a) The answer is allosteric interaction where activators or repressors bind to a protein, other than the protein active site, hence changing the protein conformation. This interaction occurs between the heme subunits of haemoglobin.
b) Through protonation. The amino group of the alpha subunits and the histidine C-terminal of beta subunits acquire H+ when the blood has low pH. Low pH in the blood is due to accumulation of carbon dioxide and consequent formation of carbonic acid. Protonation of the residues cause allosteric interaction, due to ionic interaction, of the subunits hence resulting to conformation change of haemoglobin between T-state and R-state
