Concept preview

What you will be able to do

1. 01Explain what enzymes are and why life depends on them as biological catalysts.
2. 02Define activation energy and describe how an enzyme lowers it without being used up.
3. 03Describe the active site, substrate and enzyme–substrate complex.
4. 04Compare the lock-and-key and induced-fit models and explain enzyme specificity.
5. 05State that enzymes are (mostly) proteins that are reusable and unchanged by the reaction.

Almost every chemical reaction that keeps you alive — digesting a meal, copying your DNA, releasing energy from glucose — would, left to itself, run far too slowly to be of any use. At body temperature the molecules of life are simply too stable and too sluggish to react at a meaningful pace; some reactions would take years or centuries to happen on their own. Yet inside your cells these same reactions are completed in a fraction of a second, millions of times over. The reason is a remarkable class of molecules called enzymes — biological catalysts that speed up reactions enormously without themselves being consumed. A catalyst is any substance that accelerates a reaction and emerges unchanged at the end, and enzymes are the most powerful and precise catalysts known, routinely making reactions go millions or even billions of times faster than they otherwise would.