Fundamentals of Microbiology

3.1.3 Glycolysis

Glycolysis, the oxidation of glucose to pyruvic acid, is usually the first stage in carbohydrate catabolism.

Glycolysis is also called the Embden-Meyerhof path-way. The enzymes of glycolysis catalyze the splitting of glucose, a six-carbon sugar, into two three carbon sugars. These sugars are then oxidized, releasing energy, and their atoms are rearranged to form two molecules of pyruvic acid. During glycolysis NAD⁺ is reduced to NADH, and there is a net production of two ATP molecules by substrate level phosphorylation. Glycolysis does not require oxygen it can occur under either aerobic or anaerobic conditions. This pathway is a series of ten chemical reactions, each catalyzed by a different enzyme. The steps are outlined in figure. To summarize the process, glycolysis consists of two basic stages, a preparatory stage and an energy conserving stage.

1. First, in the preparatory stage two molecules of ATP are used as a six carbon glucose molecule is phosphorylated, restructured and split into two three carbon compounds: glyceraldehyde 3-phosphate and dihydroxyacetone phosphate.

2. Then, in the energy conserving stage, the two three carbon molecules are oxidized in several steps into two molecules of pyruvic acid. In these reactions, two molecules of NAD⁺ are reduced to NADH and four molecules of ATP are formed by substrate level phosphorylation.

Because two molecules of ATP were needed to get glycolysis started and four molecules of ATP are generated by the process, there is a net gain of two molecules of ATP for each molecule of glucose that is oxidized.

Outline of the reactions of glycolysis

1. Glucose enters the cell and is phosphorylated. A molecule of ATP is invested. The product is glucose 6-phosphate.

2. Glucose 6-phosphate is rearranged to form fructose 6-phosphate.

3. The è^- from another ATP is used to produce fructose 1, 6-diphosphate, still a 6-carbon compound. (Note the total investment of two ATP molecules up to this point).

4. An enzyme cleaves (splits) the sugar into two 3-carbon molecules: dihydroxyacentone phosphate (DHAP) and glyceraldehyde 3-phosphate (GP).

5. Dihydroxyacetone phosphate and glyceraldehyde 3-phosphate can each be converted to the other.

6. The next enzyme converts glyceraldehyde 3-phosphate to another three carbon compound, 1,3-diphosphoglyceric acid. Because every dihydroxyacetone phosphate molecule can be converted to glyceraldehyde 3-phosphate, and each glyceraldehyde 3-phosphate to 1,3-diphosphoglyceric acid, the result is two molecules of 1,3-diphosphoglyceric acid for each initial molecule of glucose. Glyceraldehyde 3-phosphate is oxidized by the transfer of two hydrogen atoms to NAD⁺ to form NADH. The enzyme couples this reaction with the creation of a high-energy bond between the sugar and an è^- . The three carbon sugar now has two è^- groups.

7. The high energy è^- is moved to ADP, forming ATP, the first ATP production of glycolysis. (since the sugar splitting in step 4, all products are doubled. Therefore, this step actually repays the earlier investment of two ATP molecules).

8. An enzyme relocates the remaining è^- of 3-phosphoglyceric acid to form 2-phosphoglyceric acid in preparation for the next step.

9. By the loss of a water molecule, 2-phosphoenolpyruvic acid (PEP) in the process, the phosphate bond is upgraded to a high energy bond.

10. This high energy è­^- is transferred from PEP to ADP, forming ATP. For each initial glucose molecule, the result of this step is two molecules of ATP and two molecules of ATP and two molecules of a three carbon compound called pyruvic acid.

Alternatives to glycolysis

Many bacteria have another pathway in addition to glycolysis for the oxidation of glucose. The most common alternative is the pentose phosphate pathway another alternative is the Entner-Doudoroff pathway.