Catabolism denotes the set of biochemical processes in which large, complex molecules are broken down into simpler components and chemical energy is liberated. It forms one half of metabolism, the other half being anabolism, which uses energy to assemble molecules. Catabolic reactions provide the fuel and reducing power cells need for movement, growth, repair, and heat production.

Key characteristics and pathways

Typical catabolic pathways include digestion of dietary polymers, glycolysis (the breakdown of glucose), beta-oxidation of fatty acids, and proteolysis of proteins into amino acids. Many of these pathways feed electrons into a common system: reduced cofactors such as NADH and FADH2 donate electrons to the respiratory chain, driving ATP synthesis by oxidative phosphorylation. Enzymes catalyze each step and often operate in sequences called pathways or cycles.

Stages and common intermediates

Catabolism can be thought of in stages: extracellular digestion (large polymers to monomers), cytosolic or mitochondrial conversion of monomers into central metabolites (for example, pyruvate or acetyl groups), and final oxidation of those metabolites to CO2 with concomitant ATP production. Intermediates produced by catabolism also serve as building blocks or signals for other cellular processes.

Regulation, physiological roles, and examples

Catabolic activity is tightly regulated to match energy demand and nutrient availability. Hormones and allosteric regulators adjust flux through pathways during fasting, feeding, and exercise. Examples: glycolysis provides rapid ATP for active muscle; beta-oxidation supplies energy during prolonged fasting; proteolysis mobilizes amino acids when other fuels are scarce. Disruption of catabolic routes underlies many metabolic disorders.

Distinctions and historical context

In contrast to anabolism (biosynthesis), catabolism releases rather than consumes net free energy. Modern understanding of catabolism arose from studies of cellular respiration and enzymology in the late 19th and 20th centuries. For broader context on metabolic organization and interactions, see a general metabolism overview.

  • Common electron carriers: NAD+/NADH, FAD/FADH2
  • Outputs: ATP (or equivalent), heat, small molecule precursors
  • Clinical relevance: altered catabolism occurs in fasting, diabetes, and inherited metabolic diseases