On the left is an example of an obligatory homodimer between two folded monomers that creates an active site (coloured purple) between the subunits. On the right is an example of a transient multifunctional complex between the homodimer and an unstructured region containing two phosphorylated residues (indicated by the letter P) within a second multi-domain protein. The formation of the multifunctional complex causes the two monomers within the dimer to rotate to reveal secondary binding sites (white) and subsequently close the original active site (purple). This is an example of allostery, where binding of one protein changes the conformation of another, which may be controlled according to the presence/absence of post-translational modifications. The thermodynamic and kinetic parameters are different for the two types of interactions as indicated. The obligatory interaction may occur in a different compartment and subsequently move to another compartment (as indicated by the dashed line) to subsequently engage in the transient interaction.