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18.3A: Glycolysis

Skills to Develop

  1. Briefly describethe function of glycolysis during aerobic respiration and indicate the reactants and products.
  2. State whether or not glycolysis requires oxygen.
  3. Compare where glycolysis occurs in prokaryotic cells and in eukaryotic cells.
  4. State whether steps 1 and 3 of glycolysis are exergonic or endergonic and indicate why.
  5. State why one molecule of glucose is able to produce two molecules of pyruvate during glycolysis.
  6. Define substrate-level phosphorylation.
  7. State the total number and the net number of ATP produced by substrate-level phosphorylation during glycolysis.
  8. During aerobic respiration, state what happens to the 2 NADH produced during glycolysis.
  9. During aerobic respiration, state what happens to the two molecules of pyruvate produced during glycolysis.

Glycolysis is a partial breakdown of a six-carbon glucose molecule into two, three-carbon molecules of pyruvate, 2NADH +2H+, and 2 net ATP as a result of substrate-level phosphorylation, as shown in (Figures 1 and 2).

   

Figure 1 and 2: A Summary of Glycolysis

Steps of Glycolysis

  1. A phosphate from the hydrolysis of a molecule of ATP is added to glucose, a 6-carbon sugar, to form glucose 6-phosphate.
  2. The glucose 6-phosphate molecule is rearranged into an isomer called fructose 6-phosphate.
  3. A second phosphate provided by the hydrolysis of a second molecule of ATP is added to the fructose 6-phosphate to form fructose 1,
  4. The 6-carbon fructose 1,6-biphosphate is split into two molecules of glyceraldehyde 3-phosphate, a 3-carbon molecule. 
  5. Oxidation and phosphorylation of each glyceraldehyde 3-phosphate produces 1,3-biphosphoglycerate with a high-energy phosphate bond (wavy red line) and NADH. 
  6. Through substrate-level phosphorylation, the high-energy phosphate is removed from each 1,3-biphosphoglycerate and transferred to ADP forming ATP and 3-phosphoglycerate.
  7. Each 3-phosphoglycerate is oxidized to form a molecule of phosphoenolpyruvate with a high-energy phosphate bond.
  8. Through substrate-level phosphorylation, the high-energy phosphate is removed from each phosphoenolpyruvate and transferred to ADP forming ATP and pyruvate.

In summary, one molecule of glucose produces two net ATPs (two ATPs were used at the beginning; four ATPs were produced through substrate-level phosphorylation), two molecules of NADH + 2H+, and two molecules of pyruvate.

Glycolysis occurs in the cytoplasm of the cell. The overall reaction is:

\[glucose (6C) + 2 NAD+ 2 ADP + 2 inorganic phosphates (P_i)\]

\[ \rightarrow 2 pyruvate (3C) + 2 NADH + 2 H^+ + 2 ATP\]

Glycolysis also produces a number of key precursor metabolites, as shown in Figure 3. Glycolysis does not require oxygen and can occur under aerobic and anaerobic conditions. However, during aerobic respiration, the two reduced NADH molecules transfer protons and electrons to the electron transport chain to generate additional ATPs by way of oxidative phosphorylation.

Figure 3: Integration of Metabolism - Precursor Metabolites.Carbohydrates, proteins, and lipids can be used as energy sources; metabolites involved in energy production can be used to synthesize carbohydrates, proteins, lipids, nucleic acids, and cellular structures.

The glycolysis pathway involves 9 distinct steps, each catalyzed by a unique enzyme. You are not responsible for knowing the chemical structures or enzymes involved in the steps below. They are included to help illustrate how the molecules in the pathway are manipulated by the enzymes in order to to achieve the required products.

Step 1

To initiate glycolysis in eukaryotic cells (Figure 4), a molecule of ATP is hydrolyzed to transfer a phosphate group to the number 6 carbon of glucose to produce glucose 6-phosphate. In prokaryotes, the conversion of phosphoenolpyruvate (PEP) to pyruvate provides the energy to transport glucose across the cytoplasmic membrane and, in the process, adds a phosphate group to glucose producing glucose 6-phosphate.

Figure 4: Glycolysis, Step 1. To initiate glycolysis in eukaryotic cells, shown in this figure, a molecule of ATP is hydrolyzed to transfer a phosphate group to the number 6 carbon of glucose to produce glucose 6-phosphateIn prokaryotes, the conversion of phosphoenolpyruvate (PEP) to pyruvate provides the energy to transport glucose across the cytoplasmic membrane and, in the process, adds a phosphate group to glucose producing glucose 6-phosphate.

Step 2

The glucose 6-phosphate is rearranged to an isomeric form called fructose 6-phosphate (Figure 5).

Figure 5: Glycolysis, Step 2. The glucose 6-phosphate is rearranged to an isomeric form called fructose 6-phosphate.

Step 3

A second molecule of ATP is hydrolyzed to transfer a phosphate group to the number 1 carbon of fructose 6-phosphate to produce fructose 1,6-biphosphate (Figure 6).

Figure 6: Glycolysis, Step 3. A second molecule of ATP is hydrolyzed to transfer a phosphate group to the number 1 carbon of fructose 6-phosphate to produce fructose 1,6-biphosphate.

