Oxidative Phosphorylation Steps

  Oxidative Phosphorylation 

Most of the free energy released during the oxidation of glucose to Co2 is retained in the reduced coenzyme NADH and FADH2, generated during Glycolysis and Citric Acid Cycle. Electrons are released from NADH and FADH2 and eventually transferred to O2, forming H2O.

The standard free energy change for these exergonic reactions are - 52.6Kcal/mol. (for NADH). The large amount of free energy released during the oxidation of NADH and FADH2 is used for the formation of ATP. For this reason, the term Oxidative Phosphorylation is used to describe this energy conversion process.

Electrons are transferred from NADH/FADH2 to O2 through a series of electron carriers present on the inner mitochondrial membrane (in Prokaryotes, it is present in plasma membrane). The process of electron  transport begins when the hydride ion is removed from NADH and is converted into a proton and two electron. 

Most of the proteins (electron carriers) involving are grouped into four large respiratory enzyme complexes, each containing transmembrane proteins that hold the complex. The electrons start with very high energy and gradually lose it as they pass along the chain. Each complex in the chain has a grater affinity for electrons i.e. reduction potential than its predecessor and electrons pass sequentially from one complex to another until they are finally transferred to oxygen, which has the highest affinity for electrons. The four major respiratory enzyme complexes of electron transport chain in the inner mitochondrial membrane are: 

• NADH-Coenzyme Q reductase or NADH dehydrogenase (Complex 1)
• Succinate-Coenzyme Q reductase (Complex 2) 
• Coenzyme Q-Cytochrome c reductase or cytochrome bc1 complex (complex 3) 
• Cytochrome c oxidase (Complex 4)

Complex 1,2,and 3 appear to be associated in a supramolecular complex termed the Respirasome. All the four multiprotein enzyme complexes which act as electron carriers comprise prosthetic groups, such as Co flavins, heme, Fe-S clusters and copper. 

Electrons are transferred along the electron transport chain by the reversible reduction and oxidation of iron-Sulphur clusters, coenzyme Q , heme and copper ions. Each carrier accepts an electron or an electron pair from a carrier with a less reduction potential and transfers the electron to a carrier with a more reduction potential. Thus, reduction potentials of electron carriers favour unidirectional electron flow from NADH and FADH2 to O2.

Complex 1

It is a large, multi subunit complex causes oxidation of NADH and passes electron from NADH to coenzyme Q. It contains one molecule of flavin mononucleotide (FMN) and six to seven iron-Sulphur clusters that participate in the electron-transport process. During transport of each pair of electron from NADH to coenzyme Q, complex 1 pumps four protons across the inner mitochondrial membrane.

Coenzyme Q (also known as ubiquinone)  is a lipid soluble benzoquinone linked to a number of isoprene units. The name ubiquinone is for the ubiquitous nature of the quinone. 

Complex 2 

Succinate dehydrogenase, an inner mitochondrial membrane bound enzyme, is an integral component of the complex 2. It converts succinate to fumarate during Krebs cycle. The two electrons released in the conversion of succinate to fumarate are transferred first to FAD, then to an iron Sulphur cluster and finally to coenzyme Q. Thus, coenzyme Q draws electrons into the respiratory chain, not only from NADH, but also from FADH2. Complex 2  does not pump protons during transport of electrons across the inner mitochondrial membrane.

Complex 3 

Complex 1 and complex 2 transfer electrons to the complex 3 via coenzyme Q. Within complex, the electrons released from coenzyme Q follow two paths. In one path, electrons are transported via Rieske iron-sulphur cluster and cytochrome c1, directly to cytochrome c. In other path, electrons move through b-type cytochromes and reduce oxidized coenzyme Q as shown in the figure. During transport of each pair of electron from coenzyme Q to cytochrome c, complex 3 pumps four protons across inner mitochondrial membrane. The mechanism involved in proton pumping is called the proton-motive Q-cycle.

Q-cycle

The mechanism of the participation of ubiquinone in the electron transport process was proposed by Peter Mitchell and termed as a proton motive Q-cycle. Ubiquinone are hydrophobic and uncharged, and hence can migrate along the hydrophobic core of the membrane. Diffusion of one ubiquinol takes place to the Qp binding site adjacent to the iron-sulphur protein at the p-phase of the mitochondria membrane. One electron is transferred to Fe-S protein and the second electron is transferred to the heme bl and two protons are released to the p-face. The Fe-S protein transferred the electron along the chain to Cyt c1 and cytochrome oxidase. 

The electron moves from heme Bl to heme to bH. Ubiquinone then binds to bH at the Qn site and electron from the reduced bH forms ubisemiquinone at this site. Now, a second ubiquinol molecules is oxidized at the Qp site, the process follows as described above and the second electron formed completes the reduction of ubisemiquinone to ubiquinol. Two protons are taken from the matrix for this purpose and released to the P-face. The ubiquinol, then, goes back to the pool and the Q-cycle is completed.

Complex 4 

Cytochrome c oxidase catalyzes the transfer of electron from the reduced form of cytochrome c to molecular oxygen. It consists of 13 subunits and contains two heme groups and three copper ions, arranged as two copper centers. The two heme groups termed heme a and heme a3, have distinct properties because they are located in different environments within cytochrome c oxidase. the Two copper centers are designated as a and b. One center, CUa , contains two copper ions linked by two bridging cysteine residues. The second center, CUb , is coordinated by three histidine residues.  

Cytochrome c transports electrons, one at a time, to the complex 4. within this complex, electrons are transferred, first to a CUa center, then to Cyt a , next to CUb center and Cyt a3, and finally to O2, the ultimate electron acceptor, yielding H2O. Together, heme a3 and CUb form the active center at which O2 is reduced toH2O.

The electrons, sequentially released from molecules of reduced cytochrome c together with two protons from the matrix, combine with one O atom to form one water molecule. Additionally, foe each electron transferred from cytochrome c to oxygen, one proton is transported from the matrix to the inter membrane space, or a total of four electrons are transferred for each O2 molecules reduced to two H2O molecules.