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Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002.

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Biochemistry. 5th edition.

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Section 18.5Many Shuttles Allow Movement Across the Mitochondrial Membranes

The inner mitochondrial membrane must be impermeable to most molecules, yet much exchange has to take place between the cytosol and the mitochondria. This exchange is mediated by an array of membrane-spanning transporter proteins (Section 13.4).

18.5.1. Electrons from Cytosolic NADH Enter Mitochondria by Shuttles

Recall that the glycolytic pathway generates NADH in the cytosol in the oxidation of glyceraldehyde 3-phosphate, and NAD+ must be regenerated for glycolysis to continue. How is cytosolic NADH reoxidized under aerobic conditions? NADH cannot simply pass into mitochondria for oxidation by the respiratory chain, because the inner mitochondrial membrane is impermeable to NADH and NAD+. The solution is that electrons from NADH, rather than NADH itself, are carried across the mitochondrial membrane. One of several means of introducing electrons from NADH into the electron transport chain is the glycerol 3-phosphate shuttle (Figure 18.37). The first step in this shuttle is the transfer of a pair of electrons from NADH to dihydroxyacetone phosphate, a glycolytic intermediate, to form glycerol 3-phosphate.This reaction is catalyzed by a glycerol 3-phosphate dehydrogenase in the cytosol. Glycerol 3-phosphate is reoxidized to dihydroxyacetone phosphate on the outer surface of the inner mitochondrial membrane by a membrane-bound isozyme of glycerol 3-phosphate dehydrogenase. An electron pair from glycerol 3-phosphate is transferred to a FAD prosthetic group in this enzyme to form FADH2. This reaction also regenerates dihydroxyacetone phosphate.

Image ch18fu4.jpg

Figure 18.37. Glycerol 3-Phosphate Shuttle.

Figure 18.37

Glycerol 3-Phosphate Shuttle. Electrons from NADH can enter the mitochondrial electron transport chain by being used to reduce dihydroxyacetone phosphate to glycerol 3-phosphate. Glycerol 3-phosphate is reoxidized by electron transfer to an FAD prosthetic (more...)

The reduced flavin transfers its electrons to the electron carrier Q, which then enters the respiratory chain as QH2. When cytosolic NADH transported by the glycerol 3-phosphate shuttle is oxidized by the respiratory chain, 1.5 rather than 2.5 ATP are formed. The yield is lower because FAD rather than NAD+ is the electron acceptor in mitochondrial glycerol 3-phosphate dehydrogenase. The use of FAD enables electrons from cytosolic NADH to be transported into mitochondria against an NADH concentration gradient. The price of this transport is one molecule of ATP per two electrons. This glycerol 3-phosphate shuttle is especially prominent in muscle and enables it to sustain a very high rate of oxidative phosphorylation. Indeed, some insects lack lactate dehydrogenase and are completely dependent on the glycerol 3-phosphate shuttle for the regeneration of cytosolic NAD+.

In the heart and liver, electrons from cytosolic NADH are brought into mitochondria by the malate-aspartate shuttle, which is mediated by two membrane carriers and four enzymes (Figure 18.38). Electrons are transferred from NADH in the cytosol to oxaloacetate, forming malate, which traverses the inner mitochondrial membrane and is then reoxidized by NAD+ in the matrix to form NADH in a reaction catalyzed by the citric acid cycle enzyme malate dehydrogenase. The resulting oxaloacetate does not readily cross the inner mitochondrial membrane, and so a transamination reaction (Section 23.3.1) is needed to form aspartate, which can be transported to the cytosolic side. Mitochondrial glutamate donates an amino group, forming aspartate and α-ketoglutarate. In the cytoplasm, aspartate is then deaminated to form oxaloacetate and the cycle is restarted. This shuttle, in contrast with the glycerol 3-phosphate shuttle, is readily reversible. Consequently, NADH can be brought into mitochondria by the malate- aspartate shuttle only if the NADH/NAD+ ratio is higher in the cytosol than in the mitochondrial matrix. This versatile shuttle also facilitates the exchange of key intermediates between mitochondria and the cytosol.

Image ch18fu5.jpg

Figure 18.38. Malate-Aspartate Shuttle.

Figure 18.38

Malate-Aspartate Shuttle.

