酵素

Enzymes are biological catalysts that accelerate the rate of biochemical reactions. Most enzymes are proteins, made up of amino acids; while some are RNA molecules known as ribozymes.  Enzymes act by lowering the activation energy of a reaction, thereby increasing the reaction rate. They can catalyze both synthesis and breakdown of chemical bonds, but do not affect the direction or equilibrium of the reaction.  Each enzyme binds to a specific reactant, called the substrate, and catalyzes a particular reaction. The substrate binds to a distinct catalytic region of the enzyme called the active site through interactions such as intermolecular forces and transient covalent bonds, resulting in an enzyme-substrate complex. This binding is highly specific due to the conformational complementarity required between the enzyme and its substrate. Thus, a particular enzyme can only catalyze specific reactions based on its conformation.   A simple way of understanding complex formation is the lock-and-key model, which hypothesizes that the substrate fits into the enzyme's active site, analogous to a 'key' fitting into its corresponding 'lock'.  However, another model, the induced-fit model, takes into account the dynamic nature of the complex. This model states that when a substrate binds, it induces small conformational changes, resulting in a tighter fit that favors the reaction.  The activation energy for a reaction can be lowered by various methods.  Common mechanisms include inducing conformational changes in the substrate that allow a bond to be broken more easily or bringing the reactive groups of two substrates in close proximity, thereby promoting bond formation.\Enzyme activity can be temporarily or permanently suppressed by natural or synthetic molecules called inhibitors. For example, a competitive inhibitor competes with the substrate to bind to the enzyme's active site, thereby preventing substrate binding. On the other hand, a noncompetitive inhibitor binds to another location on the enzyme, which causes a conformational change at the active site, reducing the enzyme’s catalytic activity.