regulation of citric acid cycle Flashcards
diatery nutrients in metabolism
- Carbohydrates
- Glycolysis
- Gluconeogenesis
- Glycogenesis
- Glycogenolysis
- Fats
1. B-oxidation
2. FA synthesis
3. TG synthesis
4. Chokesterol synthesis - Proteins
1. Transamination
2. Urea cycle
3. AA catabolism
4. AA synthesis
5. Synthesis of AA derivatives
pyruvate from glycolysis
- B-oxidation of fat yields acetyl-coa
- Some AA yield acetyl-coa (leucine, threonine)
- Glycolysis yields pyruvate
** pyruvate to acetyl-coa by pyruvate dehydrogenase complex - Pryuvate to acetyl-coa: oxidative decorboxylation; irreversible
NADH generated enters respiratory chain as e- donor
pyruvate dehydrogenase complex (PDC)
- PDC - cluster of multiple copies of 3 enzymes (60 copies in bovine PDC)
- E1: Pyruvate dehydrogenase
- E2: dihydrolipoyl transacetylase
E3: dihydrolipoyl dehydrogenase
steps for formation of acetyl-coa from pyruvate
- Pyruvate is taken up by TPP which is on the E1 of the enzyme, the carboxyl group is remove to form CO2
- TPP transfer the hydroxyethyl group to the E2 which becomes acyl lipoyllysine + the 2e- that are transferred will reduce S-S of a lipoyl group on E2 to two thiol (SH) groups
- Coa- is attached to the backbone to form acetyl-coa
** transersterification in which the SH group of Coa replaces the SH group of E2 to yield acetyl-coa and the fully reduced form of the lipoyl group - E3 transfers 2 H from the reduced lipoyl group of E2 to the FAD prosthetic group of E3, restoring the oxidized form of the lipoyllysyl group of E2.
The reduced FADH2 of E3 transfers a hybride ion to NAD+ to form NADH.
** thiamine pyrophosphate (TPP), lipoate and FAD remain bound
result of TCA cycle + what is special about succinyl-coa dehydrogenase
- 9 intermediates
- 8 enzymes (steps)
- Acetyl-coa donates 2C to 4C compund (oxaloacetate)
- Acetyl-coa = 2 CO2 + 3NADH + 1FADH2 + 1ATP
2C (from the acetyl-coa) are converted to CO2
** succinal-coa synthetase (means it uses ATP) but in the TCA cycle, it is used in the opposite direction where , it generates it (either ATP or GTP) (they enzyme is reversible but it is used on the way to create energy), there are 2 isoforms of this enzyme (one generate ATP and other GTP, can be one of those in th cycle)
why citric acid is amphibolic + anaplerotic reactions
- Citrate:
- FA synthesis
- Sterols
- A-ketoglutarate: for AA synthesis - Glutamate that leads to : glutamine, proline, arginine
- Succinyl-coa: porphyrins, heme
- Oxaloacetate: - PEP which lead to a)glucose by gluconeogenesis or
B) serine, glycine, cysteine, phenylalanine, threonine, tryptophan
Anaplerotic reactions= replenishing reactions
1. Pyruvate carboxylase (pyruvate to oxaloacetate) in the liver 2. PEP carboxykinase (PEP to oxaloacetate) in heart, skeletal muscle 3. PEP carboxylase (PEP to oxaloacetate) in plants, bacteria 4. Malic enzyme (pyruvate to malate) in bacteria and eukaryotes
regulation of TCA cycle
- Enzymes of irreversible reactions are regulated
- Activated by substrate
- Inhibited by products
- PDH is phosphorylated on E1 enzyme = inactivated
- Succinyl-coa inhibits citrate synthase + KDH to regulate a-ketoglutarate for AA metabolism
Ca2+ activates TCA cycle
- Pyruvate dehydrogenase complex (pyruvate to acetyl-coa):
Inhibited by ATP, acetyl-coa, FA, NADH
Stimulates by AMP, CoA, NAD+, Ca2+
2. Citrate synthase (acetyl-coa to citrate):
Inhibited by NADH, succinyl-coa, citrate, ATP
Stimulate by AMP
- Isocitrate dehydrogenase (isocitrate to a-ketoglutarate):
Inhibits by ATP
Stimulate by Ca2+, AMP - A-ketogulatarate dehydrogenase complex (KDH) (isocitrate to a-ketoglutarate):
Inhibits by suciinyl-coa, NADH
Stimulates by Ca2+
do FA contributes to gluconeogenesis?
