Matrix space, mitochondrial

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 tricarboxylic acid cycle not only takes up acetyl CoA from fatty acid degradation, but also supplies the material for the biosynthesis of fatty acids and isoprenoids. Acetyl CoA, which is formed in the matrix space of mitochondria by pyruvate dehydrogenase (see p. 134), is not capable of passing through the inner mitochondrial membrane. The acetyl residue is therefore condensed with oxaloacetate by mitochondrial citrate synthase to form citrate. This then leaves the mitochondria by antiport with malate (right see p. 212). In the cytoplasm, it is cleaved again by ATP-dependent citrate lyase [4] into acetyl-CoA and oxaloacetate. The oxaloacetate formed is reduced by a cytoplasmic malate dehydrogenase to malate [2], which then returns to the mitochondrion via the antiport already mentioned. Alternatively, the malate can be oxidized by malic enzyme" [5], with decarboxylation, to pyruvate. The NADPH+H formed in this process is also used for fatty acid biosynthesis. 

Mitochondria are bacteria-sized organelles (about 1 X 2 im in size), which are found in large numbers in almost all eukaryotic cells. Typically, there are about 2000 mitochondria per cell, representing around 25% of the cell volume. Mitochondria are enclosed by two membranes—a smooth outer membrane and a markedly folded or tubular inner mitochondrial membrane, which has a large surface and encloses the matrix space. The folds of the inner membrane are known as cristae, and tube-like protrusions are called tubules. The intermembrane space is located between the inner and the outer membranes. 

Quantitatively the major constituents of the matrix space are a large number of proteins. These account for about 56% by weight of the matrix material and exist in a state closer to that in a protein crystal than in a true solution.19-203 Mitochondrial membranes also contain proteins, both tightly bound relatively nonpolar intrinsic proteins and extrinsic proteins bound... 

Where within the mitochondria are specific enzymes localized One approach to this question is to see how easily the enzymes can be dissociated from mitochondria. Some enzymes come out readily under hypotonic conditions. Some are released only upon sonic oscillation, suggesting that they are inside the matrix space. Others, including the cytochromes and the flavoproteins that act upon succinate and NADH, are so firmly embedded in the inner mitochondrial membranes that they can be dissociated only through the use of non-denaturing detergents. 

A suspension of mitochondria is incubated with pyruvate, malate, and l4C-labeled triphenylmethyl-phosphonium [TPP] chloride under aerobic conditions. The mitochondria are rapidly collected by centrifugation, and the amount of l4C that they contain is measured. In a separate experiment, the volume of the mitochondrial matrix space was determined so that the concentration of TPP cation in the matrix can be calculated. The internal concentration is found to be 1,000 times greater than that in the external solution. 

These organelles are the sites of energy production of aerobic cells and contain the enzymes of the tricarboxylic acid cycle, the respiratory chain, and the fatty acid oxidation system. The mitochondrion is bounded by a pair of specialized membranes that define the separate mitochondrial compartments, the internal matrix space and an intermembrane space. Molecules of 10,000 daltons or less can penetrate the outer membrane, but most of these molecules cannot pass the selectively permeable inner membrane. By a series of infoldings, the internal membrane forms cristae in the matrix space. The components of the respiratory chain and the enzyme complex that makes ATP are embedded in the inner membrane as well as a number of transport proteins that make it selectively permeable to small molecules that are metabolized by the enzymes in the matrix space. Matrix enzymes include those of the tricarboxylic acid cycle, the fatty acid oxidation system, and others. 

In mammalian cells, some 1% of the total cellular DNA is found in the mitochondria. This DNA is double stranded, circular, and small, with a molecular weight of about 10 million, which is in the same range as that of viral DNAs. Some four to ten molecules of DNA per mitochondrion, along with some ribosomes, are found in the matrix space. DNA replication, transcription, and synthesis of some mitochondrial proteins take place in the matrix space. This protein synthesis very much resembles that of bacteria. The mitochondrial genetic code differs from the "universal" genetic code (Chapter 12) used for nuclearly encoded proteins and bacteria. The reasons for this are unknown. 

The mitochondrial Ca2+ pool plays a second role in cellular Ca2+ homeostasis by serving as a sink for Ca2+ during times of excessive Ca2+ uptake by the cell. Under this circumstance, the non-ionic calcium pool in the matrix space can increase 10-fold or more, thereby protecting the cell from Ca2+ intoxication. This mechanism provides a temporary device by which the cell can protect itself, but in the long term only by regulating Ca2+ fluxes across the plasma membrane can the cell maintain Ca2+ homeostasis [14]. 