Step 4

The 6-carbon fructose 1,6 biphosphate is split to form two, 3-carbon molecules: glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. The dihydroxyacetone phosphate is then converted into a second molecule of glyceraldehyde 3-phosphate (Figure 7). Two molecules of glyceraldehyde 3-phosphate will now go through each of the remaining steps in glycolysis producing two molecules of each product.

Figure 7: Glycolysis, Step 4. The 6-carbon fructose 1,6 biphosphate is split to form two, 3-carbon molecules: glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. The dihydroxyacetone phosphate is then converted into a second molecule of glyceraldehyde 3-phosphateTwo molecules of glyceraldehyde 3-phosphate will now go through each of the remaining steps in glycolysis producing two molecules of each product.

Step 5

As each of the two molecules of glyceraldehyde 3-phosphate are oxidized, the energy released is used to add an inorganic phosphate group to form two molecules of 1,3-biphosphoglycerate, each containing a high-energy phosphate bond. During these oxidations, two molecules of NAD+ are reduced to form 2NADH + 2H+ (Figure 8). During aerobic respiration, the 2NADH + 2H+ carry protons and electrons to the electron transport chain to generate additional ATP by oxidative phosphorylation.

Figure 8: Glycolysis, Step 5. As each of the two molecules of glyceraldehyde 3-phosphate are oxidized, the energy released is used to add an inorganic phosphate group to form two molecules of 1,3-biphosphoglycerate, each containing a high-energy phosphate bond. During these oxidations, two molecules of NAD+ are reduced to form two NADH + 2H+.

Step 6

As each of the two molecules of 1,3-biphosphoglycerate are converted to 3-phosphoglycerate, the high-energy phosphate group is added to ADP producing 2 ATP by substrate-level phosphorylation, a shown in Figure 9.

Figure 9: Glycolysis, Step 6. As each of the two molecules of 1,3-biphosphoglycerate are converted to 3-phosphoglycerate, the high-energy phosphate group is added to ADP producing 2 ATP by substrate-level phosphorylation.

Step 7

The two molecules of 3-phosphoglycerate are rearranged to form two molecules of 2-phosphoglycerate (Figure 10).

Figure 10: Glycolysis, Step 7. The two molecules of 3-phosphoglycerate are rearranged to form two molecules of 2-phosphoglycerate.

Step 8

Water is removed from each of the two molecules of 2-phosphoglycerate converting the phosphate bonds to a high-energy phosphate bonds as two molecules of phosphoenolpyruvate are produced (Figure 11).

Figure 11: Glycolysis, Step 8. Water is removed from each of the two molecules of 2-phosphoglycerate converting the phosphate bonds to a high-energy phosphate bonds as two molecules of phosphoenolpyruvate are produced.

Step 9

As the two molecules of phosphoenolpyruvate are converted to two molecules of pyruvate, the high-energy phosphate groups are added to ADP producing 2 ATP by substrate-level phosphorylation, a shown in Figure 12.

Figure 12: Glycolysis, Step 9. As the two molecules of phosphoenolpyruvate are converted to two molecules of pyruvate, the high-energy phosphate groups are added to ADP producing 2 ATP by substrate-level phosphorylation.

Through an intermediate step called the transition reaction, the two molecules of pyruvate then enter the citric acid cycle to be further broken down and generate more ATPs by oxidative phosphorylation.

Glycolysis Overview

Glycolysis is a partial breakdown of a six-carbon glucose molecule into two, three-carbon molecules of pyruvate, 2NADH +2H+, and 2 net ATP as a result of substrate-level phosphorylation. Glycolysis occurs in the cytoplasm of the cell.

glycol_an.gif

The overall Glycolysis reaction is:

glucose (6C) + 2 NAD+ 2 ADP +2 inorganic phosphates (Pi)

yields 2 pyruvate (3C) + 2 NADH + 2 H+ + 2 net ATP

Summary

  1. Aerobic respiration is the aerobic catabolism of nutrients to carbon dioxide, water, and energy, and involves an electron transport system in which molecular oxygen is the final electron acceptor.
  2. Aerobic respiration involves four stages: glycolysis, a transition reaction that forms acetyl coenzyme A, the citric acid (Krebs) cycle, and an electron transport chain and chemiosmosis.
  3. Glycolysis is a partial breakdown of a six-carbon glucose molecule into two, three-carbon molecules of pyruvate, 2NADH +2H+, and 2 net ATP as a result of substrate-level phosphorylation.
  4. The overall reaction for glycolysis is: glucose (6C) + 2 NAD+ 2 ADP +2 inorganic phosphates (Pi) yields 2 pyruvate (3C) + 2 NADH + 2 H+ + 2 net ATP.
  5. Glycolysis does not require oxygen and can occur under aerobic and anaerobic conditions. However, during aerobic respiration, the two reduced NADH molecules transfer protons and electrons to the electron transport chain to generate additional ATPs by way of oxidative phosphorylation.
  6. Glycolysis also produces a number of key precursor metabolites.
  7. Through an intermediate step called the transition reaction, the two molecules of pyruvate then enter the citric acid cycle to be further broken down and generate more ATPs by oxidative phosphorylation.

Contributors

  • Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)