- Acetyl-coa cannot be converted to pyruvate BUT odd number FA to succinate
- C14-tracer experiments show flow of C from FA to glucose
- Acetyl-coa can be converted to ketone bodies
- Human fasting studies showed:
1. Acetone loss through breath accounted for 2-30%
2. Envrion 60% ended up in glucose - Acetone to glucose through methylglyoxal or 1,2-propanediol
path of e- of ubiquinone
Source of NADH to complex I:
1. B-oxidation- 1 NADH per cycle 2. Glycolysis - 2 NADH 3. Pyruvate to acetyl-coa - 1 NADH 4. TCA - 3 NADH 5. AA oxidation to pyruvate, acetyl-coa, fumarate, a-ketoglutarate, succinyl-CoA
Pathways for ubiquinone:
In complex I:
E- from NADH gives to FMN to Fe-S to Q where it will become QH2 and go through the membrane
In complex II
E- from FADH2 goes to Fe-S to Q to become QH2 where it will go to complex III
Glycerol 3-phosphate dehydrogenase (flavoprotein):
Glycerol 3-phosphate (cytolosolc) gives its e- to this flavoprotein which is on the outer face of the inner mitochondrial membrane, then they pass to Q
Acyl-CoA dehydrogenase (1st enzyme in B-oxidation) transfers its e- to ETF (electron-transfering flavoprotein) and then gives e- to Q via ETF:Q oxidoreductase
complex I (NADH dehydrogenase)
reaction: NADH + Q + 5H+ = 4 H+ + QH2+ NAD+
* * so 4 H+ are pumped out of the matrix and one + the one from NADH goes to Q to form QH2
* * largest of the 4 complexes (43 subunits encoded by mitochondrial and nuclear genes)
Complex I catalyzes the transfer of hybride ion from NADH to FMN, from which 2 electrons pass through a series of Fe-S center to the Fe-S center N-2 in the matrix arm of the complex. E- transfer from N-2 to Q to form QH2. this e- transfer also drives the explusion from the matrix of 4 H+ from the matrix to the intermembrane space per pair of e-
complex II (succinate dehydrogenase)
Difference between complex 1 and complex 2 is that complex 2 does not have H that are puting outside the intermembrane space
General steps:
1. Electrons moves from succinate to FAD (FADH2) by the oxidation of succinate to fumarate by the enzyme succinate dehydrogenase)
They are passed through 3 Fe-S centers to Q (become QH2)
** FAD act as cofactor
*** no H+ are pumped in the innermembrane
complex III (cytochrome bc1 complex)
- transfers e- from ubiquinol to cytochrome c + releases 4 H+ to inter-membrane space
Stage 1:
- Reduce ubiquiniol with 2H and 2e- they can donate - H are remove and throw in the intermembrane spase but only one e- goes to cytochrome C and the other one will be convrt to the intermediate of Q (semiquinone) - So 2H goes out, one QH2 is convert to Q and another one come and become Q- (semiquinone)
Stage 2
- Semiquinone is converted to QH2, which consume 2 H+ from the matrix - Accept 1 e- from the second Q and taken 2 H from the matrix - The other e- goes to the second cytochrome C * * so 2 cytochrome are reduced and 2 QH2 are used (one of them is synthesized, so in the net only one Q is used to reduce cytochrome) SO: - When the 2 QH2 are oxidized to Q, there is a release of 2H+ per Q so a total of 4H+ - 2 H+ are consumed to produced QH2 from the semiquinone in the first stage Each QH2 donates 1e- to cyt c and 1e- to a molecule of Q
Net equation: QH2 + 2cytc (oxidized) + 2H+ = Q + 2 cytc (reduced) + 4H+
complex IV (cytochrome oxidase)
- transfers e- from cytochrome c to O2
- transports 2 H+ to outside per e- pair (4 in total)
For every 4 e- passing through this complex (from 4 cyt c), the enzyme consume 4 H+ from the matrix in converting O2 to 2 H20
** also, it pt uses this energy to pump 1 H+ out of the matrix per e- (so 4e- = 4 4H+ thata are pumped out)
e- carriers from respirasome (4 different complexes but they function as one group of proteins)
** quantity of H+ for NADH and FADH2
(complex I,III,IV)
1 NADH + 11 H+ + 1/2 O2 = 1 NAD+ + 10H+ + H20
Resume of H+ that are pumped out:
Complex I: 4H+
Complex III: 4H+
Complex IV (for 1/2 O2) = 2H+
(complex II, III, IV)
FADH2 + 6H+ + 1/2 O2 = FAD + 6H+ + H20
** NADH produces more electrochemical gradient than FADH2
electrochemical gradient is created by what (3)
- Active transport of protons:
- Complex I and IV
2. Release of protons unto intermembrane space - Oxidation of QH2 (complex III)
3. Chemical removal of protons from the matrix
Reduction of Q (comple I,III,IV) and oxygen (IV)
- Complex I and IV
proton-motive force: chemical potential (pH due to difference in concentration of H+) + electrical potential (separation of charge) = ATP driven by proton motive force
