Final answer:
Enzyme molecules composed of multiple protein subunits are called oligomeric enzymes, with individual subunits termed apoenzymes. When combined with a cofactor, they become active holoenzymes.
Step-by-step explanation:
Enzyme molecules composed of two, three, or four protein subunits are commonly referred to as oligomeric enzymes. These enzymes are structured as complexes of multiple protein subunits, which work together to perform their catalytic function. An individual protein subunit within an oligomeric enzyme may be referred to as an apoenzyme, and when it’s combined with a cofactor—a non-protein component necessary for enzyme activity—the whole complex is known as a holoenzyme. Cofactors may be inorganic ions or organic molecules, such as coenzymes, many of which are derived from vitamins. The specific combination and the three-dimensional structure of the amino acids in the enzyme determine the enzyme's activity.