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 each subunit contains a heme moiety (iron-binding porphyrin ring) iron in heme moieties is in reduced, ferrous state (Fe2+) Fe2+ binds O2 reversibly exists in 2 states low O2 affinity T (taut) state high O2 affinity R (relaxed) state R state has 300x greater affinity for O2 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 facilitates O2 delivery from mother to fetus 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 distorts shape of red blood cells can result in occlusion of small blood vessels ↓ affinity for O2