Glycolysis pathways

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... 

Further steps in glycolysis use the D-glyceraldehyde 3-phosphate fonned in 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 glycolysis pathway for further transfonnations. 

One of the most striking features of the common fatty adds is that they have an even number of carbon atoms (Table 27.1, p. 1062). This even number results because all fatty acids are derived biosynthelically from acetyl CoA by sequential addition of two-carbon units to a growing chain. The acetyl CoA, in turn, arises primarily from the metabolic breakdown of carbohydrates in the glycolysis pathway that weTl see in Section 29.5. Thus, dietary carbohydrates consumed in excess of immediate energy needs are turned into fats for storage. 

Active Figure 29.7 MECHANISM The 10-step glycolysis pathway for catabolizing glucose to two molecules of pyruvate. Individual steps are described in the text. Sign in at www.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

Problem 29.8 Look at the entire glycolysis pathway and make a list of the kinds of organic reactions that take place—nucleophilic acyl substitutions, aldol reactions, ElcB reactions, and so forth. 

The glycolysis pathway shown in Figure 29.7 has a number of intermediates that contain phosphate groups. Why can 3-phosphoglyceryl phosphate and phosphoenolpyruvate transfer a phosphate group to ADP while glucose 6-phosphalc cannot ... 

Figure 3.15 Histogram of number of enzymes of the glycolysis pathway (from multiple organs and species) with different values of [S J/A m under physiological conditions.

Outline the reactions by which glyceraldehyde 3-phosphate is converted to 3-phosphoglycerate with coupled synthesis of ATP in the glycolysis pathway. Show important mechanistic details. 

Phosphoenolpyruvate, a key metabolic intermediate. A compound of central importance in metabolism is the phosphate ester of the enol form of pyruvate, commonly known simply as phosphoenolpyruvate (PEP).249 It is formed in the glycolysis pathway by dehydration of 2-phosphoglycerate (Eq. 13-15) or by decarboxylation of oxaloacetate. Serving as a preformed enol from which a reactive enolate anion can be released for condensation reactions,250 251 PEP... 

Figure 17-7 Outline of the glycolysis pathway by which hexoses are broken down to pyruvate. The ten enzymes needed to convert D-glucose to pyruvate are numbered. The pathway from glycogen using glycogen phosphorylase is also included, as is the reduction of pyruvate to lactate (step 11). Steps 6a-7, which are involved in ATP synthesis via thioester and acyl phosphate intermediates, are emphasized. See also Figures 10-2 and 10-3, which contain some additional information.

The polyol pathway is an active bypass of the dominant glycolysis pathway in many organisms.6 Sorbitol and other polyols such as glycerol, erythritol,... 

One gram of edianol should yield 1.304 g acetic acid. Practical yields arc 77-85%. In contrast with the well-known Embdcn-Mcycrhof-Parnass glycolysis pathway for the conversion of tiexose sugars to alcohol, lire steps in conversion of ethanol to acetic acid remain in some doubt. 

In vivo, the D-fructose 1,6-bisphosphate aldolase (FruA EC 4.1.2.13) catalyzes the pivotal reaction of the glycolysis pathway the equilibrium addition of 41 to D-glyceraldehyde 3-phosphate (GA3P, 12) to give D-fructose 1,6-bisphosphate (42) [43]. The equilibrium constant of 104 M 1 strongly favors synthesis [229]. 

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