Glycolysis (part 3)
ATP Formation Coupled to Glycolysis
During glycolysis some of the energy of the glucose molecule is conserved in ATP, while much remains in the product, pyruvate.
For each molecule of glucose degraded to pyruvate, two molecules of ATP are generated from ADP and Pi.
We can now resolve the equation of glycolysis into two processes:
1) The conversion of glucose to pyruvate, which is exergonic
2) The formation of ATP from ADP and Pi, which is endergonicThe sum of Equations 1 and 2 gives the overall standard free-energy change of glycolysis.Under standard conditions and in the cell, glycolysis is an essentially irreversible process, driven to completion by a large net decrease in free energy.
Energy Remaining in Pyruvate
- Glycolysis releases only a small fraction of the total available energy of the glucose molecule.
- The two molecules of pyruvate formed by glycolysis still contain most of the chemical potential energy of glucose, energy that can be extracted by oxidative reactions in the citric acid cycle and oxidative phosphorylation.
Importance of Phosphorylated Intermediates in glycolysis
Each of the nine glycolytic intermediates between glucose and pyruvate is phosphorylated. The phosphoryl groups appear to have three functions.
1) The phosphorylated glycolytic intermediates cannot leave the cell because the plasma membrane generally lacks transporters for phosphorylated sugars. So after the initial phosphorylation, no further energy is necessary to retain phosphorylated intermediates in the cell.
2) Phosphoryl groups are essential components in the enzymatic conservation of metabolic energy.
Energy released in the breakage of phosphor anhydride bonds (such as those in ATP) is partially conserved in the formation of phosphate esters such as glucose 6-phosphate. High-energy phosphate compounds formed in glycolysis (1,3-bisphosphoglycerate and phosphoenolpyruvate) donate phosphoryl groups to ADP to form ATP.
3) Binding energy resulting from the binding of phosphate groups to the active sites of enzymes lowers the activation energy and increases the specificity of the enzymatic reactions. The phosphate groups of ADP, ATP, and the glycolytic intermediates form complexes with Mg2+, and the substrate binding sites of many glycolytic enzymes are specific for these Mg2+ complexes. Most glycolytic enzymes require Mg2+ for activity.
Outcomes of Glycolysis
- Glycolysis starts with one molecule of glucose and ends with two pyruvate (pyruvic acid) molecules, a total of four ATP molecules, and two molecules of NADH.
- Two ATP molecules were used in the first half of the pathway to prepare the six-carbon ring for cleavage.
- Finally the cell has a net gain of two ATP molecules and 2 NADH molecules for its use.
- If the cell cannot catabolize the pyruvate molecules further (via the citric acid cycle or Krebs cycle), it will harvest only two ATP molecules from one molecule of glucose.