Glycolysis location

In eukaryotes, electron transport and oxidative phosphorylation occur in the inner membrane of mitochondria. These processes re-oxidize the NADH and FADH2 that arise from the citric acid cycle (located in the mitochondrial matrix Topic L2), glycolysis (located in the cytoplasm Topic J3) and fatty acid oxidation (located in the mitochondrial matrix Topic K2) and trap the energy released as ATP. Oxidative phosphorylation is by far the major source of ATP in the cell. In prokaryotes, the components of electron transport and oxidative phosphorylation are located in the plasma membrane (see Topic Al). 

Pathways can be illustrated in a metabolic map as linear, branched or cyclic processes (Figure 1.3) and are often compartmentalized within particular subcellular location glycolysis in the cytosol and the Krebs tricarboxylic acid (TCA) cycle in... 

The pentose phosphate pathway (PPP, also known as the hexose monophosphate pathway) is an oxidative metabolic pathway located in the cytoplasm, which, like glycolysis, starts from glucose 6-phosphate. It supplies two important precursors for anabolic pathways NADPH+H+, which is required for the biosynthesis of fatty acids and isopren-oids, for example (see p. 168), and ribose 5-phosphate, a precursor in nucleotide biosynthesis (see p. 188). 

In eukaryotes, the cytoplasm, representing slightly more than 50% of the cell volume, is the most important cellular compartment. It is the central reaction space of the cell. This is where many important pathways of the intermediary metabolism take place—e.g., glycolysis, the pentose phosphate pathway, the majority of gluconeogenesis, and fatty acid synthesis. Protein biosynthesis (translation see p. 250) also takes place in the cytoplasm. By contrast, fatty acid degradation, the tricarboxylic acid cycle, and oxidative phosphorylation are located in the mitochondria (see p. 210). 

The tightly regulated pathway specifying aromatic amino acid biosynthesis within the plastid compartment implies maintenance of an amino acid pool to mediate regulation. Thus, we have concluded that loss to the cytoplasm of aromatic amino acids synthesized in the chloroplast compartment is unlikely (13). Yet a source of aromatic amino acids is needed in the cytosol to support protein synthesis. Furthermore, since the enzyme systems of the general phenylpropanoid pathway and its specialized branches of secondary metabolism are located in the cytosol (17), aromatic amino acids (especially L-phenylalanine) are also required in the cytosol as initial substrates for secondary metabolism. The simplest possibility would be that a second, complete pathway of aromatic amino acid biosynthesis exists in the cytosol. Ample precedent has been established for duplicate, major biochemical pathways (glycolysis and oxidative pentose phosphate cycle) of higher plants that are separated from one another in the plastid and cytosolic compartments (18). Evidence to support the hypothesis for a cytosolic pathway (1,13) and the various approaches underway to prove or disprove the dual-pathway hypothesis are summarized in this paper. 

The two molecules of NADH formed by glycolysis in the cytosol are, under aerobic conditions, reoxidized to NAD+ by transfer of their electrons to the electron-transfer chain, which in eukaryotic cells is located in the mitochondria. The electron-transfer chain passes these electrons to their ultimate destination, 02 ... 

Notice that the metabolic sequences described in this and previous chapters often involve multiple locations. These locations can be at the organ (fig. 20.1) or the subcellular level (fig. 20.22). This is in contrast, for example, with glycolysis, which occurs in the cytosol, or the tricarboxylic acid cycle, which is completely mitochondrial. Can you provide a possible explanation for this multiple location phenomenon ... 

Cells must ensure that each newly synthesized protein is sorted to its correct location where it can carry out the appropriate function. This process is called protein targeting. In a eukaryotic cell, the protein may be destined to stay in the cytosol, for example an enzyme involved in glycolysis (see Topic J3). Alternatively it may need to be targeted to an organelle (such as a mitochondrion, lysosome, peroxisome, chloroplast or the nucleus) or be inserted into the plasma membrane or exported out of the cell. In bacteria such as E. coli, the protein may stay in the cytosol, be inserted into the plasma membrane or the outer membrane, be sent to the space between these two membranes (the periplasmic space) or be exported from the cell. In both prokaryotes and eukaryotes, if a protein is destined for the cytosol, it is made on free ribosomes in the cytosol and released directly into the cytosol. If it is destined for other final locations, specific protein-targeting mechanisms are involved. 

The glycolysis pathway is an intermediate metabolism pathway for glucose oxidation located in the cytosol. 

Figure 16.24. Pathway of Gluconeogenesis. The distinctive reactions and enzymes of this pathway are shown in red. The other reactions are common to glycolysis. The enzymes for gluconeogenesis are located in the cytosol, except for pyruvate carboxylase (in the mitochondria) and glucose 6-phosphatase (membrane bound in the endoplasmic reticulum). The entry points for lactate, glycerol, and amino acids are shown.

Oxidative phosphorylation is the process in which ATP molecules are formed as a result of the transfer of electrons from the reducing equivalents, NADH or FADH2 (produced by glycolysis, the citric acid cycle and fatty acid oxidation) to oxygen by a series of electron carriers in the form of a chain located in the inner membrane of mitochondria. This is the final reaction sequence of respiration. Since the electrons are transferred by a series of electron carriers in the form of a chain, it is known as electron transport system (ETS). 

The main goal of this chapter is to leam how to determine the body s overall style of energy metabolism, via respiratory quotient (RQ) measure-ntents, and to derive the daily energy requirement. A view of the stoichiometries of the glycolytic pathway, Krebs cycle, and pathways of fatty acid synthesis and oxidation will allow RQ calculations for each individual pathway. Glycolysis and the Krebs cycle were presented in Chapter 4, Fatt> add synthesis and oxidation are detailed here. The locations, at points along various metabolic pathways, whereCO2 is produced (in the Krebs cycle) and Oj is consumed (in the respiratory chain), are points of focus in this chapter... 

The crystal structure of the entire 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase, a key bifunctional regulator of both glycolysis and gluconeogenesis, has been solved at 2.0 A resolution. The entire enzyme is a homodimer of 55kDa subunits arranged in a head-to-head fashion, with each monomer consisting of an independent kinase and phosphatase domain. The location of y-5 -ATP and inorganic phosphate in the kinase and phosphatase domains,... 

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