The protein portion of an enzyme is called:

The correct answer is Apoenzyme.

To understand the different components of an enzyme, let's break down the structure of a conjugated enzyme (also known as a holoenzyme):

A holoenzyme is an active, complete enzyme that consists of a protein component and a non-protein component:
Holoenzyme = Apoenzyme (Protein part) + Cofactor (Non-protein part)

Here is a detailed breakdown of each term to explain why Apoenzyme is the correct choice:

1. Apoenzyme: This is the protein portion of the enzyme. On its own, it is inactive (an inactive enzyme precursor) and must bind with a cofactor to become active.

2. Cofactor: This is a general term for the non-protein helper substance required by an enzyme for catalytic activity. Cofactors can be inorganic ions or complex organic molecules. For example, metallic ions such is zinc or iron can act as cofactors.

3. Coenzyme: This is a specific type of cofactor. Coenzymes are organic molecules (often derived from vitamins) that bind loosely and transiently to the apoenzyme.

4. Prosthetic group: This is an organic cofactor that is tightly or covalently bound to the apoenzyme, remaining permanently attached to the enzyme structure.

Since the question specifically asks for the protein portion of the enzyme, the correct term is apoenzyme.