Reactions/Energies of Glycolysis

Reactions/Energies of Glycolysis

There are ten steps to glycolysis. Most instructors require students to memorize the molecular structures and enzyme names for each reaction of glycolysis, because it is such a universally important pathway. You should clarify this with your instructor. The Quizzing section is a convenient way to learn these things.

Highlights of the reactions:

Reaction #1

-D-Glucose + ATP <=> -D-Glucose-6-Phosphate + ADP + H+ | Enzyme: Hexokinase
Notes - ATP energy is used. Hexokinase is capable of phosphorylating other 6-carbon sugars similarly, such as galactose, fructose, and mannose. is negative, so it favors making glucose-6-phosphate (G6P), but the product of the reaction (G6P) can reach high enough concentration to inihibit hexokinase and limit glycolysis. | = -16.7 kJ/mol
Reaction Picture

Reaction #2

-D-glucose-6-phosphate <=> D-fructose-6-phosphate | Enzyme: Phosphoglucoisomerase
This is an aldose-ketose isomerization that proceeds through an enediol intermediate. G6P is the aldose and fructose-6-phosphate (F6) is the ketose. Phosphoglucoisomerase, which catalyzes this isomerization, must not be confused with phosphoglucomutase, the enzyme that interconverts G6P and glucose-1-phosphate (G1P). The for the isomerization of G6P to F6P is only slightly positive, so it strongly favors neither reactants nor products in this reaction. | = +1.7 kJ/mol
Reaction Picture

Reaction #3

D-fructose-6-phosphate + ATP <=> D-fructose-1,6-bisphosphate | Enzyme: Phosphofructokinase
ATP energy is used to phosphorylate F6P to fructose-1,6-bisphosphate (F1,6BP). This reaction is the key to understanding how regulation of glycolysis is regulated. The enzyme, phosphofructokinase (PFK), is allosterically regulated by AMP (on), ADP (on), ATP (off), citrate (off), and fructose-2,6-bisphosphate (F2,6BP) (on). The most potent of these is F2,6BP. The of -14.2 kJ/mol favors formation of F1,6BP fairly strongly. Consequently, the reaction is essentially irreversible in vivo. At this point all of the energy inputs for glycolysis are complete. | = -14.2 kJ/mol
Reaction Picture

Reaction #4

D-fructose-1,6-bisphosphate <=> Dihydroxyacetone phosphate + D-Glyceraldehyde-3-Phosphate | Enzyme: Fructose-1,6-Bisphosphate Aldolase
In this reaction, F1,6BP is cleaved to yield two three-carbon intermediates, glyceraldehyde-3-phosphate (G3P and dihydroxyacetone phosphate (DHAP). The large positive (+23.9 kJ/mol) strongly favors the reverse reaction under conditions where reactants and products are present in relatively equal quantitities. In muscle, however, the concentrations of G3P and DHAP are kept low enough that the forward reaction is favored overall . This is a good example of how a reaction that is unfavorable at standard state conditions can be made favorable in the cell by removing products as they are formed. | : = +23.9 kJ/mol
Reaction Picture Reaction Mechanism

Reaction #5

Dihydroxyacetone phosphate <=> D-Glyceraldehyde-3-Phosphate | Enzyme: Triose Phosphate Isomerase
Notes - The isomerization of DHAP to G3P, like the isomerization of G6P to F6P (reaction 2 above), proceeds through an enediol intermediate. Additionally, the isomerization of DHAP also has a positive , but the reaction is pulled to the right by keeping the cellular concentration of G3P very low. This reaction marks the end of what is referred to as the energy investment phase, although the last ATP energy was used in reaction 3.

For all reactions that follow in this section, keep in mind that the six-carbon glucose has been split into two three-carbon units. Thus, to account for everything properly, remember that there are two of each three carbon compound in the reactions shown.

