Enzymes: Life's Catalysts

Most of the chemical reactions that keep a cell alive would happen far too slowly on their own to sustain life -- they need a boost over an energy barrier called activation energy, the minimum energy required to get a reaction started. Enzymes are proteins (a few are RNA) that act as biological catalysts: they lower the activation energy needed for a specific reaction without being used up or permanently changed themselves, so the same enzyme molecule can be reused again and again. A reaction that would take years without help can finish in a fraction of a second with the right enzyme present.

An enzyme's specificity comes directly from last week's idea of protein shape: its active site, a precisely shaped pocket, fits only a particular substrate (or a small family of similar substrates), the way a specific key fits a specific lock -- though the modern, more accurate picture, called induced fit, holds that the active site actually flexes slightly to snugly hug its substrate as it binds, rather than being a perfectly rigid shape from the start. Because enzymes are proteins, their activity depends heavily on conditions: activity rises with temperature up to an optimum (for most human enzymes, around 37 degrees C, normal body temperature) and then crashes sharply past that point as heat denatures the enzyme's shape; each enzyme also has an optimum pH suited to where it works (pepsin in the strongly acidic stomach works best around pH 2, while trypsin in the small intestine works best in a nearly neutral-to-slightly-basic environment) and loses function outside that range. Higher substrate concentration increases reaction rate only up to the point where every active site is already occupied -- the enzyme is then said to be saturated, and adding more substrate does not speed things up any further.