The protein portion of an enzyme is called: 

The correct answer is Apoenzyme

Explanation:

• Enzymes are biological catalysts that speed up chemical reactions in living organisms without being consumed in the process.
• Enzymes are composed of two main parts: the protein portion and the non-protein portion.
• The protein portion of an enzyme is called an Apoenzyme, and it is inactive by itself.
• The non-protein part, which can either be a cofactor or coenzyme, binds to the apoenzyme to form an active enzyme known as a holoenzyme.

Holoenzyme = Apoenzyme + Coenzyme

Additional InformationCofactors are non-protein chemical compounds or metallic ions that are required for an enzyme's biological activity to occur. Cofactors can be broadly categorized into three main types:-

• Prosthetic Groups: Tightly bound, integral to the enzyme (Haem, FAD, Biotin).
• Coenzymes: Loosely bound, temporary carriers (NAD+, Coenzyme A, TPP).
• Metal Ions: Can be loosely or tightly bound, involved in structural and catalytic roles (Zinc, Magnesium, Iron).

• The protein portion of an enzyme, without its cofactor, is called an apoenzyme. It is inactive until the cofactor binds to it.