Mitochondria tricarboxylic acid cycle

Mitochondria Tricarboxylic acid cycle, electron transport and oxidative phosphorylation, fatty acid oxidation, urea synthesis... 

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. 

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. 

Actively respiring fungal cells possess a distinct mitochondrion, which has been described as the power-house of the cell (Fig. 4.2). The enzymes of the tricarboxylic acid cycle (Kreb s cycle) are located in the matrix of the mitochondrion, while electron transport and oxidative phosphorylation occur in the mitochondrial inner membrane. The outer membrane contains enzymes involved in lipid biosynthesis. The mitochondrion is a semiindependent organelle as it possesses its own DNA and is capable of producing its own proteins on its own ribosomes, which are referred to as mitoribosomes. 

Aerobic glycolysis Metabolism of glucose to pyruvate. Pyruvate in the presence of sufficient oxygen can be metabolized to CO via the tricarboxylic acid cycle in the mitochondrion-producing NADH and FADH, which contribute elections through the electron transfer chain to molecular oxygen producing H O and ATP. 

The oxidation reactions involved are catalyzed by a series of nicotinamide adenine dinucleotide (NAD+) or flavin adenine dinucleotide (FAD) dependent dehydrogenases in the highly conserved metabolic pathways of glycolysis, fatty acid oxidation and the tricarboxylic acid cycle, the latter two of which are localized to the mitochondrion, as is the bulk of coupled ATP synthesis. Reoxidation of the reduced cofactors (NADH and FADH2) requires molecular oxygen and is carried out by protein complexes integral to the inner mitochondrial membrane, collectively known as the respiratory, electron transport, or cytochrome, chain. Ubiquinone (UQ), and the small soluble protein cytochrome c, act as carriers of electrons between the complexes (Fig. 13.1.1). 

Mitochondria and unicellular organisms. Bacteria have no mitochondria. Being of mitochondrial size, the bacterium has to function as its own mitochondrion its plasma membrane, although it lacks cristae, has to attempt to carry out as many as it can of the complex activities of eukaryotic mitochondria. Hence many of the typical enzymes of eukaryotic mitochondria are located in the bacterial plasma membrane (De Ley and Docky, 1960). In particular, the enzymes of the tricarboxylic acid cycle are found there. More than 90% of the cell s succinic. 

The mitochondrion is an organelle where aerobic respiration occurs. Mitochondria consist of a donble membrane that is the location of the conversion of pyruvate (a metabolic componnd) and the tricarboxylic acid cycle. The nucleus stores the cell DNA and is delineated by a lipid membrane that envelopes the nucleus and is similar to the plasma membrane. The cell uses mRNA to transfer the information out into the cytoplasm for use in protein synthesis (White Zainasheff, 2010). 

Matrix of the mitochondrion This gel-like solution in the interior of mitochondria is fifty percent protein. These molecules include the enzymes responsible for the oxidation of pyruvate, amino acids, fatty acids (by p-oxidation), and those of the tricarboxylic acid (TCA) cycle. The synthesis of urea and heme occur partially in the matrix of mitochondria. In addition, the matrix contains NAD+and FAD (the oxidized forms of the two coenzymes that are required as hydrogen acceptors) and ADP and Pj, which are used to produce ATP. [Note The matrix also contains mitochondrial RNA and DNA (mtRNA and mtDNA) and mitochondrial ribosomes.]... 

The mitochondrion, in addition to being the powerhouse of the cell (because it generates more than 90% of the ATP used by the cell), is also the site of fatty acid oxidation, the tricarboxylic acid (TCA) cycle, electron transport, and amino acid metabolism. Central to the utilization of fuel molecules—carbohydrates, pro-... 

Inside the inner membrane of a mitochondrion is a viscous region known as the matrix (Fig. 1-9). Enzymes of the tricarboxylic acid (TCA) cycle (also known as the citric acid cycle and the Krebs cycle), as well as others, are located there. For substrates to be catabolized by the TCA cycle, they must cross two membranes to pass from the cytosol to the inside of a mitochondrion. Often the slowest or rate-limiting step in the oxidation of such substrates is their entry into the mitochondrial matrix. Because the inner mitochondrial membrane is highly impermeable to most molecules, transport across the membrane using a carrier or transporter (Chapter 3, Section 3.4A) is generally invoked to explain how various substances get into the matrix. These carriers, situated in the inner membrane, might shuttle important substrates from the lumen between the outer and the inner mitochondrial membranes to the matrix. Because of the inner membrane, important ions and substrates in the mitochondrial matrix do not leak out. Such permeability barriers between various subcellular compartments improve the overall efficiency of a cell. 

Krebs cycle A biochemical cycle in the second stage of cellular respiration involving eight steps that complete the metabolic breakdown of glucose molecules to carbon dioxide. Acetyl CoA is combined with oxaloac-etate to form citric acid. Citric acid is then converted into a number of other chemicals, and carbon dioxide is released. The process takes place within the mitochondrion. Also called citric acid cycle or tricarboxylic acid (TCA) cycle. Conceived and published by British scientist Sir Hans Adolf Krebs in 1957. 

In humans, oxaloacetate must be transported out of the mitochondrion to supply the cytosolic PEPCK. Because there is no mitochondrial carrier for oxaloacetate and its diffusion across the mitochondrial membrane is slow, it is transported as malate or asparate (Figure 15-2). The malate shuttle carries oxaloacetate and reducing equivalents, whereas the aspartate shuttle, which does not require a preliminary reduction step, depends on the availability of glutamate and a-ketoglutarate in excess of tricarboxylic acid (TCA) cycle requirements. 

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