 4, & 8)
3.
1 FAD is reduced to FADH2
(Step 6)
4.
1 phosphoanhydride bond is formed making GTP
(Step 5)
5. Oxaloacetate is reformed
Chemistry C483
Fall 2009
Prof Jill Paterson
31-7
Energy production from 1 pyruvate (and from 1 TCA cycle):
Every pyruvate generates:
1 NADH from pyruvate dehydrogenase
3 NADH from TCA cycle
1 ATP from TCA cycle
1 FADH2 from TCA cycle
We will see that from oxidation of:
NADH we will yield _________
FADH2 we will yield _________
Energy derived from 1 glucose molecule
We see this occur in muscles when we
exercise and they go into oxygen debt.
The reason our muscles get sore is from
lactate (lactic acid)
Chemistry C483
Fall 2009
Prof Jill Paterson
31-8
Summary of energy
ATP
NADH + H+
FADH2
Glycolysis
PDH
TCA cycle
Total
Just like glycolysis, TCA cycle is highly regulated
• Energy level of the cell regulates TCA- the goal is to keep our cellular energy constant.
• If cells are high in energy (ATP, NADH), reactions are slowed.
(generally products decrease rates, substrate increase)
• If cells are low in energy, reactions speed up.
Regulation of pyruvate dehydrogenase
• Controls the level of acetyl CoA
Chemistry C483
Fall 2009
Prof Jill Paterson
31-9
Regulation of pyruvate dehydrogenase
This is a commitment step! Pyruvate can be converted to glucose (through gluconeogenesis), but acetylCoA cannot.
Three reactions in TCA are regulated
Enzyme
Activators
Inhibitors
Others
PDH
Phosphorylation
Citrate synthase
[metabolite]
Isocitrate dehydrogenase
[metabolite]
a-ketoglutarate dehydrogenase
complex
[metabolite]
Chemistry C483
Fall 2009
Prof Jill Paterson
31-10