1. Lower the activation energy: Enzymes act as catalysts, speeding up chemical reactions by providing an alternative reaction pathway with a lower activation energy. This means less energy is needed to start the reaction, allowing it to happen faster.

2. Change the substrate's shape: Enzymes bind to their specific substrates, often inducing a change in the substrate's shape. This change can facilitate the reaction by bringing reactive groups closer together, or by straining bonds within the substrate to make it more susceptible to breaking.

So, enzymes essentially make reactions happen faster and more efficiently by lowering the energy barrier and manipulating the substrate's structure.