Hemoglobin Forms of O2 in Blood dissolved O2 2% of total O2 content O2 bound to hemoglobin 98% of total O2 content Hemoglobin a globular protein conisisting of 4 polypeptide subunits 2 α subunits and 2 β subunits binds 4 O2 molecules login to view 3 more bullets exists in 2 states low O2 affinity T (taut) state high O2 affinity R (relaxed) state login to view 1 more bullet exhibits positive cooperativity binding of 1 O2 molecule to 1 subunit of deoxyhemoglobin increases affinity for O2 in adjacent subunits sigmoidal shape of oxygen-hemoglobin dissociation curve is characteristic of positive cooperativity Hemoglobin Variants Hemoglobin F fetal hemoglobin 2 α subunits and 2 γ subunits (α2γ2) ↑ affinity for O2, ↓ affinity for 2,3-BPG login to view 1 more bullet Hemoglobin S an abnormal variant of hemoglobin that causes sickle cell disease α subunits are normal, β subunits are abnormal forms sickle-shaped robs in red blood cells in deoxygenated form login to view 2 more bullets ↓ affinity for O2