Cellular Metabolism, Enzymes and Aerobic Respiration ATP Yield

Cellular Metabolism and Aerobic Respiration ATP Yield

The metabolism includes all chemical transformations and energy changes that occur in living beings. Each of these transformations requires the participation of an enzyme; enzymes themselves may be modified as products of other reactions, including protein synthesis.

Enzymes biological catalysts

Enzyme classes include:

• Hydrolases
• Lyases
• Transferases
• Isomerases
• Oxidoreductases
• Synthetases

Vitamins essential for metabolism

Vitamins are essential biomolecules for metabolism. They include:

• Water-soluble: complex B and vitamin C
• Fat-soluble: vitamins A, D, E, K

Aerobic respiration: stages

Stage 1: Glucose to acetyl-CoA

Glucose to acetyl-CoA. Glucose is converted to pyruvate and then to acetyl-CoA.

Stage 2: Krebs cycle

Second, the Krebs cycle: acetyl-CoA enters the Krebs cycle in the mitochondrial matrix, where it combines with oxaloacetic acid to form citric acid. After successive transformations, the two carbon atoms of the acetyl group are fully oxidized to two CO2 molecules. One ATP (or GTP) is formed per acetyl unit, and three molecules of NADH and one FADH2 are produced.

Stage 3: Respiratory chain and oxidative phosphorylation

Respiratory chain: the NADH and FADH2 produced in the Krebs cycle are oxidized by a redox chain of electron carriers located in the mitochondrial cristae. Oxygen from the atmosphere is the final electron acceptor, forming H2O. This process releases a large amount of energy via proton pumping; protons return through ATP synthase, synthesizing ATP (oxidative phosphorylation).

Energy balance and ATP yield

Balance energy: In the first phase, for each molecule of glucose converted to pyruvate you get 2 ATP (from glycolysis) plus additional ATP equivalents from NADH reoxidation. Depending on the shuttle used to reoxidize cytosolic NADH (for example, via the dihydroxyacetone phosphate shuttle), the yield can be counted as 4 ATP from extramitochondrial NADH reoxidation (yielding intramitochondrial FADH2 equivalents) and 6 ATP from intramitochondrial NADH formed when the two molecules of pyruvate are converted to acetyl-CoA. In this accounting, the first stage yields 12 ATP per glucose.

In the second stage: only 2 ATP are produced in the Krebs cycle (one per acetyl unit, two acetyl units per glucose), but six NADH and two FADH2 are formed, which will be reoxidized in the respiratory chain.

In step 3: for each NADH oxidized in the respiratory chain about 3 ATP are obtained; for each FADH2 (which donates electrons via ubiquinone) about 2 ATP are obtained. Using the numbers above, this can lead to a total of 22 ATP in that accounting.

In the aerobic degradation of glucose by cells, 36 ATP molecules are obtained (alternative accounting gives 38 or 36 depending on shuttle and organism; the text above reflects one common textbook accounting).