How is it possible for two proteins in the same cell to perform different functions

Enzymes are regulated by more than the binding of small molecules. A second method that is used all the time by eucaryotic cells to regulate a protein's function is the covalent addition of a phosphate group to one of its amino acid side chains. These phosphorylation events can affect the protein in two important ways.

protist eukaryotes that are not plants, animals, or fungi ("misfits")