Answer
It is given that PAH can form one active site based on the amino acid sequence and that the functional form of the enzyme has four identical active sites. It is evident that PAH is made up of four identical protein subunits, with each subunit containing the same active site.
Work Step by Step
The primary sequence determines the active site. If one active site can be formed from a particular sequence, protein folding will always yield only one active site.
Some enzymes are made up of many subunits. Sometimes these subunits may be identical (as in PAH), and sometimes they may differ (as in insulin).
