Download chap18 oxidative phosphorylation

Oxidative phosphorylation Chap 18.
Oxidative phosphorylation is a process in which ATP is synthesized as a result of transfer
of electrons from NADH and FADH2 to oxygen. It is a major source of energy under
aerobic conditions.
Oxidative pphosphorylation produces 30 of the 32 molecules of ATP that are formed
when glucose is oxidized to CO2 and H2O.
The process is conceptually easy but mechanistically very difficult. The electron flow
from NADH and FADH2 to oxygen through protein complexes leads to pumping of
protons outside the mitochondrion membrane. A proton motive force is generated
consisting of pH gradient and transmembrane electron potentials. ATP is synthesized
when protons flow back into the mitochondria through another enzyme complex.
Oxidative phosphorylation consists of two phases.
1. Electron motive force is converted into proton motive force. This is achieved by
three different enzyme complexes a) NADH Coenzyme Q reductase (complex I) b)
cytochrome reductase (Complex III) and c) cytochrome c oxidase (Complex IV). During
the process protons are pumped outside. (Complex II is succinate dehydrogenaseCoenzyme Q reductase).
2. Proton motive force is used to synthesize ATP. ATP synthase drives protons
inside and makes ATP. (Complex V)
----X- + H+  X + 1/2 H2
This can be written in two equations.
X-  X + eH+ + e-  1/2 H2
A substance is said to have negative reduction potential if it has lower affinity for
electrons in comparison to H2. Strong reducing agents such as NADH have negative
reduction potentials. A positive reduction potential means that the substance has high
affinity for electrons. Strong oxidants such as oxygen have positive reduction potentials.
NADH  NAD + 2e Eo’ = - 0.32 V
1/2 O2 + 2H+ + 2e  H2O Eo’ = + 0.82 V
difference is 1.14 volts. This difference is used to make ATP.
-The electron transport system consists of the following components.
NADH transfers its electrons through NADH Coenzyme Q reductase complex (called
complex I) to coenzyme Q first. [Electrons from FADH2 (succinate dehydrogenase
reaction) is directly transferred to Coenzyme Q via succinate – Coenzyme Q reductase
(called complex II)]. Reduced QH2 transfers electrons to cytochrome c through
cytochrome reductase (complex III). Reduced cytochrome c transfers electrons to oxygen
via cytochrome c oxidase (complex IV).
NADH - Coenzyme Q reductase (Complex I) acts as follows:
Reaction catalyzed by NADH -Coenzyme Q reductase
Fe-S is called iron sulfur protein. For simplicity I used Fe-SH; but that is not the way the
reaction occurs. This step leads to the pumping of 4 protons outside of inner
mitochondrial membrane. But the mechanism is not known.
Complex II is succinate – Coenzyme Q reductase – the electrons from FADH2 are
transferred to coenzyme Q as shown in the above figure.
Cytochrome c reductase – Complex III transfers electrons from QH2 to cytochrome c as
Shown in Figure. 18.12. You must write the cartoon diagram to show how 4 protons are
pumped out; Two QH2 is converted to two Q and one QH2 is produced with two
cytochrome c reduction as shown in the figure.
Cytochrome c reductase has two components - cytochrome b and cytochrome c1.
Reduced coenzyme Q gives one electron to FeS, which is transferred to cytochrome c
The free radical Q transfers one electron to cytochrome b, which holds it temporarily.
When the next QH2 comes and gives its one electron to Fe-S protein and generates the
free radical Q, it gives that electron to the free radical Q reducing it to QH2.
This step leads to the pumping of a total of 4 protons outside of inner mitochondrial
membrane.
Final step (third) is carried out by cytochrome c oxidase – Complex IV.
4 cytochrome c (2+) + 4 H + O2  4 cytochrome c (3+) + 2H2O
+
This enzyme has 13 subunits. Two heme groups and two copper sites (cu A is a dimer)
and Cu b is with heme a3)
Cytochrome c donates the electrons to heme a, which then donates it to cuA-CuB. Which
then donates it to hem a3. Hem a3 finally reduces oxygen to water as shown in Figure.
18.4. You must present the cartoon diagram and show how 4protons and one oxygen
molecule is reduced to 2 water molecules.
See figure 18.17 for summary.
At no time one electron reduced species of oxygen is given out. If they come out they are
very dangerous. Superoxide, peroxide and hydroxy radicals are sequential one electron
reduction intermediates formed when oxygen is reduced to water. All three of them are
extremely dangerous.
ATP synthesis:
ATP synthesis is carried out by ATP synthase (Complex V). The inner mitochondrial
membrane has ATP synthase. It uses ADP and Pi and makes ATP freely. But the ATP
formed cannot leave freely. The protons that were pumped outside, rotate the ATP pump
and gets inside. This rotation releases the bound ATP.
You must show on of the structure in figure 18.28 to explain the ATP synthetase.
You should also explain the subunits C and “a” of the proton pump and explain how
it works to rotate (Fig. 18.31).
ATP balance: p555 table 18.4
Glycolysis (in the cytosol)
Phosphorylation of glucose -1 ATP
Phosphorylation of fructose 6 P -1 ATP
Dephosphorylation of two -1,3 Bis P glycerate +2 ATP
Dephosphorylation of two phosphoenolpyruvate +2 ATP
2 NADH from 2 glyceraldehyde 3 P
pyruvate to Acetyl CoA (inside mitochondrion)
2 NADH formed from pyruvate to acetyl CoA
Citric acid cycle (inside mitochondrion)
2 GTP formed from 2 succinate CoA = +2 ATP
2 NADH formed from isocitrate dehydrogenase
2 NADH formed from α-ketoglutarate
2 NADH formed from malate
2 FADH2 formed from succinate dehydrogenase
Oxidative phosphorylation (inside mitochondrion)
2 NADH from glycolysis gives 2.5 ATP each +5 ATP
2 NADH from pyruvate dehydrogenase gives 2.5 ATP each +5 ATP
2 FADH2 from citric acid cycle gives 1.5 ATP each +3 ATP
6 NADH from citric acid cycle gives 2.5 ATP each +15 ATP
Net ATP gain during glucose - CO2 + H2O = 32 ATP
NADH does not freely enter or get out of mitochondria. If malate aspartate shuttle
is used, you get as mentioned above, 32 ATP. If it uses glycerol 3-Phospahte shuttle,
(which works via FAD) you get only 3 ATP instead of 5 and hence the total is
reduced to 30 ATP.
Glyoxylate cycle: Figure 17. 23.
In plants bacteria and fungi but not in animals there are group of enzymes that convert
acetylCoA into oxaloacetate and use it for gluconeogenesis. In plants these enzymes are
called glyoxylate cycle enzymes. Glyoxalate cycle consists of five enzymes. Two new.
Reaction 1. Oxaloacetate + acetyl CoA  citrate. Same as TCA cycle. - Citrate synthase
Reaction 2. Citrate is isomerized to isocitrate. Same as TCA cycle. - aconitase
Reaction 3. Isocitrate lyase cleaves isocitrate to succinate and glyoxylate.
Reaction 4. Malate synthase condenses glyoxylate and acetylCoA to give malate.
Reaction 5. Malate dehydrogenase converts malate to oxaloacetate. Same as TCA cycle.
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