Lecture 21 Flashcards
What is a reduction potential?
- A measure of the potential energy of electrons.
- Electron transfer potential.
X + 2H+—-> + 2e- XH2
ox form red form
- XH2 and X are a redox pair shown in a “half-reaction”
- “Reduction Potential” reflects how much a molecule prefers to be in the reduced state.
- Positive means it prefers to be reduced
- Negative means it prefers to be oxidized
E0
1 M everything, (1 atm for gas)
• Reference half cell: H+
+ e- —-> ½ H2
• E0 for the reference cell = 0
• When E0 is < 0 then X has less need than H+ to be reduced so electrons flow towards the reference cell.
• When E0 is > 0 then X wants electrons more than H2
so electrons flow away from the reference cell.
Some Standard Reduction Potentials
Acceptors Donors E’0
acceptors: .5O2, FAD, fumarate, oxaloacetate, NAD+, α-ketoglutarate
Donors: H2O,FADH2, succinate, malate, NADH, isocitrate
How to calculate ∆E0’
in other words how do you determine the “driving force” for a reaction?
Acetaldehyde + NADH + H+ —–> ethanol + NAD+
two half rxns
Acetaldehyde + 2H+ + 2e- —-> ethanol -0.197
NAD+ + 2e- + H+ —-> NADH -0.32
∆E0’ = E0’ acceptor - E0’donor
Which is the electron acceptor? Acetaldehyde
Which is the electron donor? NADH
What is ∆E0’ for this reaction? (-0.197) – (-0.32) = +0.123
What is the ΔE0’ for reduction of O2 by NADH?
NAD+ + 2e- + H+ ——> NADH Eo’ = -0.32V
½ O2 + 2e- +2H+ ——> H2O Eo’ = +0.82V
½ O2 + NADH + H+ ——> H2O + NAD+’
ΔE0’ = Eacceptor – Edonor = +0.82V - (-0.32V) = +1.14 V
ΔG0’ = -nFΔE0’ = -2(96 kJ mol-1 V-1)(1.14 V) = -219kJ/mol
In summary
• If ∆G0’ is <0, ∆E0’ is >0
–the reaction is exergonic.
• If ∆G0’ is >0, ∆E0’ is <0
–the reaction is endergonic.
Overall reaction catalyzed by complex 1:
NADH + Q + 5 H+ matrix —->NAD+ + QH2 + 4H+ cytoplasm
Complex 1:
“NADH-coenzyme Q Oxidoreductase”
46 subunits, 900 kDal
Flavin Mononucleotides
- A derivative of Riboflavin (Vitamin B2)
- Tightly bound to proteins in the respiratory complexes.
Electron carriers:
Ubiquinone (Q)
- Soluble in the membrane
- Can accept two electrons
- Pass them on one at a time.
- Forms a radical or semiquinone
Type of electron carriers in the respiratory chain
- Flavin mononucleotides
- Iron-sulfur proteins
- Ubiquinones
- Cytochromes
“Q cycle”:
“Q pool” means a mix of Q and QH2 floating in the membrane
Why is the Q cycle important?
- The Q cycle allows a 2 electron carrier (QH2) to
- transfer electrons one a time to a one electron carrier (Cyt c)
Cytochromes: Electron carriers
- All have heme prosthetic groups
- Pass one electron Fe3+ –> Fe2+
- Reduction potentials vary from 0.077 to 0.55 volts
- Different micro-environments in the protein.
Cytochrome Oxidase (Complex IV)
2cyt(c red) + 4H+(matrix)+ 1/2O2 —–> 2cyt(c ox) + 1H2O +2H+ (cytoplasm)
idk add after lecture
ΔE0’ = Eacceptor – Edonor = +0.82V - (-0.32V) = +1.14 V
ΔG0’ = -nFΔE0’ = -2(96 kJ mol-1 V-1)(1.14 V) = -219 kJ/mol
Phosphorylation of ADP to ATP requires 30.5 kJ/mol so this releases plenty of energy to synthesize many ATP’s
Inhibitors of Respiration
- Rotenone- (plant toxin used as an insecticide)
- Amytal
- Cyanide
- Antimycin A (an antibiotic)
Where do these inhibitors act?
• Treat mitochondria with amytal
– Find that NADH is reduced.– Q, cyt b, cyt c1, cyt c, cyt a, and cyt a3 are oxidized.
