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Change in protein subunit activity in a multimeric protein as mediated by other protein subunits.
Classically, cooperativity was observed in terms of an increase in hemoglobin binding to oxygen – that is, hemoglobin affinity for oxygen increases – which occurs in response to hemoglobin binding of oxygen.
The increased affinity can occur because hemoglobin can bind four molecules of oxygen simultaneously. Therefore, once one molecule of O2 is bound, then affinity of the additional O2 binding sites for oxygen increases. Thus, one protein subunit – hemoglobin consists of four – modifies the activity of a different protein subunit.
Cooperativity as seen with hemoglobin binding of oxygen is an example of positive cooperativity. Alternatively, with negative cooperativity a decrease in function is seen such as receptor protein affinity for ligands following ligand binding to one of the receptor binding sites (for that ligand). The latter can serve as a means of reducing the potential for a receptor to saturate with ligand binding.
Cooperativity with hemoglobin, by contrast, serves as a means of rapidly picking up oxygen when oxygen is abundant (such as in pulmonary capillaries) and then rapidly releasing oxygen where oxygen is relatively lacking (such as in oxygen-depleted muscle tissues).
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