Glycolytic pathway, scheme

Scheme 2.9 Glycolytic pathway leading to the L-malic, fumaric, and succinic acids.

Scheme 9.4 Biochemical retrosynthesis of 2 -deoxyribo-nucleosides from glucose, acetaldehyde and a nucleobase (adenine) through the glycolytic pathway and the reverse reactions of 2 -deoxyribonucleoside degradation.

Scheme 9.5 Multi-step enzymatic process for 2 -deoxyribo-nucleoside production from glucose, acetaldehyde and a nucleobase through glycolysis, reverse reactions of 2 -deoxy-ribonucleoside degradation and ATP regeneration by the yeast glycolytic pathway recycling the phosphate generated by nucleoside phosphorylase.

This shows the production of NADPH in the pathway and its return of all but one of the carbon atoms of glucose 6-phosphate to the glycolytic pathway. However, the overall scheme belies its adaptability to produce and to deal with a variety of sugar phosphates. 

Occurrence. D-Glyceraldehyde (1) and dihydroxyacetone phosphate (3) are biological products of the retro-aldol reaction of fructose and enter the glycolytic pathway as glyceraldehyde 3-phosphate (Scheme l).11... 

The best-known glycolytic pathway is that studied especially in muscle tissue and yeast cells. This pathway, sometimes known as the Embden-Meyerhof-Parnas (EMP) scheme, is shown in Fig. 8. The reactions take place under anaerobic conditions. In yeast the end-products are ethanol and carbon dioxide, whereas in muscle tissue the end-product is L-lactic acid. The EMP scheme is operative in a great many tissues and organisms and apparently represents the major pathway of carbohydrate breakdown. 

A recent study (Bloom, B., Stetten, M. R., and Stetten, D., J. Biol. Chem. 204, 681 (1953)) of the catabolism of isotopic glucose in mammalian systems has confirmed this utilization of the Embden-Meyerhof scheme in muscle. Even more interesting, however, is the result that at least 75% of glucose utilization in liver proceeds by a non-glycolytic pathway wherein the C, of glucose is preferentially converted to CO2. 

Figures 4 and 5 illustrates the use of NMR spectroscopy to study the metabolism of (l-i C)glu-cose in primary cultures of neurons and astrocytes. A simplified scheme of the metabolism of (l-i C)glu-cose in neural cells is given in Figure 4. Briefly, (1-i C)glucose is metabolized to (3- C)pyruvate through the Embden Meyerhoff glycolytic pathway. The (3-i C)pyruvate produced can be transaminated to (3- C)alanine, reduced to (3-i C)lactate or enter the tricarboxylic acid (TCA) cycle through the pyruvate dehydrogenase (PDH) or pyruvate carboxylase (PC) activities. A net increase in (3-i C)lactate reveals increased aerobic glycolysis and is normally observed under hypoxic conditions in normal cells. If (3-i C)pyruvate enters the TCA cycle though PDH it produces (2-i C)acetyl-coenzyme A first, and subsequently (4-i C)a-ketoglutarate. In contrast, if (3-i C)pyruvate is carboxylated to (3-i C)oxalacetate by...

Fig. 4.7. A scheme for the organic pathways in the cytoplasm of all cells illustrating their interconnected nature (after Kauffman see Further Reading), (a) Glycolytic (b) Krebs cycle pathways.

The association of the +AId with active metabolism was further confirmed by experiments in which the glucose substrate was replaced by fructose (as fructose-6-phosphate) and gluconolactone (as 6-phosphogluconolactone) which are intermediates in the glycolytic and pentose phosphate pathways respectively. Pyruvate was also incorporated into this series to determine what effect this intermediate, which precedes the formation of the terminal product of metabolism, namely lactate, would have on the Aid value. It was expected that pyruvate, because of its position in the metabolic scheme, would show either no effect or manifest an adverse effect on the metabolic behavior of the RBC. The adverse... 

These reactions have been designated the Embden-Meyerhof scheme, in recognition of the pioneer discoveries that led to the acceptance of this pathway, but workers in many other laboratories have contributed heavily to the study of the glycolytic enzymes. The reactions indicated as unidirectional are in fact reversible, but under ordinary circumstances the reverse reactions do not occur to significant extents. In order to reverse the glycolytic system, additional reactions by-pass the irreversi-O. H. Lowry, N. R. Roberts, and C. Lewis, J. Biol. Chem. 220, 879 (1956). 

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