Cooperative hemoglobins: conserved fold, diverse quaternary assemblies andallosteric mechanisms

Assembly of hemoglobin subunits into cooperative complexes produces a remar kable variety of architectures, ranging in oligomeric state from dimers to complexes containing 144 hemoglobin subunits. Diverse stereochemical mechan isms for modulating ligand affinity through intersubunit interactions have been revealed from studies of three distinct hemoglobin assemblages. This m echanistic diversity, which occurs between assemblies of subunits that have the same fold, provides insight into the range of regulatory strategies th at are available to protein molecules.