Final answer:
An enzyme is a protein that acts as a biological catalyst and speeds up biochemical reactions. It consists of an apoenzyme (protein portion) and a cofactor (non-protein portion). When they combine, they form an active enzyme.
Step-by-step explanation:
An enzyme is a protein that acts as a biological catalyst and speeds up biochemical reactions. It does this by reducing the activation energy required for the reaction to occur.
Enzymes can be categorized into two main parts: the apoenzyme, which is the protein portion of the enzyme, and the cofactor, which is the non-protein portion. Cofactors can either be metallic ions, such as Zn²+ or Mg²+, or organic compounds known as coenzymes.
When the apoenzyme combines with its cofactor, it forms the active enzyme, also known as the holoenzyme.