A steady flow of metabolites both in and out of the mitochondrial matrix space is necessary for mitochondria to perform functions which involve the participation of enzymes inside the membrane permeability barrier. These functions include oxidative phosphorylation and therefore O2, ADP, phosphate and electron-rich substrates such as pyruvate, fatty acids and ketone bodies must enter the mitochondria, and the products, HjO, CO2 and ATP must leave. Although Oj, HjO and CO2 are permeable to the inner mitochondrial membrane [1,2], most metabolites are not, because of their highly hydrophiUc nature. The outer mitochondrial membrane does not present a significant barrier to hydrophilic metabolites because of the presence of large unregulated channels composed of the membrane protein, porin [3]. The inner mitochondrial membrane has a much larger surface area [4] than the outer membrane and a much higher ratio of protein to lipid [5]. It is composed not only of proteins involved in electron transport and oxidative phosphorylation but also specialized proteins which facilitate, and in many cases provide, directionality to the transport of metabolites [6]. 

In some mammalian cells, enzymes comprising partial spans of biosynthetic pathways are inside and some outside the mitochondrial matrix space. Therefore, in the liver, six mitochondrial membrane transport proteins are required for urea synthesis, three for gluconeogenesis [7,8], and three others participate in ammonia-genesis [9] in the kidney. The synthesis of neurotransmitter substances such as acetylcholine, glutamate and y-amino butyric acid requires the participation of metabolite transporters in mitochondrial membranes of nervous tissue [9,10]. 

Because medium- and short-chain fatty acids can freely diffuse across the mitochondrial inner membrane and conversion to their respective CoA esters takes place in the mitochondrial matrix, no carnitine-dependent translocase is required to shuttle these substrates from the cytoplasm to the mitochondrial matrix space (Guzman and Geelen, 1993). 

In normal myocardium, the first step in fatty acid utilization is thioesterification catalyzed by acylCoA synthetase. There are three potential metabolic fates of the synthesized acylCoA including 1) transport into the mitochondrial matrix for subsequent P-oxidation, 2) utilization as an intermediate in polar and nonpolar lipid synthesis, and 3) hydrolysis by acylCoA hydrolase (i.e., a net futile cycle). In normal myocardium, the major fraction of synthesized acylCoA is transported into the mitochondrial matrix by sequential transesterification reactions catalyzed by carnitine acyltransferase. AcylCoA in the mitochondrial matrix space is sequentially oxidized in two-carbon units to produce acetylCoA, which is accompanied by the production of the reducing equivalents, NADH and FADH2. 

Current evidence suggests that a total of 10 protons are transported from the matrix space across the inner mitochondrial membrane for every electron pair that is transferred from NADH to O2 (see Figure 8-17). Since the succinate-CoQ reductase complex does not transport protons, only six protons are transported across the membrane for every electron pair that is transferred from succinate (or FADH2) to O2. Relatively little is known about the coupling of electron flow and proton translocation by the NADH-CoQ reductase complex. More is known about operation of the cytochrome c oxidase complex, which we discuss here. The coupled electron and proton movements mediated by the CoQH2-cytochrome c reductase complex, which involves a unique mechanism, are described separately. 

Mitochondria are football-shaped organelles that are roughly the size of a bacterial cell. They are surrounded by an outer mitochondrial membrane and an inner mitochondrial membrane (Figure 22.1). The space between the two membranes is the intermembrane space, and the space inside of the inner membrane is the matrix space. The enzymes of the citric acid cycle, of the (3-oxidation pathway for the breakdown of fatty acids, and for the degradation of amino acids are all found in the mitochondrial matrix space. 

The inner membrane is highly folded to create a large surface area. The folded membranes are known as cristae. The inner mitochondrial membrane is almost completely impermeable to most substances. For this reason it has many transport proteins to bring particular fuel molecules into the matrix space. Also embedded within the inner mitochondrial membrane are the protein electron carriers of the electron transport system and ATP synthase. ATP synthase is a large complex of many proteins that catalyzes the s)mthesis of ATP. 

The enzymes for the citric acid cycle are found in the mitochondrial matrix space. The first enzyme catalyzes a reaction that joins the acetyl group of acetyl CoA (two carbons) to a four-carbon molecule (oxaloacetate) to produce citrate (six carbons). The remaining enzymes catalyze a series of rearrangements, decarboxylations (removal of CO2), and oxidation-reduction reactions. The eventual products of this cyclic pathway are two CO2 molecules and oxaloacetate—the molecule we began with. 

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 mitochondria are aerobic cell organelles that are responsible for most of the ATP production in eukaryotic cells. They are enclosed by a double membrane. The outer membrane permits low-molecular-weight molecules to pass through. The inner mitochondrial membrane, by contrast, is almost completely impermeable to most molecules. The inner mitochondrial membrane is the site where oxidative phosphorylation occurs. The enzymes of the citric acid cycle, of amino acid catabolism, and of fatty acid oxidation are located in the matrix space of the mitochondrion. 

The enzymes that catalyze the p-oxidation of fatty acids are located in the matrix space of the mitochondria. Special transport mechanisms are required to bring fatty acid molecules into the mitochondrial matrix. Once inside, the fatty acids are degraded by the reactions of p-oxidation. As we will see, these reactions interact with oxidative phosphorylation and the citric acid cycle to produce ATP. 

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