Glycolysis cycle

In addition to glucose, other monosaccharides and nutrients may be converted to intermediates in the glycolysis cycle and enter the cycle as these intermediates. 

Precursors. Precursors for this reaction are compounds exhibiting keto-enol tau-tomerism. These compounds are usually secondary metabolites derived from the glycolysis cycle of yeast metabolism during fermentation. Pyruvic acid is one of the main precursor compounds involved in this type of reaction. During yeast fermentation it is decarboxylated to acetaldehyde and then reduced to ethanol. Acetone, ace-toin (3-hydroxybutan-2-one), oxalacetic acid, acetoacetic acid and diacetyl, among others, are also secondary metabolites likely to participate in this kind of condensation reaction with anthocyanins. 

Electrophoretic Analysis of Metabolic Profiles of Glycolysis Cycle Metabolites in Human Erythrocytes... 

The validated method was subsequently applied for determination of metabolic profiles of metabolites involved in the glycolysis cycle from blood samples derived from 22 healthy volunteers. In order to isolate erythrocytes, the samples were centrifuged, followed by lysis and ultrafiltration. The determined mean concentrations of metabolites in erythrocytes ranged from 49.6 23 pM for F6P to 3.1 0.9 mM for 2,3-DPG. 

Markuszewski, M.J., Szczykowska, M., Siluk, D., Kaliszan, R. Human red blood cells targeted metabolome analysis of glycolysis cycle metabolites by capillary electrophoresis using an indirect photometric detection method. J. Pharm. Biomed. Anal. 39, 636-642 (2005)... 

By the glycolysis cycle, the oxidation of glucose yields two molecules of pyruvate, two molecules of reduced nicotinamide adenine dinucleotide (2 NADH), and two molecules of adenosine triphosphate (2 ATP). 

In the Gray-Scott model PI of this system, both reactions are considered to be irreversible. This reaction scheme is a simplification of the autocatalytic model of the glycolysis cycle (see Chapter 7). A is a feed term and B an inert product. PearsonI °l has shown that as a function of kinetic and diffusion parameters this system leads to the formation of local regions of concentration defined by sharp boundaries. These local regions take on cell-like characteristics, thus undergoing multiplication and division behavior. We discuss some of the results in detail, also because of the discussion in the next chapter on self replication and the origin of protocellular systems. As a function of feed (F) and rate parameter (fc), a state phase diagram can be constructed (see Fig. 8.5). 

PRACTICE EXAMPLE A Glycolysis involves ten biochemical reactions. The first two reactions of the glycolysis cycle are... 

Calculate the equilibrium concentration of F6P(aq) generated in the glycolysis cycle at normal body temperature, 37 °C, starting with [CgHi206(aq)] = 1.20 x 10 M [ATP(aq)] = 10 M and [ADP(aq)] = 10 M. The equilibrium constant for the first reaction is 4.630 X 10 for the second reaction it is 2.76 X 10. During a fever body temperature increases. Will [G6P] increase or decrease with an increase with temperature ... 

FIGURE 18.2 The metabolic map as a set of dots and lines. The heavy dots and lines trace the central energy-releasing pathways known as glycolysis and the citric acid cycle. 

FIGURE 18.16 Compartmentalization of glycolysis, the citric acid cycle, and oxidative phosphorylation. 

Glycolysis and the citric acid cycle (to be discussed in Chapter 20) are coupled via phosphofructokinase, because citrate, an intermediate in the citric acid cycle, is an allosteric inhibitor of phosphofructokinase. When the citric acid cycle reaches saturation, glycolysis (which feeds the citric acid cycle under aerobic conditions) slows down. The citric acid cycle directs electrons into the electron transport chain (for the purpose of ATP synthesis in oxidative phosphorylation) and also provides precursor molecules for biosynthetic pathways. Inhibition of glycolysis by citrate ensures that glucose will not be committed to these activities if the citric acid cycle is already saturated. 

The glycolytic pathway described in this chapter begins with the breakdown of glucose, but other sugars, both simple and complex, can enter the cycle if they can be converted by appropriate enzymes to one of the intermediates of glycolysis. Figure 19.32 shows the mechanisms by which several simple metabolites can enter the glycolytic pathway. Fructose, for example, which is pro-... 

Discuss the cycling of NADH and NAD in glycolysis and the related fermentation reactions. 

Braun, L., Puskas, F., Csala, M., et al., 1997. Ascorbate as a substrate for glycolysis or glnconeogenesis Evidence for an interorgan ascorbate cycle. Free Radical Biology and Medicine 23 804—808. 

FIGURE 20.1 Pyruvate produced hi glycolysis is oxidized in the tricarboxylic acid (TCA) cycle. Electrons liberated in this oxidation flow through the electron transport chain and drive the synthesis of ATP in oxidative phosphorylation. In eukaryotic cells, this overall process occurs in mitochondria. 

In 1937 Krebs found that citrate could be formed in muscle suspensions if oxaloacetate and either pyruvate or acetate were added. He saw that he now had a cycle, not a simple pathway, and that addition of any of the intermediates could generate all of the others. The existence of a cycle, together with the entry of pyruvate into the cycle in the synthesis of citrate, provided a clear explanation for the accelerating properties of succinate, fumarate, and malate. If all these intermediates led to oxaloacetate, which combined with pyruvate from glycolysis, they could stimulate the oxidation of many substances besides themselves. (Kreb s conceptual leap to a cycle was not his first. Together with medical student Kurt Henseleit, he had already elucidated the details of the urea cycle in 1932.) The complete tricarboxylic acid (Krebs) cycle, as it is now understood, is shown in Figure 20.4. 

Pyruvate produced by glycolysis is a significant source of acetyl-CoA for the TCA cycle. Because, in eukaryotic ceils, glycolysis occurs in the cytoplasm, whereas the TCA cycle reactions and ail subsequent steps of aerobic metabolism take place in the mitochondria, pyruvate must first enter the mitochondria to enter the TCA cycle. The oxidative decarboxylation of pyruvate to acetyl-CoA,... 

Glucose metabolized via glycolysis produces two molecules of pyruvate and thus two molecules of acetyl-CoA, which can enter the TCA cycle. Combining glycolysis and the TCA cycle gives the net reaction shown ... 

Pyruvate carboxylase is the most important of the anaplerotie reactions. It exists in the mitochondria of animal cells but not in plants, and it provides a direct link between glycolysis and the TCA cycle. The enzyme is tetrameric and contains covalently bound biotin and an Mg site on each subunit. (It is examined in greater detail in our discussion of gluconeogenesis in Chapter 23.) Pyruvate carboxylase has an absolute allosteric requirement for acetyl-CoA. Thus, when acetyl-CoA levels exceed the oxaloacetate supply, allosteric activation of pyruvate carboxylase by acetyl-CoA raises oxaloacetate levels, so that the excess acetyl-CoA can enter the TCA cycle. 

Situated as it is between glycolysis and the electron transport chain, the TCA cycle must be carefully controlled by the ceil. If the cycle were permitted to run unchecked, large amounts of metabolic energy could be wasted in overproduction of reduced coenzymes and ATP conversely, if it ran too slowly, ATP would not be produced rapidly enough to satisfy the needs of the cell. Also, as just seen, the TCA cycle is an important source of precursors for biosynthetic processes and must be able to provide them as needed. 

---