Glycolysis

Glycolysis is a process that results in the conversion of a molecule of glucose into two molecules of pyruvate. It is a primitive metabolic pathway since it operates in even the simplest cells and does not require oxygen. The pathway of glycolysis performs five main functions in the cell  

Glucose is converted to pyruvate, which can be oxidized in the citric acid cycle (Chap. 

Many compounds other than glucose can enter the pathway at intermediate stages. 

In some cells the pathway is modified to enable glucose to be synthesized. 

The pathway contains intermediates that are involved in alternative metabolic reactions. 

Glycolysis, also called as Embden-Meyerhof-Pamas pathway (EMP pathway). Glycolysis is of central importance to the metabolism of eukaryotic cells. It links the metabolism of sugars to that of organic acids in the Krebs cycle, and in anaerobic organisms, provides the principle route of energy (ATP) generation. The reactions are rather complex, but can be seen as four basic processes (Fig. 9.2)  

Isomerisation (catalysed by an isomerase) the intramolecular rearrangement of a molecule. This may be the transfer of a carboxyl group (C=0) from the end of a molecule (such as an aldose) to the middle (such as a ketose), or it may be the transfer of a phosphate group (the enzymes catalysing this latter sort of isomerisation are generally termed mutases). 

Phosphorylation (catalysed by kinases) the movement of a phosphate group from one molecule (such as a phosphorylated sugar) to cmother (such as ATP). 

Dehydration (catalysed by a dehydratase or hydrolase acting in reverse) the removal of water from a molecule. 

Aldol cleavage (catalysed by an aldolase) the splitting of the carbon-carbon bond in a -(C=0)-CH(0H)-CH(0H)- molecule to generate a free aldehyde. 

Glycolysis is the simplest and oldest method of PET depolymerization. The first patents on PET glycolysis were filed more than 30 years ago.3-9 The method involves the reaction of PET, under pressure and at temperatures in the range 180-240 °C, with an excess of glycol, usually ethylene glycol, which promotes the formation of BHET.10-13 This monomer has to be purified, normally by melt filtration under pressure, prior to its use in the production of new PET polymer. Colours present in the starting PET wastes are not usually removed by glycolysis. The depolymerization is carried out in the presence of a transesterification catalyst, usually zinc or lithium acetate. 

The following phenomena have been proposed to describe the mechanism of PET glycolysis 14,15 glycol diffusion into the polymer, polymer swelling which increases the diffusion rate, and reaction of the glycol with an ester bond in the chain. Because the reaction rate is proportional to the polymer surface area, it is advisable to first reduce the size of the raw PET waste to small particles by grinding, cutting, etc. 

Malik and Most16 described a process for the depolymerization of PET with ethylene glycol catalysed by sodium acetate, with several improvements over previous methods. PET is first mixed with a certain amount of BHET to promote its dissolution and the formation of diethylene glycol during the reaction is inhibited by continuously introducing water into the reaction mixture. The production of glycol ethers, mainly diethylene glycol, is undesir- 

The formation of diethylene glycol can be limited by using a combination of lithium and zinc acetates and antimony trioxide as catalyst.17 In addition to diethylene glycol, other secondary products have been identified during PET glycolysis, such as ethylene terephthalate, dioxane, aldehydes and cyclic 

The effect of the type of glycol used in the chemical depolymerization of PET has been studied at 200 °C, comparing the results obtained with ethylene glycol, 

A Glycolysis is the process by which glucose is broken down to pyruvate in order to begin obtaining some of the enei stored in the glucose molecule for use by the body. 

The energy released in this process results in the direct formation of ATP. 

The further metabolism of pyruvate also yields ATP synthesis through oxidative phosphorylation (see Chapter 7). 

Disruption of glycolysis causes disease and death due to the reliance of some tissues (RBCs and neurons, for example) on glucose metabolism for their energy needs. 

Energy (ATP) is expended in the phosphorylation of intermediates in these reactions. 

Solvolysis (chemolysis) is defined as a techniqne which obtains the raw materials, snch as TPA, DMT, EG monomers and oligomers as the reverse process of PET prodnction by saponification or transesterification with water, methanol or EG. For the process of PET production raw materials of high purity are obligatory. Polycondensation is not a chain reaction, but a step reaction. Impurities can stop the polycondensation at low molar masses, so high-purity standards are also reqnired for the solvolysis prodncts. The choice of process depends on the starting material and the demand for solvolysis prodncts. 

dnring the production and the glycolysis process diethylene glycol is formed, diethylene glycol is accumulated in the BHET. This diethylene glycol has a negative influence on the properties of PET. Hence, the contribntion of glycolysis BHET is limited to 20-30% in the prodnction of new PET. 

Transesterification performed at 170-200°C in the presence of aliphatic carboxylic acids, such as adipic acid, after the depolymerization of PET in EG and propylene glycol leads to unsaturated polyesters. These materials are used in foam production or for the production of polyurethanes and polyester polyol copolymers [7-9]. 

