Glycolysis reversible reactions

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.

Therefore, since step-growth polymers are often prepared by reversible reactions, it is feasible to convert them back to their monomers or ohgomers/chemicals by solvolytic processes such as hydrolysis, glycolysis,... 

Transesterification is the main reaction of PET polycondensation in both the melt phase and the solid state. It is the dominant reaction in the second and subsequent stages of PET production, but also occurs to a significant extent during esterification. As mentioned above, polycondensation is an equilibrium reaction and the reverse reaction is glycolysis. The temperature-dependent equilibrium constant of transesterification has already been discussed in Section 2.1. The polycondensation process in the melt phase involves a gas phase and a homogeneous liquid phase, while the SSP process involves a gas phase and two solid phases. The respective phase equilibria, which have to be considered for process modelling, will be discussed below in Section 3.1. 

The three control steps in glycolysis are reactions catalysed by HK, PFK-1 and PK. All three reactions involve ADP/ATP cycling and are strongly exergonic suggesting they operate far from the true equilibrium position. Such reactions are physiologically difficult to reverse and so act as metabolic one-way streets . 

Many, if not most, step polymerizations involve equilibrium reactions, and it becomes important to analyze how the equilibrium affects the extent of conversion and, more importantly, the polymer molecular weight. A polymerization in which the monomer(s) and polymer are in equilibrium is referred to as an equilibrium polymerization or reversible polymerization. A first consideration is whether an equilibrium polymerization will yield high-molecular-weight polymer if carried out in a closed system. By a closed system is meant one where none of the products of the forward reaction are removed. Nothing is done to push or drive the equilibrium point for the reaction system toward the polymer side. Under these conditions the concentrations of products (polymer and usually a small molecule such as water) build up until the rate of the reverse reaction becomes equal to the polymerization rate. The reverse reaction is referred to generally as a depolymerization reaction other terms such as hydrolysis or glycolysis may be used as applicable in specific systems. The polymer molecular weight is determined by the extent to which the forward reaction has proceeded when equilibrium is established. 

Figure 6-8. Conversion of phosphoenolpyruvate to glucose during gluconeogenesis. Except for the indicated enzymes that are needed to overcome irreversible steps of glycolysis, all other steps occur by the reverse reactions catalyzed by the same enzymes as those used in glycolysis.

Gluconeogenesis is a ubiquitous multistep process in which pyruvate or a related three-carbon compound (lactate, alanine) is converted to glucose. Seven of the steps in gluconeogenesis are catalyzed by the same enzymes used in glycolysis these are the reversible reactions. 

Scheme 4.—The Reversible Reaction Catalyzed by the Glycolysis Aldolase.

As a result of the reversal of Step 4 in glycolysis, the equivalent of two molecules of pyruvate is condensed to give one molecule of fructose 1,6-bisphosphate. This compound is the product of the irreversible Step 3 in glycolysis. Gluconeogenic cells have the enzyme fructose-1,6-bisphosphatase, which catalyzes the reverse reaction (Step 3, p. 313). 

Fructose-6-phosphate formed from the isomerization discussed above is further phos-phorylated during glycolysis to fructose-1,6-diphosphate (108), which is then cleaved by fructose-1,6-bisphosphate aldolase to afford dihydroxy acetone phosphate (109) and glyceraldehyde-3-phosphate (110). This cleavage reaction is the reverse of an aldol condensation discussed in Section II.C and during gluconeogenesis. In the latter case, fructose-1,6-bisphosphate aldolase catalyzes the reverse reaction herein via aldol condensation of the ketose 109 and the aldose 110 to form linear fructose-1,6-bisphosphate (108) . 

On formation, phosphoenolpyruvate is metabolized by the enzymes of glycolysis but in the reverse direchon. These reactions are near equilibrium under intracellular condihons so, when condihons favor gluconeogenesis, the reverse reactions will take place until the next irreversible step is reached. This step is the hydrolysis of fructose 1,6-bisphosphate to fructose 6-phosphate and Pj. 

It is difficult to suppress the reverse (glycolysis) reaction that occurs during the preparation of polyesters, though Griehl and Schnock [63] virtually eliminated it. Their results and later data of Challa [69], who took the reverse reaction into account, agree quite well. Griehl and Schnock proposed first-order kinetics however, if account is taken of the gradual increase in over-all rate coefficient, their data fit second-order kinetics better [69]. 

G6P isomerase (GPI EC 5.3.1.9) (also known as phospho-giucose isomerase [PGI]), catalyzes the interconversion of G6P and fructose 6-phosphate (F6P), the second step of the EMP. As a result of this reversible reaction, products of the hexose monophosphate pathway can be recycled to G6P. Besides being a housekeeping enzyme of glycolysis, GPI exerts outside the cell cytokine properties and is involved in several extracellular processes. In addition, autoantibodies against GPI seem to be involved in rheumatoid arthritis. GPI is a crucial enzyme, since GPI knockout mice die in the embryological state. ... 

This reversible reaction is the final step of glycolysis and is catalyzed by (LDH) ... 

In glycolysis, glucose is converted into pyruvate in gluconeogenesis, pyruvate is converted into glucose. However, gluconeogenesis is not a reversal of glycolysis. Several reactions must differ because the equilibrium of glycolysis lies far on the side of pyruvate formation. The actual AG for the formation... 

In glycolysis each glucose molecule is converted into two pyruvate molecules. In addition, two molecules each of ATP and NADH are produced. Reactions with double arrows are reversible reactions and those with single allows are irreversible reactions that serve as control points in the pathway. 

Conversion of glucose-6-phosphate to fructose-6-phosphate. During reaction 2 of glycolysis, the aldose glucose-6-phosphate is converted to the ketose fmctose-6-phosphate by phosphoglucose isomerase (PGI) in a readily reversible reaction ... 

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