Transport across mitochondrial membrane

A FIGURE 8-10 The malate shuttle. This cyclical series of reactions transfers electrons from NADH in the cytosol (intermembrane space) across the inner mitochondrial membrane, which is impermeable to NADH itself. StepH Cytosolic malate dehydrogenase transfers electrons from cytosolic NADH to oxaloacetate, forming malate. StepH An antiporter (blue oval) in the inner mitochondrial membrane transports malate into the matrix in exchange for a-ketoglutarate. StepH Mitochondrial malate dehydrogenase converts malate back to oxaloacetate, reducing NAD in the matrix to NADH in the process. StepH Oxaloacetate, which cannot directly cross the inner membrane, is converted to... 

Electron transport and ATP production are coupled to each other by the same mechanism in mitochondria and chloroplasts. In both cases, the coupling depends on the generation of a proton gradient across the inner mitochondrial membrane or across the thylakoid membrane, as the case may be. 

As with Complex 1, passage of electrons through the Q cycle of Complex 111 is accompanied by proton transport across the inner mitochondrial membrane. The postulated pathway for electrons in this system is shown in Figure 21.12. A large pool of UQ and UQHg exists in the inner mitochondrial membrane. The Q cycle is initiated when a molecule of UQHg from this pool diffuses to a site (called Q, ) on Complex 111 near the cytosolic face of the membrane. 

Thus, Og and cytochrome c oxidase are the final destination for the electrons derived from the oxidation of food materials. In concert with this process, cytochrome c oxidase also drives transport of protons across the inner mitochondrial membrane. These important functions are carried out by a transmembrane protein complex consisting of more than 10 subunits (Table 21.2). 

Complex IV Also Transports Protons Across the Inner Mitochondrial Membrane... 

The reduction of oxygen in Complex IV is accompanied by transport of protons across the inner mitochondrial membrane. Transfer of four electrons through this complex drives the transport of approximately four protons. The mechanism of proton transport is unknown but is thought to involve the steps from state P to state O (Figure 21.20). Four protons are taken up on the matrix side for every two protons transported to the cytoplasm (see Figure 21.17). 

In 1961, Peter Mitchell, a British biochemist, proposed that the energy stored in a proton gradient across the inner mitochondrial membrane by electron transport drives the synthesis of ATP in cells. The proposal became known as... 

FIGURE 21.22 The proton and electrochemical gradients existing across the inner mitochondrial membrane. The electrochemical gradient is generated by the transport of protons across the membrane. 

FIGURE 21.31 Structures of several uiicouplers, molecules that dissipate the proton gradient across the inner mitochondrial membrane and thereby destroy the tight coupling between electron transport and the ATP synthase reaction. 

Most of the NADH used in electron transport is produced in the mitochondrial matrix space, an appropriate site because NADH is oxidized by Complex I on the matrix side of the inner membrane. Furthermore, the inner mitochondrial membrane is impermeable to NADH. Recall, however, that NADH is produced in glycolysis by glyceraldehyde-3-P dehydrogenase in the cytosol. If this NADH were not oxidized to regenerate NAD, the glycolytic pathway would cease to function due to NAD limitation. Eukaryotic cells have a number of shuttle systems that harvest the electrons of cytosolic NADH for delivery to mitochondria without actually transporting NADH across the inner membrane (Figures 21.33 and 21.34). 

The second electron shuttle system, called the malate-aspartate shuttle, is shown in Figure 21.34. Oxaloacetate is reduced in the cytosol, acquiring the electrons of NADH (which is oxidized to NAD ). Malate is transported across the inner membrane, where it is reoxidized by malate dehydrogenase, converting NAD to NADH in the matrix. This mitochondrial NADH readily enters the electron transport chain. The oxaloacetate produced in this reaction cannot cross the inner membrane and must be transaminated to form aspartate, which can be transported across the membrane to the cytosolic side. Transamination in the cytosol recycles aspartate back to oxaloacetate. In contrast to the glycerol phosphate shuttle, the malate-aspartate cycle is reversible, and it operates as shown in Figure 21.34 only if the NADH/NAD ratio in the cytosol is higher than the ratio in the matrix. Because this shuttle produces NADH in the matrix, the full 2.5 ATPs per NADH are recovered. 

The thylakoid membrane is asymmetrically organized, or sided, like the mitochondrial membrane. It also shares the property of being a barrier to the passive diffusion of H ions. Photosynthetic electron transport thus establishes an electrochemical gradient, or proton-motive force, across the thylakoid membrane with the interior, or lumen, side accumulating H ions relative to the stroma of the chloroplast. Like oxidative phosphorylation, the mechanism of photophosphorylation is chemiosmotic. 

FIGURE 24.9 The formation of acylcar-nitines and their transport across the inner mitochondrial membrane. The process involves the coordinated actions of carnitine acyltrans-ferases on both sides of the membrane and of a translocase that shuttles O-acylcarnitines across the membrane. 

