Enzymes: Structure, Mechanism, and Control

Enzyme definition and catalytic role

An enzyme is a biological macromolecule—usually a protein—that functions as a catalyst by accelerating chemical reactions without being consumed. The reactant molecules that enzymes act on are called substrates, which are converted into products. Enzymes are essential for nearly all metabolic processes because they enable reactions to occur at biologically relevant rates. They work by lowering the activation energy of reactions and do not change the overall equilibrium; instead, they are regenerated after each catalytic cycle. Enzymes are highly specific due to their unique three-dimensional structures and are sensitive to conditions such as temperature and pH. Catalytically, enzymes increase reaction rates through several mechanisms: stabilizing the transition state, providing an alternative reaction pathway (often via transient intermediates), destabilizing the substrate’s ground state, and orienting substrates into a productive arrangement (with entropy effects playing a smaller role). Substrate binding is a prerequisite for catalysis and is achieved through complementary binding pockets that can distinguish between similar molecules. Models such as “lock and key” and the more flexible “induced fit” describe how the enzyme’s active site accommodates substrates, often reshaping during binding to form an enzyme–substrate complex. The catalytic function depends on the enzyme’s active site, which includes both catalytic residues and substrate-binding residues. Only a small portion of the enzyme’s structure directly participates in catalysis, while the rest helps maintain the active site’s precise orientation and dynamics. Enzymes may also use cofactors (including coenzymes) or undergo allosteric modulation, where binding at an allosteric site changes activity. Enzyme activity can be enhanced by activators or reduced by inhibitors, and improper enzyme function (due to mutations or altered expression) can lead to disease.

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Enzymology and pseudoenzymes

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