Mitochondria Transport systems

Figure 9.13 Examples of mitochondrial transport systems for anions. 0 The anb port system transfers malate into but oxo-glutarate out of the mitochondrion. The symport system transfers both pyruvate and protons into the mitochondrion across the inner membrane. Both transport processes are electroneutral.

Opening of leads to a local increase in the cytosolic Ca concentration from 10 M to 10 M. In this concentration region, the Ca transport systems mentioned above work very efficiently. However, if an increase in Ca concentration over lO M takes place, e.g., due to cell damage, a level critical for the cell is reached. In this case, Ca is pumped into the mitochondria with the help of Ca transport systems localized in the iimer membrane of the mitochondrion. 

It has been known for many years that the mitochondrion shows a respiration-linked transport of a number of ions. Of these, calcium has attracted the most attention since it depends on a specific transport system with high-affinity binding sites. The uptake of calcium usually also involves a permeant anion, but in the absence of this, protons are ejected as the electron transfer system operates. The result is either the accumulation of calcium salts in the mitochondrial matrix or an alkalinization of the interior of the mitochondrion. The transfer of calcium inwards stimulates oxygen utilization but provides an alternative to the oxidative phosphorylation of ADP618 ... 

The combined effect of exchanging extramitochon-drial ADP-3 and H2P04 for mitochondrial ATP-4 and OH is to move one proton into the mitochondrial matrix for every molecule of ATP that the mitochondrion releases into the cytosol. This proton translocation must be considered, along with the movement of protons through the ATP synthase, to account for the P-to-O ratio of oxidative phosphorylation. If three protons pass through the ATP synthase, and the adenine nucleotide and Pj transport systems move one additional proton, then four protons in total move into the matrix for each ATP molecule provided to the cytosol. 

In eukaryotes, most of the reactions of aerobic energy metabolism occur in mitochondria. An inner membrane separates the mitochondrion into two spaces the internal matrix space and the intermembrane space. An electron-transport system in the inner membrane oxidizes NADH and succinate at the expense of 02, generating ATP in the process. The operation of the respiratory chain and its coupling to ATP synthesis can be summarized as follows ... 

The next step in building a biochemical network model for the TCA cycle is determining the governing differential equations. Since we are not treating transport of material into or out of the mitochondrion, the reactants of the overall reaction of Equation (6.31) are held clamped in this model. In addition, ASP and GLU are held fixed because there are no sources or sinks for the metabolites other than the aspartate aminotransferase reaction built into the model at this stage. Since the electron transport system is not modeled, proton transport is not included and pH is held fixed. 

Almost all cells have an active transport system to maintain nonequilibrium concentration levels of substrates. For example, in the mitochondrion, hydrogen ions are pumped into the intermembrane space of the organelle as part of producing ATP. Active transport concentrates ions, minerals, and nutrients inside the cell that are in low concentration... 

As mentioned, the battery cell process is analogous to the living cell process of the mitochondrion, the chloroplast, and the electron-transport system in the cytoplasmic membrane of the procaryotes. Thus, Equation (15.11) can represent the basic thermodynamics of a microbial system. 

Figure 16.28. Compartmental Cooperation. Oxaloacetate utilized in the cytosol for gluconeogenesis is formed in the mitochondrial matrix by carboxylation of pyruvate. Oxaloacetate leaves the mitochondrion by a specific transport system (not shovm) in the form of malate, -which is reoxidized to oxaloacetate in the cytosol.

Carnitine is used mainly for facilitating the transport of long-chain fatty adds into the mitochondria. As shown in Figure4.53, this transport system requires the participation of two different carnitine acyl transferases. One is located on the outside of the mitochondrial membrane, the other on the inner side. Once fatty acyl-camitine is inside the organelle, its carnitine is released. A separate transport system is used to transport this carnitine from the interior of the mitochondrion back to the cytoplasm for reuse. 

Transsport of fatty adds into the mitochondrion is regulated by a mechanism that plays a major role in controlling the overall rate of oxidation of fatty acids. This mechanism is in "communication" with the pathw ay for fatty acid synthesis. The fatty acid transport system is sensitive to the concentration of one fatty acid synthesis intermediate, malonyl-CoA. 

The uiea cycle may be considered to be a mitochondrial pathway, as carbamyl phosphate synthase and ornithine transcarbamylase are mitochondrial enzymes however, the enzymes catalyzing subsequent steps of the pathway arc cytosolic-The steps leading to conversion of citrulline to ornithine occur in the cytosol. Hence, the pathway is shared by the mitochondrial and cytosolic compartments. The fumarate produced by the urea cycle is converted to malate by a cytoplasmic form of fumarase. Mittxihondrial fumarase is part of the Krebs cycle. Cytoplasmic malate can enter the mitochondrion by means of a transport system, such as the malate/phosphate exchanger or the ma ate/a-ketoglutaratc exchanger. These transport systems are membrane-bound proteins. 

Krebs cycle Citric acid cycle, TCA cycle, the mitochondrial process by which acetyl groups from acetyl-CoA are oxidized to CO. The reducing equivalents are captured as NADH and FADH, which feed into the electron transport system of the mitochondrion to produce ATP via oxidative phosphorylation. 

The electron transport chain (ETC) or electron transport system (ETS) shown in Figure 16-1 is located on the inner membrane of the mitochondrion and is responsible for the harnessing of free energy released as electrons travel from more reduced (more negative reduction potential, E to more oxidized (more positive carriers to drive the phosphorylation of ADP to ATP. Complex 1 accepts a pair of electrons from NADH ( = -0.32 V)... 

How does brown fat generate heat and burn excess calories For the answer we must turn to the mitochondrion. In addition to the ATP synthase and the electron transport system proteins that are found in all mitochondria, there is a protein in the inner mitochondrial membrane of brown fat tissue called thermogenin. This protein has a channel in the center through which the protons (H ) of the intermembrane space could pass back into the mitochondrial matrix. Under normal conditions this channel is plugged by a GDP molecule so that it remains closed and the proton gradient can continue to drive ATP synthesis by oxidative phosphorylation. 

NADH carries electrons to the electron transport system inside the mitochondrion via a shuttle system (Figure 15.11). Electrons that enter via the shuttle in Figure 15.11a bypass complex I of the electron transport system, whereas electrons that enter via the shuttle in Figure 15,11b enter at complex L... 

These enzymes contain FAD, and the reduced coenzyme FADH2 that is formed is reoxidized by an electron transferring flavoprotein (Chapter 15), which also contains FAD. This protein carries the electrons abstracted in the oxidation process to the inner membrane of the mitochondrion where they enter the mitochondrial electron transport system, as depicted in Fig. 10-5 and as discussed in detail in... 

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