What is the term for the inactive protein-only portion of an enzyme?

The term for the inactive protein-only portion of an enzyme is "apoenzyme." An apoenzyme is the protein component of an enzyme without its necessary cofactor, which may be either a metal ion or an organic molecule (coenzyme). When an apoenzyme combines with its cofactor, it becomes an active enzyme known as a holoenzyme, which can effectively catalyze biochemical reactions.

Understanding the distinction between apoenzymes and holoenzymes is crucial for comprehending enzyme functionality. The apoenzyme, by itself, lacks the necessary elements to perform catalysis, whereas the holoenzyme is the complete functional unit capable of substrate interaction and facilitating reactions. This differentiation is an important concept in enzymology and biochemical reactions, as it illustrates how cofactors play a critical role in the activity of enzymes.