GLYCOLYSIS AND THE KREBS CYCLE

Glycolysis is the anaerobic catabolism of glucose. In other words, as its name implies, the pathway uses several enzyme catalyzed reactions to split (lysis) a sugar (glyco). Glycolysis occus in the cytosol.

The Individual Reactions of Glycolysis

The pathway of glycolysis can be seen as consisting of 2 separate phases. The first is the chemical priming phase requiring energy in the form of ATP, and the second is considered the energy-yielding phase. In the first phase, 2 equivalents of ATP are used to convert glucose to fructose 1,6-bisphosphate (F1,6BP). In the second phase F1,6BP is degraded to pyruvate, with the production of 4 equivalents of ATP and 2 equivalents of NADH.

GLYCOLOSYS YIELDS: 8 ATP; 2NADH + H+, 4 ATP, -2 ATP

  • C6H12O6 + 2NAD+ -> 2C3H4O3 + 2NADH + 2H+

    A great animation of glycolysis can be found HERE

    The Krebs Cycle

    The pyruvate molecules produced during glycolysis contain a lot of energy in the bonds between their molecules. In order to use that energy, the cell must convert it into the form of ATP. To do so, pyruvate molecules are processed through the Krebs Cycle, also known as the citric acid cycle. This cycle occurs in the mitochondria.

    1. Prior to entering the Krebs Cycle, pyruvate must be converted into acetyl CoA. This is achieved by removing a CO2 molecule from pyruvate and then removing an electron to reduce an NAD+ into NADH. An enzyme called coenzyme A is combined with the remaining acetyl to make acetyl CoA which is then fed into the Krebs Cycle. The steps in the Krebs Cycle are summarized below:

    2. Citrate is formed when the acetyl group from acetyl CoA combines with oxaloacetate from the previous Krebs cycle.

    3. Citrate is converted into its isomer isocitrate.

    4. Isocitrate is oxidized to form the 5-carbon α-ketoglutarate. This step releases one molecule of CO2 and reduces NAD+ to NADH2+.

    5. The α-ketoglutarate is oxidized to succinyl CoA, yielding CO2 and NADH2+.

    6. Succinyl CoA releases coenzyme A and phosphorylates ADP into ATP.

    7. Succinate is oxidized to fumarate, converting FAD to FADH2.

    8. Fumarate is hydrolized to form malate.

    9. Malate is oxidized to oxaloacetate, reducing NAD+ to NADH2+.

    We are now back at the beginning of the Krebs Cycle. Because glycolysis produces two pyruvate molecules from one glucose, each glucose is processes through the kreb cycle twice. For each molecule of glucose, six NADH2+, two FADH2, and two ATP.

    KREBS CYCLE YIELDS (2 CYCLES, 1 FOR EACH PYRUVATE): 24 ATP; 6 NADH + H+, 2 GTP, 2FADH

    Click HERE for an excellent animation of the Krebs Cycle.