Enzymes are proteins which catalyse (speed up) metabolic reactions. Like all other catalysts (e.g. in chemistry), enzymes achieve this by lowering the activation energy (energy needed for a reaction to occur) of a reaction, by forming an enzyme-substrate complex.
This can be described by the lock and key, and induced fit models of enzyme action. The lock and key model is based on complementary shapes between the enzyme and substrate. The substrate fits into the enzyme.
The induced fit model: (the enzyme changes shape to “hug” the substrate)
The enzyme’s shape is not exactly matched to the substrate, but it is able to accommodate the substrate with a close enough shape into an enzyme-substrate complex and carry out catalysing that reaction. Here is a video of an enzyme catalysing a reaction between two molecules into one molecule.
This is different to the above scenario in the diagrams, where one molecule is broken down into two molecules.
Enzymes are crucial to proper metabolic function, and ultimately life. They are found acting both intracellularly and extracellularly, for example in DNA replication and digestion. Some fungi excrete enzymes into their environment in order to break down nutrients and feed on the products.