Chapter 10: Introduction to Metabolism

Metabolism comprises three fundamental categories: anabolism, which constructs complex molecules from simpler precursors using energy input; catabolism, which breaks down large molecules to release energy and building blocks; and amphibolic reactions that participate in both synthetic and degradative processes. Despite extraordinary diversity across organisms, metabolic pathways display remarkable conservation, with variations arising from species-specific enzymes added to a shared foundational set of reactions. Pathways operate as sequences of enzyme-catalyzed steps where products of one reaction become substrates for the next, proceeding through linear, cyclic, or spiral configurations. This stepwise organization permits controlled energy release or consumption through carriers like ATP and NADH rather than wasteful energy bursts, and allows precise regulation at metabolically irreversible reactions far from equilibrium. Regulation mechanisms include feedback inhibition, where end products suppress earlier steps, and feed-forward activation, where early intermediates promote later enzymes through allosteric modulation or covalent modification. The direction of all metabolic reactions depends on actual Gibbs free energy change under cellular conditions, not standard values, with negative values driving spontaneous forward progression. ATP serves as the primary energy currency, its hydrolysis releasing substantial energy through electrostatic repulsion and product stabilization, while thioesters like acetyl CoA provide comparable energy storage. Electron transfer reactions depend on reduced coenzymes including NADH, NADPH, and ubiquinone, with reduction potential differences determining energy availability. Cellular compartmentation in eukaryotes—glycolysis in cytosol, citric acid cycle in mitochondria, nucleic acid synthesis in nucleus—permits simultaneous operation of opposing pathways and maintains concentrated metabolite pools. Experimental investigation employs isotopic tracers, spectrophotometry, nuclear magnetic resonance, and mutant organism analysis to elucidate metabolic transformations and pathway mechanisms.