Oxygen binding changes everything
When the iron binds an oxygen molecule, it triggers a series of adjustments. The iron's favorite bond angles change, flattening the heme and 'pulling' on that histidine we've been watching. This in turn triggers a number of shifts as some parts of the molecule are forced out of the way of the heme, or are forced to move by virtue of their attachment to one of two key histidines.
add helices histidines + the helices that house them
everything ribbons view
everything wireframe view