Most molecules of deoxyhemoglobin are in the T form. The binding of successive molecules of oxygen makes the R form increasingly probable. Thus, the first molecule of oxygen binds to the T form, whereas the fourth molecule of oxygen binds to the R form. The T from has a 300-fold lower affinity for oxygen than does the R form because the movement of the proximal histidine into the heme plane is more constrained in the T form. Specifically, the proximal histidine is more tilted in the T form and so there is greater steric repulsion in moving toward the heme plane. The more symmetric position of the proximal histidine in the R form allows it to approach the heme with less hindrance.
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