Biochemical reactions, such as those in cellular respiration, are catalyzed (made faster and more specific) by enzymes, which are proteins (=polymers of amino acids).

 Enzymes have one or more active sites that bind the substrate(s) and carry out the chemical reaction. Enzymes, in particular the active site(s), have very specific three-dimensional shapes that are sensitive to its physical (temperature) and chemical (pH) environment. Up to a point, higher temperature = faster reaction.

 Chemicals called inhibitors can attach to a specific enzyme and inactivate it by changing its shape or blocking the active site. Rotenone is an example of an inhibitor, in this case one that disables a critical enzyme in the mitochondrial electron transport chain. No ETC = inability to use oxygen and to make ATP. The mechanism of action of an inhibitor can be determined by the specific biochemical effects on metabolism.

Enzyme catalytic cycle quiz