How does an enzyme speed a reaction without being used up?

An enzyme speeds a reaction by helping molecules pass through a difficult temporary arrangement. It is a catalyst: it participates in the process but is restored at the end, ready to act again.

Before reactants become products, some bonds and electron arrangements must change. Passing through the transition requires activation energy. At a given temperature, molecular encounters have different energies and orientations, so only some lead to reaction.

An enzyme’s active site interacts with particular substrate molecules. It may bring them together in a helpful orientation, change their local chemical surroundings or stabilize the difficult transition. The protein can flex during this process; the drawing’s simple shapes are not a rigid lock-and-key description of every enzyme.

The energy profile compares two routes with the same starting and ending levels. The uncatalyzed route rises to ten invented units above the starting level. The enzyme route rises to four. These are illustrative numbers, not measured energy or a calculated reaction speed. The lower barrier allows a greater fraction of encounters to produce reaction under the same conditions.

After products leave, the enzyme can help another substrate. It supplies no free energy and does not change the reaction’s overall energy difference or equilibrium position. A lower mountain pass is a useful comparison: crossing becomes easier without moving the valleys. The enzyme changes the route molecules can take, while remaining available for another cycle.