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 A
4 to A
2B
2 to B
4. 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:
zymogen
allostery
modulator protein