Oxygen binding on hemoglobin - all or none?

[thRob]

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So I have a professor who insists that hemoglobin either binds zero oxygen molecules or binds 4 - "there are no intermediates." Going by the Kaplan Physiology book, this is incorrect. In Kaplan, it says that:

Site 1 always has oxygen bound
Site 2 has oxygen bound when PO2 > 26 mm Hg
Site 3 has oxygen bound when PO2 > 40 mm Hg
Site 4 has oxygen bound when PO2 ~ 100 mm Hg

The fact that it says that site 1 always has oxygen bound (under physiological conditions) makes me think that the "all or none" idea is bogus. Thoughts? Has there been new research on this or something?

 

[thRob]

looks like there is both a sequential and a concerted model. The sequential seems easier to understand / apply practically, so thats probably why Kaplan went with it.