How Enzymes Speed Up Biological Reactions

Imagine you need to chop a log — the axe does not burn along with the wood; it just makes the job easier. That is exactly what an enzyme does inside a cell. All chemical reactions require a minimum amount of energy to get started — called the **activation energy**. Enzymes are biological catalysts, almost always proteins, that lower this energy barrier so reactions proceed fast enough to support life. Crucially, the enzyme is not consumed: it is released unchanged after each reaction and can catalyze reactions at vastly different speeds depending on the enzyme — from a few reactions per second to millions per second. Every enzyme has a precisely shaped region called the **active site** — a pocket or groove on the protein surface. Only a molecule with a complementary shape, the **substrate**, fits there. This fit is not rigid like a lock-and-key; the enzyme slightly changes shape when the substrate approaches, a model called **induced fit**. This snug embrace strains bonds in the substrate and stabilizes the transition state, which is the unstable peak the reaction must pass through — lowering how high that peak is. Once the reaction completes, the products no longer fit the active site and are released. The enzyme returns to its original shape, ready for the next substrate. **Factors that affect enzyme activity:** • **Temperature** — warming increases molecular motion and collision frequency, so reaction rate rises. But each enzyme has an optimal temperature (around 37 °C for most human enzymes). Above that optimum, excess heat disrupts the weak interactions (hydrogen bonds, hydrophobic interactions) that hold the protein in shape. The protein **denatures** — its 3-D shape is lost — and activity drops sharply. Mild shifts in temperature can sometimes be reversed, but at extreme temperatures the protein often aggregates and the denaturation becomes permanent. • **pH** — the active site depends on specific amino acid side chains that must carry the right charge. Changing pH alters those charges, distorting the active site. A mild pH shift may cause a reversible conformational change; a drastic shift causes irreversible denaturation. Pepsin in the stomach works best near pH 2; salivary amylase works near pH 7. Move each enzyme far from its optimum and activity plummets. • **Substrate concentration** — at low concentrations, adding more substrate increases the reaction rate because more active sites are occupied. At high concentrations all active sites are busy (the enzyme is **saturated**), and rate plateaus at the maximum velocity (Vmax). Adding still more substrate produces no further gain — the enzyme, not the substrate, is now the limiting factor. Remember: enzymes are specific, reusable, and sensitive to their environment. When conditions shift too far, structure breaks down and function disappears — a direct demonstration that protein shape IS protein function.