In many proteins, the cooperative effects arise through a transition between two or more alternative structures with different substrate affinities. These structures are distinguished by the arrangement of the subunits and the number and kinds of bonds between them. If there are only two alternative structures, the one with fewer and weaker bonds between the subunits would be free to develop its full catalytic activity or oxygen affinity. It is therefore called R, for "relaxed." The activity would be damped in the structure with more and stronger bonds between the subunits; this form is called T, for "tense." In the absence of oxygen or substrate, nearly all of the protein molecules have the T structure; if the protein molecules are saturated with oxygen or substrate, nearly all of them have the R structure. Proteins that exhibit cooperativety and change their structures in response to chemical stimuli are called allosteric. Compounds other than oxygen or substrates that change the equilibrium between T and R states, such as DPG in red blood cells, are called allosteric effectors. In hemoglobin the heterotropic ligands all act as effectors that shift the allosteric equilibrium towards the T structure.
Return to tutorial