Let's think ProteinA can phosphorylate proteinB, proteinC and proteinD.
Condition1: All proteins are expressed and proteinA phosphorylates proteinB, proteinC and proteinD.
Condition2: Only proteinA and proteinB are expressed for that reason protein A phosphorylates only proteinB.
Which is more realistic?
1) In condition1; proteinB, proteinC and proteinD will be 3 times less phosphorylated in comparison to proteinB in condition2.
2) The proteins of both conditions will be phosphorylated same.