Deoxyhemoglobin has a higher pKa than oxyhemaglobin and thus will accept hydrogen ions more readily. This is important in the ability to "carry" CO2 as the main mechanism of CO2 transport is actually conversion of CO2 to HCO3- (CO2 + H2O -> HCO3- + H+), thus deoxyhemaglobin carries the H+ while HCO3- is transported in the plasma to the lungs. Carbaminohemaglobin is actually only about 20% of the CO2 trasnport and CO2 dissolved as CO2 is 10%; trasnport as HCO3- is around 70% of the CO2 transport.

Here is another image of the process.

Another important aspect of the process to take note of that they like to ask about is the chloride shift. Chloride moves the opposite direction as HCO3- in the RBC. Thus at tissues where we are generating HCO3- from CO2 and need to pump it out of the cell, chloride moves into the cell.At the alveoli where we need to pump HCO3- back into the RBC to turn it into CO2, chloride moves out of the cell.

Other answers:

• 2,3BPG will bind and stabilizes deoxyhemaglobin making it less attracted to oxygen, shifting the hemaglobin dissosiation curve right.
• Deoxyhemaglobin has a higher capacity to form carbanimohemaglobin
• Deoxyhemaglobin has a higher pKA than oxyhemaglobin (this is why it can accept hydrogen ions more readily)
• Competition for binding site is not a major mechanism for the ability to carry CO2 as most CO2 is "carried" by conversion to HCO3-
• Nitric oxide binding is not a mechanism of CO2 carry.