Oxidative Phosphorylation

The reductive power generated by the citric acid cycle in form of NADH/H⁺ can be utilized by the inner mitochondrial membrane to generate ATP. Reducing equivalents harnessed in the Krebs cycle, fatty acid oxidation, and pyruvate dehydrogenase activity are transferred to membrane bound electron transport chain. Four classes of electron transport complexes segregate electron and protons promoting proton pumping across the inner membrane using molecular oxygen as a final electron acceptor ('sink') reducing oxygen to water. The proton gradient across the inner membrane drives the ATP synthase to regenerate ATP from ADP and inorganic phosphate. This process is also known as respiration.

The Mitochondrion

Mitochondria are small organelles with possible common origin to bacteria (endosymbiotic theory). They contain a double membrane system. The outer membrane is relatively permeable to small metabolites. This is due to the presence of a porin channel type VDAC (voltage dependent anion channel) with a high copy number (thousands of channel in a single mitochondrion) with preferences for negatively charged ions (nucleotides, phosphorylated compounds). Large molecules like proteins, however, are not able to cross the outer membrane. The size limit for permeable molecules is about 1,000 Dalton. The mitochondrial inner membrane functions as electrical insulator and capacitor and is impermeable for ions and small hydrophilic metabolites. This membrane maintains the ion (proton) gradient essential for ATP synthesis. The matrix side is negatively charged relative to the outside (inter-membrane space; connected to cytoplasm through VDAC) because of the proton gradient (high out; low in) generated during the electron transport chain reaction. This membrane contains many of the substrate specific transport systems like citrate, glycerol, and malate shuttles, but also the ATP-ADP exchange protein. It is the latter exchange transport that controls the speed of citric acid cycle, oxidative phosphorylation and thus all precursor pathways like glycolysis, protein degradation and fatty acid oxidation.

Electron carriers in oxidative phosphorylation

Electron carriers along the electron transfer chain come in two forms; they bind reducing equivalents or electrons only. Combining both types within a chain forces the separation of protons from electrons. While the electrons stay within the membrane, protons are captured and released from and into the surrounding compartments. This process is unidirectional and always picks up protons in the matrix compartments and releases them in the inter-membrane space thus creating a proton gradient which stores energy in form of electrochemical potential.

The first category of reducing equivalent acceptors riboflavin (vitamin B2) and nicotinamine (vitamin B3 or nicacin). Riboflavin is part of FAD and FMN and is analogous to that of nicotinamide in NAD(P) by accepting hydrogens and electrons into its heterocyclic ring structures. Two important differences to NAD(P), however, exist. First, flavin-adenine dinucleotide (FAD; or FADH₂, the reduced flavin- adenine- dinucleotide; C01352) and flavin mono nucleotide (FMN oxidized C00061; reduced FMN C01847) are prosthetic groups (covalent link to enzyme) and do not carry reducing equivalents by diffusion. Second, the reduction of FAD and FMN by NADH is not reversible. Flavoproteins participate at several points where electrons are first funneled into the respiratory chain:

NADH + H⁺ + Enz-FMN Þ NAD⁺ + Enz-FMNH₂

The reduction potential of this reaction D E₀' = 0.3V corresponding to about -46kcal/mol of standard free energy change, enough for the synthesis of two to three mols of ATP per mol of oxidized NADH.

The second category of reducing equivalent carriers is the benzoquinone (oxidized) conatining compound coenzyme Q. Like flavoproteins they accept two hydrogens along with two electrons upon reduction of the hydroquinone ring structure to ubiquinole or CoQH₂. The quinone-hydroquinone redox couple serves as a diffusible transport system within the inner membrane of mitochondria coupling the electron flux with a proton flux across the dielectric barrier by providing a non-charged carrier system. Quinones serve as a collector molecule of reducing equivalents (e^-/H⁺) from NADH and succinate donors.

Redox systems of the respiratory chain that only bind electrons are the Fe-S complexes and heme groups (Fe-proto-porphyrin ring; protoheme C00032). The irons of the heme groups serve as redox partners by reversibly changing their redox state between the reduced Fe(II) and the oxidized Fe(III) form. Synthesis of heme can be found in KEGG as MAP00860 (Porphyrin and chlorophyll metabolism).

Table of individual protein complex systems in oxidative phosphorylation

NADH dehydrogenase (complex I) ENTRY EC 1.6.5.3: Complex I is the first coupling site in the mitochondrial membrane meaning that the redox reaction is coupled to a proton pumping activity across the membrane. It is the energy stored in the electrochemical proton gradient that is used for ATP synthesis by the H⁺-ATPase or complex V. Complex I is the entry point for NADH reducing power and involves FMN, Fe-S complexes, and ubiquinone.

Succinate dehydrogenase (complex II) ENTRY EC 1.3.5.1 is the mitochondrial succinate dehydrogenase and unlike complex I does not directly contribute to proton pumping. The reducing equivalents from succinate are transferred by FAD to ubiquinone.

Cytochrome-c reductase (complex III) Complex III or cytochrome bc1 complex (EC 1.10.2.2) transfers electrons from quinones to cytochrome c, a small peripheral membrane protein (cytochrome c; C00524) in the inter-membrane space. This complex contributes to proton pumping in a mechanism known as Q-cycle. A similar mechanism is also hypothesized to work in complex I.

Cytochrome-c oxidase (complex IV) Cytochrome c transfer single electrons from complex III to complex IV, or cytochrome-c oxidase (EC 1.9.3.1). As can be seen from the KEGG entry for cytochrom c oxidase, the human complex IV contains up to eight subunits. For some subunits, cell type specific homologues exists such as COX7A1 in muscle and COX7A2 in liver. Cytochrome oxidase catalyzes a cyclic reaction in which the electrons extracted from metabolites are finally transferred to molecular oxygen in the presence of 4 protons to form 2 molecules of H2O. This is a carefully controlled mechanism because the intermediate oxygen radicals are highly reactive and damaging to the cell. Oxygen is tightly bound to an iron-copper containing coenzyme.

H⁺-ATPase (complex V) The mitochondrial ATP synthase (EC 3.6.1.34) is a multi-subunit protein complex that couples a proton channel (F₀ portion, integral membrane protein complex) with an ATP synthesizing unit (F₁ portion, soluble mitochondrial matrix component) and is a member of the so called F-type ATPases.

For an on-line demonstration of the Q-cycle go to Science Media and choose the 'Biochemical Interactions' thumbnail.

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