Metabolic pathways are sequences of enzyme-catalyzed chemical reactions that occur within a cell. These pathways convert substrates into products, generate energy, and synthesize the building blocks needed for cellular growth and maintenance.

Types of Metabolic Pathways

Catabolism

Catabolic pathways break down large molecules into smaller ones, releasing energy. The breakdown of glucose through glycolysis and the citric acid cycle is a classic example. The energy released is captured in the form of ATP and reduced electron carriers such as NADH and FADH2.

Anabolism

Anabolic pathways use energy to build complex molecules from simpler ones. Examples include the synthesis of proteins from amino acids, nucleic acids from nucleotides, and fatty acids from acetyl-CoA. These pathways require the input of ATP and reducing power from NADPH.

Amphibolic Pathways

Some pathways serve both catabolic and anabolic functions. The citric acid cycle, for example, oxidizes acetyl-CoA to generate energy while also providing intermediates for amino acid and nucleotide synthesis.

Key Features of Metabolic Pathways

Enzyme Regulation

Pathways are tightly regulated to meet the cell’s needs. Key enzymes are controlled through feedback inhibition, allosteric regulation, and covalent modification. The rate-limiting enzyme of a pathway is usually the first committed step.

Compartmentalization

In eukaryotic cells, metabolic pathways are often compartmentalized in specific organelles. Glycolysis occurs in the cytoplasm, the citric acid cycle in the mitochondria, and fatty acid synthesis in the cytoplasm. This separation allows for independent regulation.

Energy Currency

ATP is the universal energy currency of the cell. NADH and FADH2 carry electrons for oxidative phosphorylation, while NADPH provides reducing power for biosynthetic reactions. The balance of these molecules determines the metabolic state of the cell.