The protein portion of an enzyme is known as the:

1. coenzyme

2. cofactor

3. apoenzyme

4. allosteric site

3. apoenzyme

Explanation:

Apoenzymes -

It is the inactive form of the enzyme, and it binds with the substrate which have affinity like of the holoenzyme, but is does not have the capability to convert the substrate to product.

The binding process may lead to some conformational changes in the apoenzyme, which the method of fluorescence can detect.

hence, the protein portion of the enzyme is the 3. apoenzyme.