Mitochondrial membranes, proton conductance

As in an electrical circuit, where the current of electrons flowing through a resistive element is related to the electrical potential difference and the resistance by Ohm s law, the proton current flowing back into the mitochondrial matrix through a leak pathway will be given by the product of the membrane proton conductance and the proton electrochemical potential ... 

Proton movement back out through the thylakoid membrane is conducted by an ATP-synthase, and this movement drives the formation of ATP. The chloroplast ATP-synthase is structurally very similar to the mitochondrial ATP-synthase (see fig. 14.24). Its head-piece (CF ... 

Figure 2 The chemiosmotic theory of respiration. The mitochondrial or bacterial membrane (yellow) provides resistance to proton conduction. The respiratory chain generates a proton electrochemical gradient across the membrane by redox-coupled proton translocation (Figure 1). This gradient is used as the driving force for synthesis of ATP, as catalyzed by the H+-ATP synthase in the same membrane...

In order to maintain a A/1h+ across a membrane, and to ensure that it is used for the synthesis of ATP and not dissipated by leakage, the membrane must be closed and not leaky to protons. From the rate at which a pH gradient across the membrane decayed, it was shown that the effective proton conductance of the mitochondrial inner membrane [8], bacterial plasma membrane [9], and chloroplast thylakoid membrane [10] have a value of only some 0.5 jttS2/cm, or a million-fold less than the aqueous phases on either side. 

With our present understanding, the thermogenic qualities of brown adipose tissue mitochondria are a consequence of the existence in the mitochondrial inner membrane of a polypeptide, thermogenin, uniquely [13-15] found in brown adipose tissue. (For technical and historical reasons, thermogenin is also known under several other names, such as the GDP-binding protein, the 32000 protein, the purine-nucleotide-binding protein (NbP), the uncoupling protein (UCP), the proton conductance pathway, etc.)... 

An adequate proton supply to an F0 subunit channel in the mitchondrial ATP synthase is less certain. Using the same logic as before, proton supply from the cytosol at pH 7.5 would be only 20 H+ per channel per second. This discrepancy might be overcome by a much wider channel mouth, a slower rate of ATP synthesis per enzyme, or some additional mechanism by which protons are supplied to the mitochondrial ATP synthase. One possibility is that in mitochondria, where ATP synthesis (and therefore proton flux) is driven by a membrane potential, hydrolysis of water at the channel mouth could be a major source for protons. Kasianowicz et al. (46) found it necessary to invoke this possibility to account for the observed rates of protonophore-mediated proton conductance across lipid bilayers. 

Uncoupling proteins (UCPs) form channels through the inner mitochondrial membrane that are able to conduct protons from the intermembrane space to the matrix, thereby short-circuiting ATP synthase. 

In the inner mitochondrial membrane there reside many copies of a molecular complex that is a marvel of nanoengineering the FoFiATPase. This consists of an intramembranous channel, the Fq section, that conducts protons across the membrane, and another section of the complex located on the internal side of the inner mitochondrial membrane, FiATPase it phosphorylates ADP with P, to generate ATP. The three-dimensional structure and the molecular mechanism of this incredible ATP generator are discussed further in Sec. 10.11. The mechanism of synthesis of ATP by the FqF, ATPase that is coupled to the flow of electrons from NADH and FADHj along the ETC is called oxidative phosphorylation. 

Fig. 13.1.6. Schematic representation of the structure and function of the mitochondrial Fi FoATP synthase (Complex V). Rotation of the c subunits is believed to be driven by proton conduction through the membrane domain, which in turn drives rotation of the...

Fig. 3. Primary carbon metabolism in a photosynthetic C3 leaf. An abbreviated depiction of foliar C02 uptake, chloroplastic light-reactions, chloroplastic carbon fixation (Calvin cycle), chloroplastic starch synthesis, cytosolic sucrose synthesis, cytosolic glycolysis, mitochondrial citric acid cycle, and mitochondrial electron transport. The photorespiration cycle spans reactions localized in the chloroplast, the peroxisome, and the mitochondria. Stacked green ovals (chloroplast) represent thylakoid membranes. Dashed arrows near figure top represent the C02 diffusion path from the atmosphere (Ca), into the leaf intercellular airspace (Ci), and into the stroma of the chloroplast (Cc).SoHd black arrows represent biochemical reactions. Enzyme names and some substrates and biochemical steps have been omitted for simplicity. The dotted line in the mitochondria represents the electron transport pathway. Energy equivalent intermediates (e.g., ADP, UTP, inorganic phosphate Pi) and reducing equivalents (e.g., NADPH, FADH2, NADH) are labeled in red. Membrane transporters Aqp (CO2 conducting aquaporins) and TPT (triose phosphate transporter) are labeled in italics. Mitochondrial irmer-membrane electron transport and proton transport proteins are labeled in small case italics.

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