Metabolism 2: Glycolysis

From Glucose to Pyruvate

The breakdown of glucose is termed glycolysis, and occurs in many small steps. The purpose of having so many small steps is that energy is released at each step, which is used to convert ADP into ATP. For the complete breakdown of one glucose molecule, the cell gains 36 molecules of ATP.

Glycolysis occurs in the cytoplasm of the cell. Glycolysis involves the conversion of glucose, a 6-carbon molecule, into two 3-carbon molecules called pyruvate. This occurs through 10 individual steps. Although overall we can 36 ATP in glucose catabolism, the conversion of glucose to two pyruvate molecules yields 2 ATP molecules. This is through substrate level phosphorylation.

There are 5 factors that affect glycolysis:

1. Glucose molecules
2. Cytoplasmic enzymes
3. ATP and ADP
4. Inorganic phosphates
5. NAD (coenzyme)

10 Step Process

1. Glucose is phosphorylated using a kinase, consuming one ATP molecule and leaving it as ADP. This means we have a net ATP of -1.
2. Carbon molecules within the phosphorylated glucose are rearranged and a phosphate is added, producing fructose-1,6-biphosphate. Another ATP molecule is consumed to enable phosphorylation, becoming ADP. Net ATP: -2
3. Cleavage occurs, meaning fructose-1,6-biphosphate is split into two 3-carbon molecules, resulting in 2 molecules of glyceraldehyde-3-phosphate.
4. A phosphate is added to each molecule of G-3-P, and two NAD molecules are reduced to    NADH (one per G-3-P).
5. One phosphate from each of the molecules is transferred to 2 ADP molecules, generating two ATP. Net ATP: 0
6. Water is produced as a by-product, yielding phosphoenolpyruvate (PEP).
7. Substrate level phosphorylation occurs, where PEP transfers a phosphate to ADP, producing one ATP for each of the two PEP molecules. PEP becomes pyruvate, which can then enter the mitochondria. Net ATP: +2

The next steps are purely dependent on whether or not oxygen is present. These first 7 steps occur regardless of whether oxygen is present. However, the next steps are when respiration becomes aerobic or anaerobic. When oxygen is present, the following steps occur:

1. Pyruvate is oxidised in a decarboxylation reaction.
2. Pyruvate, a 3-carbon molecule, is converted to a 2-carbon molecule, called acetyl-CoA. The remaining carbon is used to form carbon dioxide, CO2.
3. NAD+ is reduced to NADH, and this reaction is aided by the energy released from the conversion of pyruvate to acetyl-CoA as described in Step 2 (step 9).

The above reactions involve co-enzyme A, a small organic non-protein molecule that carries chemical groups between enzymes. This specific enzyme helps pyruvate to oxidise to acetyl-CoA.