18.5.2. The Entry of ADP into Mitochondria Is Coupled to the Exit of ATP by ATP-ADP Translocase

The major function of oxidative phosphorylation is to generate ATP from ADP. However, ATP and ADP do not diffuse freely across the inner mitochondrial membrane. How are these highly charged molecules moved across the inner membrane into the cytosol? A specific transport protein, ATP-ADP translocase (also called adenine nucleotide translocase or ANT), enables these molecules to traverse this permeability barrier. Most important, the flows of ATP and ADP are coupled. ADP enters the mitochondrial matrix only if ATP exits, and vice versa. The reaction catalyzed by the translocase, which acts as an antiporter, is

Image ch18e27.jpg

ATP-ADP translocase is highly abundant, constituting about 14% of the protein in the inner mitochondrial membrane. The translocase, a dimer of identical 30-kd subunits, contains a single nucleotide-binding site that alternately faces the matrix and cytosolic sides of the membrane (Figure 18.39). ATP and ADP (both devoid of Mg2+) are bound with nearly the same affinity. In the presence of a positive membrane potential (as would be the case for an actively respiring mitochondrion), the rate of binding-site eversion from the matrix to the cytosolic side is more rapid for ATP than for ADP because ATP has one more negative charge. Hence, ATP is transported out of the matrix about 30 times as rapidly as is ADP, which leads to a higher phosphoryl transfer potential on the cytosolic side than on the matrix side. The translocase does not evert at an appreciable rate unless a molecule of ADP is bound at the open, cytosolic site, which then everts to the mitochondrial matrix side. This feature ensures that the entry of ADP into the matrix is precisely coupled to the exit of ATP. The other side of the coin is that the membrane potential and hence the proton-motive force are decreased by the exchange of ATP for ADP, which results in a net transfer of one negative charge out of the matrix. ATP-ADP exchange is energetically expensive; about a quarter of the energy yield from electron transfer by the respiratory chain is consumed to regenerate the membrane potential that is tapped by this exchange process. The inhibition of this process leads to the subsequent inhibition of cellular respiration as well (Section 18.6.3).

Figure 18.39. Mechanism of Mitochondrial ATP-ADP Translocase.

Figure 18.39

Mechanism of Mitochondrial ATP-ADP Translocase. The translocase catalyzes the coupled entry of ADP and exit of ATP into and from the matrix. The reaction cycle is driven by membrane potential. The actual conformational change corresponding to eversion (more...)

18.5.3. Mitochondrial Transporters for Metabolites Have a Common Tripartite Motif

ATP-ADP translocase is but one of many mitochondrial transporters for ions and charged metabolites (Figure 18.40). For historical reasons, these transmembrane proteins are sometimes called carriers. Recall that some of them function as symporters and others as antiporters (Section 13.4). The phosphate carrier, which works in concert with ATP-ADP translocase, mediates the electroneutral exchange of H2PO4- for OH- (or, indistinguishably, the electroneutral symport of H2PO4- and H+). The combined action of these two transporters leads to the exchange of cytosolic ADP and Pi for matrix ATP at the cost of an influx of one H+. The dicarboxylate carrier enables malate, succinate, and fumarate to be exported from mitochondria in exchange for Pi. The tricarboxylate carrier transports citrate and H+ in exchange for malate. Pyruvate in the cytosol enters the mitochondrial matrix in exchange for OH- (or together with H+) by means of the pyruvate carrier. These mitochondrial transporters and more than five others have a common structural motif. They are constructed from three tandem repeats of a 100-residue module, each containing two putative transmembrane segments (Figure 18.41).

Figure 18.40. Mitochondrial Transporters.

Figure 18.40

Mitochondrial Transporters. Transporters (also called carriers) are transmembrane proteins that move ions and charged metabolites across the inner mitochondrial membrane.

Figure 18.41. Structure of Mitochondrial Transporters.

Figure 18.41

Structure of Mitochondrial Transporters. Many mitochondrial transporters consist of three similar 100-residue units. These proteins contain six putative membrane-spanning segments. [After J. E. Walker. Curr. Opin. Struct. Biol. 2(1992):519.]

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2002, W. H. Freeman and Company.
Bookshelf ID: NBK22470

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