In order to sustain life functions, every cell must carry out two general types of reactions, carefully balancing thermodynamically favorable reactions that release free energy, with thermodynamically unfavorable reactions that require free energy. However, very few molecules react spontaneously under ordinary physiological conditions because of the stability of molecular covalent bonds. The energy required to excite molecules from these stable ground states into reactive transition states is called activation energy, and it is this factor, rather than the thermodynamic nature of a reaction, that determines the rate at which a reaction will occur. Most cellular reactions have large activation-energy barriers, and thus even exergonic reactions would occur so slowly under unassisted conditions that cellular activity would cease.
Within the cell, certain molecules, called enzymes, act as catalysts, lowering this activation-energy barrier to a level surmountable by the thermal motion of molecules under ordinary physiological temperatures. Like all catalysts, enzymes act to speed up a reaction, and, although enzymes might temporarily be altered while a reaction is in progress, they remain unchanged when the reaction is complete. Unlike inorganic catalysts, enzymes interact with only one set of reactants, referred to as substrates, and speed up only one of the numerous possible reactions the substrates could undergo. An enzyme's extreme specificity is due to the fact that each type of enzyme has its own distinctive three-dimensional contours, determined by the
