Mechanism of Tertiary Structural Change in Hemoglobin

A reaction path is presented by which the effects of oxygen binding in hemoglobin are transmitted from a heme group to the surface of its subunit. Starting from the known deoxy geometry, it is shown by calculations with empirical energy functions and comparisons with available data how the change in heme geometry on ligation introduces a perturbation that leads to the tertiary structural alterations essential for cooperativity. It is found that there is little strain on the unliganded heme; instead, the reduced oxygen affinity of hemoglobin results from the strain on the liganded subunit in a tetramer with the deoxy quaternary structure.