Some proteins exist in multiple quaternary forms, depending on a mixture of different structurally equivalent, but catalytically different forms. For example, in lactate dehydrogenase, there are two structurally equivalent subunits, A and B. The protein exisists as a tetramer (four components) which can be anywhere from A4 to A2B2 to B4. These different isozymes vary in affinity to their natural substrate and their sensitivity to inhibitors.

The cell is therefore able to control the function of a protein like lactate dehydrogenase by controlling the relative ratios of A and B in the cytoplasm.

Other ways of regulating protein activity:
modulator protein