Mechanistic studies have shown that TBT and certain other forms of trialkyltin have two distinct modes of toxic action in vertebrates. On the one hand they act as inhibitors of oxidative phosphorylation in mitochondria (Aldridge and Street 1964). Inhibition is associated with repression of ATP synthesis, disturbance of ion transport across the mitochondrial membrane, and swelling of the membrane. Oxidative phosphorylation is a vital process in animals and plants, and so trialkyltin compounds act as wide-ranging biocides. Another mode of action involves the inhibition of forms of cytochrome P450, which was referred to earlier in connection with metabolism. This has been demonstrated in mammals, aquatic invertebrates and fish (Morcillo et al. 2004, Oberdorster 2002). TBTO has been shown to inhibit P450 activity in cells from various tissues of mammals, including liver, kidney, and small intestine mucosa, both in vivo and in vitro (Rosenberg and Drummond 1983, Environmental Health Criteria 116). 

Uncouplers of oxidative phosphorylation Compounds that uncouple oxidative phosphorylatiou from electron transport in the inner mitochondrial membrane. Most are weak lipophilic acids that can run down the proton gradient across this membrane. 

Because the inner mitochondrial membrane is impermeable to protons and other ions, special exchange transporters span the membrane to allow passage of ions such as OH, Pf, ATP , ADP, and metabo-htes, without discharging the electrochemical gradient across the membrane. 

Allelopathic inhibition of mineral uptake results from alteration of cellular membrane functions in plant roots. Evidence that allelochemicals alter mineral absorption comes from studies showing changes in mineral concentration in plants that were grown in association with other plants, with debris from other plants, with leachates from other plants, or with specific allelochemicals. More conclusive experiments have shown that specific allelochemicals (phenolic acids and flavonoids) inhibit mineral absorption by excised plant roots. The physiological mechanism of action of these allelochemicals involves the disruption of normal membrane functions in plant cells. These allelochemicals can depolarize the electrical potential difference across membranes, a primary driving force for active absorption of mineral ions. Allelochemicals can also decrease the ATP content of cells by inhibiting electron transport and oxidative phosphorylation, which are two functions of mitochondrial membranes. In addition, allelochemicals can alter the permeability of membranes to mineral ions. Thus, lipophilic allelochemicals can alter mineral absorption by several mechanisms as the chemicals partition into or move through cellular membranes. Which mechanism predominates may depend upon the particular allelochemical, its concentration, and environmental conditions (especially pH). 

Figure 8.1 Transport of fatty acids across the mitochondrial membrane.

Production of Malonyl-CoA for the Fatty Acid Biosynthesis. Acetyl-CoA serves as a substrate in the production of malonyl-CoA. There are several routes by which acetyl-CoA is supplied to die cytoplasm. One route is the transfer of acetyl residues from the mitochondrial matrix across the mitochondrial membrane into the cyto-plasm. This process resembles a fatty acid transport and is likewise effected with the participation of carnitine and the enzyme acetyl-CoA-camitine transferase. Another route is the production of acetyl-CoA from citrate. Citrate is delivered from the mitochondria and undergoes cleavage in the cytoplasm by the action of the enzyme ATP-citrate lyase ... 

Defects of mitochondrial transport interfere with the movement of molecules across the inner mitochondrial membrane, which is tightly regulated by specific translocation systems. The carnitine cycle is shown in Figure 42-2 and is responsible for the translocation of acyl-CoA thioesters from the cytosol into the mitochondrial matrix. The carnitine cycle involves four elements the plasma membrane carnitine transporter system, CPT I, the carnitine-acyl carnitine translocase system in the inner mitochondrial membrane and CPT II. Genetic defects have been described for each of these four steps, as discussed previously [4,8,9]. 

To explain how H+ transfer occurred across the membrane Mitchell suggested the protons were translocated by redox loops with different reducing equivalents in their two arms. The first loop would be associated with flavoprotein/non-heme iron interaction and the second, more controversially, with CoQ. Redox loops required an ordered arrangement of the components of the electron transport system across the inner mitochondrial membrane, which was substantiated from immunochemical studies with submitochondrial particles. Cytochrome c, for example, was located at the intermembranal face of the inner membrane and cytochrome oxidase was transmembranal. The alternative to redox loops, proton pumping, is now known to be a property of cytochrome oxidase. 

Examples of such intra cellular membrane transport mechanisms include the transfer of pyruvate, the symport (exchange) mechanism of ADP and ATP and the malate-oxaloacetate shuttle, all of which operate across the mitochondrial membranes. Compartmentalization also allows the physical separation of metabolically opposed pathways. For example, in eukaryotes, the synthesis of fatty acids (anabolic) occurs in the cytosol whilst [3 oxidation (catabolic) occurs within the mitochondria. 

Fatty acid utilized by muscle may arise from storage triglycerides from either adipose tissue depot or from lipid stores within the muscle itself. Lipolysis of adipose triglyceride in response to hormonal stimulation liberates free fatty acids (see Section 9.6.2) which are transported through the bloodstream to the muscle bound to albumin. Because the enzymes of fatty acid oxidation are located within subcellular organelles (peroxisomes and mitochondria), there is also need for transport of the fatty acid within the muscle cell this is achieved by fatty acid binding proteins (FABPs). Finally, the fatty acid molecules must be translocated across the mitochondrial membranes into the matrix where their catabolism occurs. To achieve this transfer, the fatty acids must first be activated by formation of a coenzyme A derivative, fatty acyl CoA, in a reaction catalysed by acyl CoA synthetase. 

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