Question

Itemize the reactions of glycolysis. Include reference to reactants, catalysts, products, control points and ultimate change in free energy.

Answer 1

1. Conversion of glucose into glucose-6-phosphate.

Reactant: glucose

Product: glucose-6-phosphate

The enzyme that catalyze this reaction is hexokinase enzyme, which phosphorylates the glucose. One ATP molecule is consumed. Magnesium is also involved to help shield the negative charges from the phosphate groups on the ATP.

One point of control is at this reaction. The free energy is high and it is an irreversible reaction.

2. Rearrangement of glucose-6-phosphate (reactant) to fructose-6-phosphate (product).

Catalyst: Phosphoglucose isomerase, which helps in isomerization.

3. Conversion of fructose-6-phosphate (reactant) to fructose- 1,6-bisphosphate (product).

The enzyme that catalyzes this reaction is phosphofructokinase (PFK). Magnesium atom is involved as a cofactor to help shield negative charges.

This is the committed step and the most important point of control is at this reaction. The committed step of the pathway is defined as the first highly exergonic step that is unique to glycolysis pathway.

4. The enzyme Aldolase splits fructose 1, 6-bisphosphate (reactant) into two sugars that are isomers of each other. These two sugars are dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP).

Products : DHAP and GAP.

5. The enzyme triosephosphate isomerase inter- converts the molecules dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP).

6. Glyceraldehyde-3-phosphate (reactant) is oxidized by the coenzyme nicotinamide adenine dinucleotide (NAD) and it is phosphorylated by the addition of a free phosphate group.

Product: 1,3-bisphosphoglycerate.

The enzyme that catalyzes this reaction is glyceraldehyde-3-phosphate dehydrogenase (GAPDH).

The phosphate group of this compound has a very high free energy of hydrolysis (ΔG°′= -11.5 kcal/mol), so it is used in the next reaction to drive the synthesis of ATP from ADP.

7. 1,3 bisphoglycerate (reactant)is converted to 3-phosphoglycerate (product) by the enzyme phosphoglycerate kinase (PGK) through dephosphorylation. Magnesium is involved to shield the negative charges on the phosphate groups of the ATP.

Two ATP are synthesized in this step.

8. Rearrangement of the position of the phosphate group on the 3 phosphoglycerate (reactant) molecule, making it 2 phosphoglycerate (product).

The molecule responsible for catalyzing this reaction is called phosphoglycerate mutase (PGM).

9. Conversion of 2 phosphoglycerate (reactant)to phosphoenolpyruvate (PEP). The reaction is catalyzed by the enzyme enolase by dehydrating the 2 phosphoglycerate.   

Product : PEP

10. Convertion of phosphoenolpyruvate (reactant) into pyruvate (product) with the help of the enzyme pyruvate kinase (catalyst). Transfer of phosphate group us involved and two molecules of ATP are generated.

In the hydrolysis of the high-energy phosphate of phosphoenolpyruvate, ΔG°′= -14.6 kcal/mol.

This is an irreversible reaction.

Total, four ATPs are synthesized from each molecule of glucose. Since two ATPs were required to prime the initial reactions, the net gain is two ATP molecules.