OXIDATIVE PHOSPHORYLATION:

OXIDATIVE PHOSPHORYLATION:




Oxidative phosphorylation involves the reduction of O2 to H2O with electrons donated by NADH and FADH2; it occurs equally well in light or darkness.
Photophosphorylation involves the oxidation of H2O to O2, with NADP+ as ultimate electron acceptor; it is absolutely dependent on the energy of light.
In eukaryotes, oxidative phosphorylation occurs in mitochondria, Photophosphorylation in chloroplasts.

Electron-Transfer Reactions in Mitochondria:
The outer mitochondrial membrane is readily permeable to small molecules (Mr <5,000) and ions, which move freely through transmembrane channels formed by a family of integral membrane proteins called porins.
The inner membrane is impermeable to most small molecules and ions, including protons (H+); the only species that cross this membrane do so through specific transporters.
The inner membrane bears the components of the respiratory chain and the ATP synthase.
The mitochondrial matrix, enclosed by the inner membrane, contains the pyruvate dehydrogenase complex and the enzymes of the citric acid cycle, the fatty acid ,B-oxidation pathway, and the pathways of amino acid oxidation—all the pathways of fuel oxidation except glycolysis, which takes place in the cytosol.




Electrons Are Funneled to Universal Electron Acceptors:
universal electron acceptors—nicotinamide nucleotides (NAD+or NADP+) or flavin nucleotides (FMN or FAD).
 Nicotinamide nucleotide–linked dehydrogenases catalyze reversible reactions of the following general types-




NAD+ linked dehydrogenases remove two hydrogen atoms from their substrates. One of these is transferred as a hydride ion (: H-) to NAD+; the other is released as H+ in the medium.
NADH and NADPH are water-soluble electron carriers that associate reversibly with dehydrogenases.
NADH carries electrons from catabolic reactions. NADPH generally supplies electrons to anabolic reactions.
Cells maintain separate pools of NADPH and NADH, with different redox potentials.
This is accomplished by holding the ratios of [reduced form]/[oxidized form] relatively high for NADPH and relatively low for NADH. Neither NADH nor NADPH can cross the inner mitochondrial membrane, but the electrons they carry can be shuttled across indirectly,
Flavoproteins contain a very tightly, sometimes covalently, bound flavin nucleotide, either FMN or FAD.
Electron transfer occurs because the flavoproteins have a higher reduction potential than the compound oxidized. The standard reduction potential of a flavin nucleotide, unlike that of NAD or NADP, depends on the protein with which it is associated.

Electrons Pass through a Series of Membrane-Bound Carriers:

Three types of electron transfers occur in oxidative phosphorylation:
(1) Direct transfer of electrons, as in the reduction of Fe3+ to Fe2+;
(2) transfer as a hydrogen atom (H+ + e-).
(3) Transfer as a hydride ion (:H-), which bears two electrons.
 The term reducing equivalent is used to designate a single electron equivalent transferred in an oxidation-reduction reaction.
 In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the respiratory chain: a hydrophobic quinine (ubiquinone) and two different types of iron-containing proteins (cytochromes and iron sulfur proteins).
Ubiquinone (also called coenzyme Q, or simply Q) is a lipid-soluble benzoquinone with a long isoprenoid side chain.
The closely related compounds plastoquinone (of plant chloroplasts) and menaquinone (of bacteria) play roles analogous to that of ubiquinone, carrying electrons in membrane-associated electron-transfer chains.
Ubiquinone can accept one electron to become the semiquinone radical (◦QH) or two electrons to form ubiquinol (QH2) and, like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor.it carries both electrons and protons, it plays a central role in coupling electron flow to proton movement.

The cytochromes are proteins with characteristic strong absorption of visible light, due to their iron containing heme prosthetic groups.
Mitochondria contain three classes of cytochromes, designated a, b, and c, which are distinguished by differences in their light-absorption spectra.
The longest wavelength band is near 600 nm in type a cytochromes near 560 nm in type b, and near 550 nm in type c.
The heme cofactors of a and b cytochromes are tightly, but not covalently, bound to their associated proteins; the hemes of c-type cytochromes are covalently attached through Cys residues.

In iron-sulfur proteins the iron is present not in heme but in association with inorganic sulfur atoms or with the sulfur atoms of Cys residues in the protein, or both.

These iron-sulfur (Fe-S) centers range from simple structures with a single Fe atom coordinated to four Cys -SH groups to more complex Fe-S centers with two or four Fe atoms.
Rieske iron-sulfur proteins (named after their discoverer, John S. Rieske) are a variation on this theme, in which one Fe atom is coordinated to two His residues rather than two Cys residues.
All iron-sulfur proteins participate in one-electron transfers in which one iron atom of the iron-sulfur cluster is oxidized or reduced.








