Conspectus Many enzymes that produce or transform small molecules such as O 2 , H 2 , and CO 2 embed inorganic cofactors based on transition metals. The active site, where the chemical reaction occurs, is buried in and protected by the protein matrix, and connected to the solvent in several ways: chains of redox cofactors mediate long range electron transfer; static or dynamic tunnels guide the substrate, product and inhibitors; amino acids and water molecules transfer protons. The catalytic mechanism of these enzymes is therefore delocalised over the protein and involves many different steps, some of which determine the response of the enzyme under conditions of stress (extreme redox conditions, presence of inhibitors, light), the catalytic rates in the two directions of the reaction, and their ratio (the "catalytic bias"). Understanding all the steps in the catalytic cycle-including those that occur on sites of the protein that are remote from the active site-requires a combination of biochemical, structural, spectroscopic, theoretical and kinetic methods. Here we argue that kinetics should be used to the 1