Enzyme Catalysis

Enzymes are biological catalysts -- large protein molecules, produced by living

cells, that catalyze the countless biochemical reactions of metabolism with a speed, and under such

remarkably mild conditions of temperature and pH, that no ordinary inorganic catalyst can match.

Mechanistically they work by the same underlying principle as any catalyst -- providing an

alternative pathway of lower activation energy -- but they do so through an extraordinarily precise

three-dimensional binding pocket, the active site, into which the reacting molecule (the

substrate) fits.

Specificity. The single most distinctive feature of enzyme catalysis is its exceptionally high

specificity: a given enzyme will typically catalyze only one particular reaction, or act on only

one substrate (or a very narrow family of closely related substrates), because the substrate must fit

the enzyme's active site with a precision often compared to a key fitting a specific lock. This stands

in sharp contrast to an ordinary inorganic catalyst such as finely divided platinum or iron, which

typically catalyzes a broad range of chemically similar reactions with comparatively little

selectivity among the reactants presented to it. Familiar examples include invertase, which catalyzes

specifically the hydrolysis (inversion) of cane sugar into glucose and fructose, diastase, which

converts starch specifically to maltose, and urease, which hydrolyzes urea and essentially nothing

else.

Saturation kinetics. At low substrate concentration, an enzyme-catalyzed reaction's rate rises

roughly in proportion to substrate concentration, behaving much like an ordinary first order reaction,

because most of the enzyme's active sites are empty and available to bind more incoming substrate

molecules. But as substrate concentration keeps rising, an increasing fraction of the enzyme's (fixed,

comparatively small) population of active sites becomes occupied at any instant, and the rate of

product formation levels off toward a maximum, constant value -- the reaction becomes effectively

zero order in substrate once every active site is continuously occupied, since adding still more

substrate beyond that point cannot make the enzyme molecules work through the reaction any faster than

they already are. This saturation behaviour, in which rate rises and then plateaus rather than

increasing indefinitely, is the elementary, qualitative essence of what is formalized quantitatively in

Michaelis-Menten enzyme kinetics.

Optimum temperature and pH. Unlike an ordinary reaction, whose rate keeps rising with temperature

throughout the range in which the reactants remain chemically stable (as described by the Arrhenius

equation), an enzyme-catalyzed reaction's rate rises with temperature only up to a certain

optimum temperature (often close to $37\,^\circ\text{C}$ for human enzymes) and then falls away …