An enzyme’s protein portion without its required nonprotein component is the __________.

### Why this is right An apoenzyme is the protein portion of an enzyme without its required cofactor. If the missing component is needed for catalysis, the apoenzyme alone is not the complete active enzyme. ### The concept When the appropriate cofactor binds, the full enzyme is called a holoenzyme. The cofactor can be a metal ion or an organic molecule such as a coenzyme. ### Terms - **Holoenzyme**: an enzyme together with its required cofactor. ### Remember “Apo” refers to the protein portion without its required partner.