NADH — nicotinamide adenine dinucleotide, reduced form 

Still other substances and minutiae may enter, but are not shown. Generally speaking, the various enzymes encountered are named according to their functions, using the suffix -ase, and these organic catalysts may be regarded as proteinaceous. 

3-phosphate dehydrogenase, (7) phosphoglycerate kinase, (8) phosphoglyceromu-tase, (9) enolase, and (10) pyruvate kinase. 

Fructose 6 phosphate phosphofructokinase i ATP— ADP Fructose-1, 6-biphosphate aldolase i 

FIGURE 3.1 Glycolysis conversion seqnence yielding pyruvic acid or pyruvate, with enzymes and supportive reactions shown. (Based on information in Voet and Voet, Biochemistry, 1995, p. 446 Harris, in Textbook of Biochemistry, 1986, p. 334.) 

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In all these reactions, the conversion of glucose to product is an oxidation reaction, requiring an accompanying reduction reaction in which NAD is converted to NADH, a point to which we shall return when we discuss the pathway in detail. The breakdown of glucose to pyruvate can be summarized as follows  

For athletes, efficient use of carbohydrates can provide the margin of victory. 

2 Conversion of Six-Carbon Glucose to Three-Carbon Glyceraldehyde-3-Phosphate 

Glucose (Six carbon atoms) 2 Pyruvate (Three carbon atoms) 

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This is not the place to expose in detail the problems and the solutions already obtained in studying biochemical reaction networks. However, because of the importance of this problem and the great recent interest in understanding metabolic networks, we hope to throw a little light on this area. Figure 10.3-23 shows a model for the metabolic pathways involved in the central carbon metabolism of Escherichia coli through glycolysis and the pentose phosphate pathway [22]. 

Figure 10.3-23. Metabolic model of glycolysis and tbe pentose phosphate pathway in E. coli. Squares Indicate enzyme activities circles indicate regulatory effects,...

This cleavage is a retro aldol reaction It is the reverse of the process by which d fruc tose 1 6 diphosphate would be formed by aldol addition of the enolate of dihydroxy acetone phosphate to d glyceraldehyde 3 phosphate The enzyme aldolase catalyzes both the aldol addition of the two components and m glycolysis the retro aldol cleavage of D fructose 1 6 diphosphate... 

Further steps m glycolysis use the d glyceraldehyde 3 phosphate formed m the aldolase catalyzed cleavage reaction as a substrate Its coproduct dihydroxyacetone phosphate is not wasted however The enzyme triose phosphate isomerase converts dihydroxyacetone phosphate to d glyceraldehyde 3 phosphate which enters the glycol ysis pathway for further transformations... 

In spite of the number of different structural types lipids share a common biosyn thetic origin m that they are ultimately derived from glucose During one stage of car bohydrate metabolism called glycolysis glucose is converted to lactic acid Pyruvic acid IS an intermediate... 

In most biochemical reactions the pH of the medium is close to 7 At this pH car boxylic acids are nearly completely converted to their conjugate bases Thus it is common practice m biological chemistry to specify the derived carboxylate anion rather than the carboxylic acid itself For example we say that glycolysis leads to lactate by way of pyruvate... 

Glycolysis is claimed to be somewhat less cosdy than methan olysis (33). Depolymerization is not taken completely to monomers (34). Rather, recovered PET is depolymerized to low molecular weight oligomers. Contaminants are removed using proprietary technology. The oligomers are then fed to a melt polymerization vessel in which PET is produced. 

Process costs for the methanolysis and glycolysis of PET to produce monomers are similar. Cost estimates are summarized in Table 8. 

Potassium [7440-09-7] K, is the third, element ia the aLkaU metal series. The name designation for the element is derived from potash, a potassium mineral the symbol from the German name kalium, which comes from the Arabic qili, a plant. The ashes of these plants al qili) were the historical source of potash for preparing fertilisers (qv) or gun powder. Potassium ions, essential to plants and animals, play a key role in carbohydrate metaboHsm in plants. In animals, potassium ions promote glycolysis, Hpolysis, tissue respiration, and the synthesis of proteins (qv) and acetylcholine. Potassium ions are also beheved to function in regulating blood pressure. 

In contrast with the well-known Embden-Meyerhof-Pamass glycolysis pathway for the conversion of hexose sugars to alcohol, the steps in conversion of ethanol to acetic acid remain in some doubt. Likely, ethanol is first oxidized to acetaldehyde and water (39). For further oxidation, two alternative routes are proposed more likely, hydration of the acetaldehyde gives CH2CH(OH)2, which is oxidized to acetic acid. An alternative is the Cannizzaro-type disproportionation of two molecules of acetaldehyde to one molecule of ethanol and one molecule of acetic acid. Jicetobacter... 

Glycolysis Metabolic pathway involving the conversion of glucose to lactic acid or ethanol. 

Glycolysis (Section 25.21) Biochemical process in which glucose is converted to pyruvate with release of energy. 

We can predict whether pairs of coupled reactions will proceed spontaneously by simply summing the free energy changes for each reaction. For example, consider the reaction from glycolysis (discussed in Chapter 19)... 

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