Reaction 1: Citrate Synthase
The first reaction of the citric acid cycle is catalyzed by the enzyme citrate synthase. In this step, oxaloacetate is joined with acetyl-CoA to form citric acid. Once the two molecules are joined, a water molecule attacks the acetyl leading to the release of coenzyme A from the complex
Acetyl-CoA + Oxaloacetate + H2O <=> Citrate + CoASH + H+ (Enzyme: <>, DeltaG0’= -32.2 kJ/mol)
Reaction 2: Acontinase
The next reaction of the citric acid cycle is catalyzed by the enzyme acontinase. In this reaction, a water molecule is removed from the citric acid and then put back on in another location. The overall effect of this conversion is that the –OH group is moved from the 3’ to the 4’ position on the molecule. This transformation yields the molecule isocitrate.
Citrate <=> cis-Aconitate + H2O <=> Isocitrate (Enzyme: <>, DeltaG0’= +6.3 kJ/mol)
Reaction 3: Isocitrate Dehydrogenase
Two events occur in reaction 3 of the citric acid cycle. In the first reaction, we see our first generation of NADH from NAD. The enzyme isocitrate dehydrogenase catalyzes the oxidation of the –OH group at the 4’ position of isocitrate to yield an intermediate which then has a carbon dioxide molecule removed from it to yield alpha-ketoglutarate.
Reaction 4: Alpha-ketoglutarate deydrogenase
In reaction 4 of the citric acid cycle, alpha-ketoglutarate loses a carbon dioxide molecule and coenzyme A is added in its place. The decarboxylation occurs with the help of NAD, which is converted to NADH. The enzyme that catalyzes this reaction is alpha-ketoglutarate dehydrogenase. The mechanism of this conversion is very similar to what occurs in the first few steps of pyruvate metabolism. The resulting molecule is called succinyl-CoA.
Reaction 5: Succinyl-CoA Synthetase
The enzyme succinyl-CoA synthetase catalyzes the fifth reaction of the citric acid cycle. In this step a molecule of guanosine triphosphate (GTP) is synthesized. GTP is a molecule that is very similar in its structure and energetic properties to ATP and can be used in cells in much the same way. GTP synthesis occurs with the addition of a free phosphate group to a GDP molecule (similar to ATP synthesis from ADP). In this reaction, a free phosphate group first attacks the succinyl-CoA molecule releasing the CoA. After the phosphate is attached to the molecule, it is transferred to the GDP to form GTP. The resulting product is the molecule succinate.
Reaction 6: Succinate Dehydrogenase
The enzyme succinate dehydrogenase catalyzes the removal of two hydrogens from succinate in the sixth reaction of the citric acid cycle. In the reaction, a molecule of FAD, a coenzyme similar to NAD, is reduced to FADH2 as it takes the hydrogens from succinate. The product of this reaction is fumarate.
FAD, like NAD, is the oxidized form while FADH2 is the reduced form. Although FAD and NAD perform the same oxidative and reductive roles in reactions, FAD and NAD work on different classes of molecules. FAD oxidizes carbon-carbon double and triple bonds while NAD oxidizes mostly carbon-oxygen bonds.
Reaction 7: Fumarase
In this reaction, the enzyme fumarase catalyzes the addition of a water molecule to the fumarate in the form of an –OH group to yield the molecule L- malate.
Reaction 8: Malate Dehydrogenase
In the final reaction of the citric acid cycle, we regenerate oxaloacetate by oxidizing L–malate with a molecule of NAD to produce NADH.
Conclusion
We have now concluded our discussion of the reactions that compose the citric acid cycle. It is helpful at this point to take a minute to take stock of what the citric acid cycle has generated from one acetyl-CoA molecule.
The acetyl-CoA, has been oxidized to two molecules of carbon dioxide.
Three molecules of NAD were reduced to NADH.
One molecule of FAD was reduced to FADH2.
One molecule of GTP (the equivalent of ATP) was produced.
reaction order
Citrate Synthase Acontinase Isocitrate Dehydrogenase Alpha-ketoglutarate deydrogenase Succinyl-CoA Synthetase Fumarase Malate Dehydrogenase
Key concepts for electron transport
- NADH is oxidized stepwise through three large protein complexes to ultimately transfer electrons to oxygen.
- Complexes I, III and IV translocate protons from the matrix to the cytoplasmic side of the mitochondrial inner membrane.
- FMN, Iron-sulfur clusters, Ubiquinone, Cu ions and the heme of cytochromes act as electron carriers.
- The energy of oxidation is stored in a proton gradient
- The Q cycle allows a 2 electron carrier to transfer one electron at a time to a 1 electron carrier.
- Q and cytochrome c are “couriers” between complexes.