Effects of inhibitors of electron transfer:




Complex I: NADH to Ubiquinone :
Complex I, also called NADH:ubiquinone oxidoreductase or NADH dehydrogenase, is a large enzyme composed of 42 different polypeptide chains, including an FMN-containing flavoprotein and at least six iron sulfur centers.
Complex I catalyzes two simultaneous and obligately coupled processes:
(1) The exergonic transfer to ubiquinone of a hydride ion from NADH and a proton from the matrix, expressed by


(2) The endergonic transfer of four protons from the matrix to the intermembrane space.
Complex I is therefore a proton pump driven by the energy of electron transfer, and the reaction it catalyzes is vectorial: it moves protons in a specific direction from one location (the matrix, which becomes negatively charged with the departure of protons) to another (the intermembrane space, which becomes positively charged).


NADH:ubiquinone oxidoreductase (Complex I). Complex I catalyzes the transfer of a hydride ion from NADH to FMN, from which two electrons pass through a series of Fe-S centers to the ironsulfur protein N-2 in the matrix arm of the complex. Electron transfer from N-2 to ubiquinone on the membrane arm forms QH2, which diffuses into the lipid bilayer. This electron transfer also drives the expulsion from the matrix of four protons per pair of electrons.

Amytal (a barbiturate drug), rotenone (a plant product commonly used as an insecticide), and piericidin A (an antibiotic) inhibit electron flow from the Fe-S centers of Complex I to ubiquinone and therefore block the overall process of oxidative phosphorylation.


Complex II: Succinate to Ubiquinone:
Complex II in Chapter 16 as succinate dehydrogenase, the only membrane-bound enzyme in the citric acid cycle.
Although smaller and simpler than Complex I, it contains five prosthetic groups of two types and four different protein subunits.
The path of electron transfer from the succinate-binding site to FAD, then through the Fe-S centers to the Q-binding site, is more than 40 Å long, but none of the individual electron-transfer distances exceeds about 11 Å—a reasonable distance for rapid electron transfer.


Complex III: Ubiquinone to Cytochrome c:
Complex III, also called cytochromes bc1 complex or ubiquinone:cytochrome c oxidoreductase, couples the transfer of electrons from ubiquinol (QH2) to cytochrome c with the vectorial transport of protons from the matrix to the intermembrane space.
The net equation for the redox reactions of this Q cycle

The Q cycle accommodates the switch between the two electron carrier ubiquinone and the one-electron carriers— cytochromes b562, b566, c1, and c—and explains the measured stoichiometry of four protons translocated per pair of electrons passing through the Complex III to cytochrome c.
QH2 is oxidized to Q and two molecules of cytochrome c are reduced.
cytochrome c moves to Complex IV to donate the electron to a binuclear copper center.



Complex IV: Cytochrome c to O2:
Complex IV, also called cytochromes oxidase, carries electrons from cytochrome c to molecular oxygen, reducing it to H2O.
• Path of electrons through Complex IV.
The three proteins critical to electron flow are subunits I, II, and III. The larger green structure includes the other ten proteins in the complex.
Electron transfer through Complex IV begins when two molecules of reduced cytochromes c (top) each donate an electron to the binuclear center CuA. From here electrons pass through heme a to the Fe-Cu center (cytochromes a3 and CuB). Oxygen now binds to heme a3 and is reduced to its peroxy derivative (O2 2-by two electrons from the Fe-Cu center. Delivery of two more electrons from cytochrome c (making four electrons in all) converts the O2 2_ to two molecules of water, with consumption of four “substrate” protons from the matrix. At the same time, four more protons are pumped from the matrix by an as yet unknown mechanism.






Plant Mitochondria Have Alternative Mechanisms for Oxidizing NADH:

In the light, the principal source of mitochondrial NADH is a reaction in which glycine, produced by a process known as photorespiration, is converted to serine.


For reasons, plants must carry out this reaction even when they do not need NADH for ATP production. To regenerate NAD+ from unneeded NADH, plant mitochondria transfer electrons from NADH directly to ubiquinone and from ubiquinone directly to O2, bypassing Complexes III and IV and their proton pumps.

Unlike cytochrome oxidase (Complex IV), the alternative QH2 oxidase is not inhibited by cyanide.
Cyanide-resistant NADH oxidation is therefore the hallmark of this unique plant electron-transfer pathway.