Factors Affecting Enzyme Activity

The rate at which an enzyme catalyses its reaction is not constant but is sensitive to several physical and chemical conditions, each of which a student of enzymology needs to understand individually.

Temperature affects enzyme activity in two opposing ways. Over a lower range of temperatures, activity rises as temperature increases, because the enzyme and substrate molecules move faster and collide more frequently, producing more successful enzyme-substrate encounters; this rise continues up to a characteristic optimum temperature (close to 37 degrees Celsius, human body temperature, for most human enzymes). Beyond this optimum, however, activity falls away sharply, because the additional heat energy begins to disrupt the weak hydrogen bonds, ionic interactions and hydrophobic interactions that hold the enzyme's secondary, tertiary and quaternary structure in its correct, functional shape -- a process called denaturation, which is generally irreversible and abolishes the enzyme's activity by destroying the precise shape of its active site.

pH similarly affects the ionisation state of the amino acid side chains within and around the active site, which in turn affects both substrate binding and catalysis, so that every enzyme has its own characteristic optimum pH at which it works fastest. Most enzymes that act within the cytoplasm have an optimum near neutral pH (about 7), but some enzymes that operate in unusual chemical environments have very different optima -- pepsin, which digests protein in the strongly acidic environment of the stomach, has an optimum near pH 2, while trypsin, which acts in the mildly alkaline environment of the small intestine, has an optimum near pH 8-9. As with temperature, pH values far from the optimum, in either direction, denature the enzyme and abolish its activity.

Substrate concentration also influences reaction rate: at a fixed enzyme concentration, increasing the substrate concentration increases the reaction rate, because more substrate molecules are available to encounter the limited number of active sites present -- but this increase is not indefinite. Once the substrate concentration is high enough that essentially every enzyme active site is continuously occupied, the reaction rate levels off at a maximum value, called Vmax, beyond which adding still more substrate produces no further increase, since the enzyme itself has become the limiting factor. The substrate concentration at which the reaction proceeds at exactly half of Vmax is called the Michaelis constant, Km, and serves as a measure of how readily an enzyme binds its substrate -- a low Km indicating high affinity. Enzyme concentration has a comparable, generally proportional, effect on rate whenever substrate is present in excess. …