Specific Pathways: From Complex Nutrients to Simple Molecules
In the human body, the initial stages of breakdown for major energy substrates—proteins, fats, and carbohydrates—are carried out by enzyme systems that are strictly specific to each class of compounds. This phase is known as the specific catabolic pathway. Its purpose is to convert a diverse array of molecules into intermediate metabolites.
Despite the structural differences among nutrients, the outcome of the specific pathways is always the same. The reactions lead to the formation of just two key substances:
- Pyruvic acid (or pyruvate), whose carbon skeleton consists of three carbon atoms ($C_3$).
- Acetic acid in the form of Acetyl-CoA, containing two carbon atoms ($C_2$).
How these universal molecules are formed:
- Proteins are first cleaved into amino acids, which are subsequently converted either into pyruvate or directly into Acetyl-CoA.
- Carbohydrates are degraded to glucose, which yields pyruvate through intracellular transformations.
- Fats break down into glycerol (converted to pyruvate) and fatty acids (oxidized to Acetyl-CoA).
Common Catabolic Pathway (CCP)
Once the specific reactions are complete and pyruvate is formed, the common catabolic pathway (CCP) begins. At this stage, degradation proceeds in a unified manner, regardless of whether the original substrate originated from glucose, an amino acid, or glycerol.
Functionally, the common pathway is divided into two sequential processes:
- Oxidative decarboxylation of pyruvate. At this stage, pyruvate is converted into the two-carbon Acetyl-CoA.
- The tricarboxylic acid (TCA) cycle, also known as the Krebs cycle, which utilizes the generated Acetyl-CoA.
All CCP reactions occur in the mitochondrial matrix. The end products of this global process are carbon dioxide ($CO_2$) and water ($H_2O$), alongside the release of free coenzyme HS-CoA.
Connection of CCP Reactions to the Electron Transport Chain
The biological purpose of catabolism is to supply the cell with energy. CCP reactions generate primary hydrogen donors necessary for the function of the electron transport chain (ETC). While the Krebs cycle operates in the matrix, the components of the ETC are embedded in the inner mitochondrial membrane.
Substrate oxidation in the cycle involves dehydrogenases:
- NAD⁺-dependent dehydrogenases operate in four reactions: the conversion of pyruvate to Acetyl-CoA, isocitrate to $\alpha$-ketoglutarate, $\alpha$-ketoglutarate to succinyl-CoA, and malate to oxaloacetate. The first three reactions are accompanied by the release of $CO_2$. This yields NADH, which donates electrons to Complex I of the respiratory chain.
- FAD-dependent dehydrogenase (succinate dehydrogenase) catalyzes the conversion of succinate to fumarate. Reduced FAD transfers electrons to ubiquinone (Q), bypassing Complex I.
In addition to generating hydrogen donors, the TCA cycle features a substrate-level phosphorylation reaction: the conversion of succinyl-CoA to succinate produces a molecule of ATP (or GTP).