enzyme catalysis
Imagine a key-cutting machine so precisely shaped that only one kind of blank slides in, and once it is held in place the cutting happens almost effortlessly. Living cells are full of such machines, made of protein, that grab one specific molecule, ease it through a chemical change, then let go and grab the next. These protein machines are enzymes, and the speed-up they give is enzyme catalysis.
More precisely, enzyme catalysis is the acceleration of a biochemical reaction by an enzyme — usually a protein, occasionally an RNA — that binds its reactant (the substrate) in a precisely shaped pocket called the active site, lowers the activation energy of the reaction, releases the product, and emerges unchanged to act again. Enzymes are extraordinarily fast and extraordinarily selective, often speeding reactions by millions or billions of times while acting on just one molecule out of a crowded cellular soup.
Why it matters: essentially every reaction that keeps you alive — digesting food, copying DNA, releasing energy — is run by an enzyme, because uncatalysed these reactions would be far too slow at body temperature. The honest caveat is that enzymes are delicate: heat, extreme acidity, or certain poisons can change their shape and stop them working, and like any catalyst they only speed reactions up; they never make a thermodynamically uphill reaction run on its own.
Hydrogen peroxide builds up as a toxic by-product in your cells. The enzyme catalase grabs it and converts it to harmless water and oxygen so quickly that a single catalase molecule can handle millions of peroxide molecules every second — which is why a cut dabbed with peroxide foams instantly on contact with living tissue.
One catalase molecule clears millions of peroxide molecules per second.
Enzymes are biological catalysts, so all the usual catalyst rules apply: not consumed, do not shift equilibrium, only speed things up. Their special features are extreme selectivity and sensitivity to temperature and pH.