Hydrogen carriers, mitochondrial

A third hydrogen carrier of mitochondrial membranes, and the only one that is not unequivocally... 

Transfer of protons and electrons between complexes I and III is mediated by the hydrophobic hydrogen carrier ubiquinone. This compound, which is free to move around the inner mitochondrial membrane, is also called Coenzyme Q and Coenzyme Qiq. It carries hydrogens and is also involved in shuttling hydrogens from complex II to complex III. (It received its particular name because it is a quinonoid compound that is ubiquitous throughout the inner mitochondrial membrane. It has 10 linked isoprenoid units, and since it was originally thought to be a coenzyme, it became known as coenzyme Qiq.)... 

When an electron carrier reduces a hydrogen carrier, there is a need for a proton to accompany the electron that is transferred. This is acquired from the mitochondrial matrix, thus shifting the equilibrium between H O + OH, resulting in an... 

Ubiquinone, known also as coenzyme Q, plays a crucial role as a respiratory chain electron carrier transport in inner mitochondrial membranes. It exerts this function through its reversible reduction to semiquinone or to fully hydrogenated ubiquinol, accepting two protons and two electrons. Because it is a small lipophilic molecule, it is freely diffusable within the inner mitochondrial membrane. Ubiquinones also act as important lipophilic endogenous antioxidants and have other functions of great importance for cellular metabolism. ... 

NAD-linked dehydrogenases remove two hydrogen atoms from their substrates. One of these is transferred as a hydride ion ( II ) to NAD+ the other is released as H+ in the medium (see Fig. 13-15). NADH and NADPH are water-soluble electron carriers that associate reversibly with dehydrogenases. NADH carries electrons from catabolic reactions to their point of entry into the respiratory chain, the NADH dehydrogenase complex described below. 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, as we shall see. 

Electrons flow from NADH to molecular oxygen through a series of electron carriers embedded in the inner mitochondrial membrane. Protons are pumped from the mitochondrial matrix space into the intermembrane space. This results in a hydrogen ion reservoir in the intermembrane space. As protons pass through the channel in ATP synthase, their energy is used to phosphorylate ADP and produce ATP. 

The other important subtlety is that some carriers, such as NADH, carry electrons and hydrogens in their reduced forms others, such as the iron-sulfur protein we just saw, can carry only electrons. This is the basis of the proton pumping that ultimately leads to ATP production. When a carrier such as NADH reduces the iron-sulfur protein, it passes along its electrons, but not its hydrogens. The architecture of the inner mitochondrial membrane and the electron carriers allows the hydrogen ions to pass out on the opposite side of the membrane. We shall look more closely at this in Section 20.5. 

Mitochondrial structure confines the reduced electron carriers produced by the citric acid cycle to the matrix. There they are close to the respiratory complexes of the electron transport chain that will pass the electrons from the carriers produced by the citric acid cycle to oxygen, the ultimate recipient of electrons and hydrogens. 

Rey, M., Man, R, aemencon, B., et al. (2010) Conformational dyneimics of the bovine mitochondrial ADP/ATP carrier isoform 1 revetiled by hydrogen/deuterium exchange coupled to mass spectrometry. Journal of Biological Chemistry, 285 (45), 34981-34990. 

Because the mitochondrial membrane is impermeable to ATP/ADP and NAD/NADH, two transport systems are necessary for the function of the R.c. One is the ATP/ADP carrier, which effects facilitated exchange diffusion, and the other is a metabolic shuttle (see Hydrogen metabolism) which allows reducing equivalents generated in the cytoplasm to enter the mitochondrion. 

Oxidation of metabolites in the cell leads to the generation of reduced NAD in the mitochondria. The mitochondrial inner membrane contains a series of hydrogen and electron carriers which together constitute the respiratory chain. This catalyses the overall reaction ... 

Figure 3.21 Hydrogen and electron carriers in the mitochondrial electron transport chain — generation of a transmembrane proton gradient.

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