= +7.6 kJ/mol

Reaction Picture

Reaction #6

D-Glyceraldehyde-3-Phosphate + NAD+ + Pi <=> 1,3 bisphosphoglycerate + NADH + H+ | Enzyme: Glyceradehyde-3-Phosphate Dehydrogenase
Notes - In this reaction, G3P is phosphorylated and oxidized, so something (NAD+) must be concomitantly reduced. As a result, the NAD+/NADH balance in the cell is important. If the concentration of NAD+ is low, the reverse reaction is favored, preventing glycolysis from occurring aerobically. Instead it must occur anaerobically. Thus this reaction determines whether glycolysis occurs aerobically or anaerobically. 1,3-bisphosphoglycerate (1,3BPG), the reaction product, contains an acylphosphate group, which has a standard free energy of hydrolysis of 49.4kJ/mol. Thus, 1,3BPG is capable of synthesizing ATP via a substrate-level phosphorylation. The slightly positive shows that the reverse reaction is slightly favored at standard conditions. In the cell, however, the forward reaction is favored, thanks partly to the high NAD+/NADH ratio normally present. Note also that the NADH produced in this reaction can be used to make three molecules of ATP in aerobic glycolysis (when oxidative phosphorylation is occurring). Finally, glyceraldehyde-3-phosphate dehydrogenase uses a thiol group in catalysis, which can be inhibited by iodoacetate and heavy metals, such as mercury. | = +6.3 kJ/mol
Reaction Picture Show Mechanism

Reaction #7

1,3 bisphosphoglycerate + ADP <=> 3-phosphoglycerate + ATP | Enzyme: Phosphoglycerate Kinase
Notes - This reaction is a substrate-level phosphorylation of ADP to produce 3-phosphoglycerate (3PG) and the first ATP of glycolysis. Because two molecules of ATP are produced per molecule of glucose, the net yield of ATP is zero at this stage of glycolysis. | = -18.8 kJ/mol
Reaction Picture

Reaction #8

3-phosphoglycerate <=> 2-phosphoglycerate | Enzyme: Phosphoglycerate Mutase
Notes - The for the isomerization slightly favors formation of 3PG over 2PG under standard conditions, but in the cell the concentration of 3PG is kept high relative to the concentration of 2PG, which drives the reaction to the right. | = +4.4 kJ/mol
Reaction Picture

Reaction #9

2-phosphoglycerate <=> Phosphoenolpyruvate + H2O | Enzyme: Enolase
Notes - This reaction is a simple dehydration (or elimination) of 2PG to form phosphoenolpyruvate (PEP), but it has the effect of increasing the energy of hydrolysis of the phosphate bond almost four fold (from -15.6 kJ/mol in 2PG to -61.9 kJ/mol in PEP). This high free energy of hydrolysis is necessary for the next step in glycolysis, which is another substrate level phosphorylation of ADP to form ATP. | = +1.7 kJ/mol
Reaction Picture

Reaction #10

Phosphoenolpyruvate + ADP + H+ <=> Pyruvate + ATP | Enzyme: Pyruvate Kinase
Notes - This reaction is important for several reasons. First, it generates ATP from the substrate-level phosporylation of ADP, putting the balance for glycolysis at a net gain of two molecules of ATP per molecule of glucose. Second, it is very favorable energetically, serving to "pull" the two preceding reactions (both of which have slightly positive values) forward. Third, the enzyme catalyzing the reaction, pyruvate kinase, is allosterically inactivated by ATP, alanine, and acetyl-CoA, allosterically activated by F1,6BP, and is inactivated by covalent modification (phosphorylation) from the kinase cascade. | = -31.4 kJ/mol
Reaction Picture

See also: Gluconeogenesis and Glycolysis Interregulation Link Page, Alanine, Acetyl-CoA, Fructose-2,6-Bisphosphate, AMP, Oxidative Phosphorylation (from Chapter 15)

INTERNET LINKS:

> 1. Glycolysis Enzymes in the Yeast Genome

> 2. Glycolysis/Gluconeogenesis