- 20% of the students are/is present.
- The remaining 20% of the protein form/forms enzymes.
Singular verb or plural — which one is correct?
When we are looking at a sentence with words that indicate portions (percent in your example, also: fraction, some...), to choose the correct subject-verb agreement, we simply have to look at the noun after "of".
In the first case, the noun is "students" - a countable noun and it is in plural, therefore the verb should be in plural as well:
20% of the students are present
Even if there are 100 students, and you want to infer 1%, you still use plural:
1% of 100 students are present
In the second case, the noun is "protein" - countable noun in singular, the verb is singular as well:
The remaining 20% of the protein forms enzymes