<Dl> <Dd> CO + H O → H CO → HCO + H </Dd> </Dl> <Dd> CO + H O → H CO → HCO + H </Dd> <P> Hence, blood with high carbon dioxide levels is also lower in pH (more acidic). Hemoglobin can bind protons and carbon dioxide, which causes a conformational change in the protein and facilitates the release of oxygen . Protons bind at various places on the protein, while carbon dioxide binds at the α - amino group . Carbon dioxide binds to hemoglobin and forms carbaminohemoglobin . This decrease in hemoglobin's affinity for oxygen by the binding of carbon dioxide and acid is known as the Bohr effect . The Bohr effect favors the T state rather than the R state . (shifts the O - saturation curve to the right). Conversely, when the carbon dioxide levels in the blood decrease (i.e., in the lung capillaries), carbon dioxide and protons are released from hemoglobin, increasing the oxygen affinity of the protein . A reduction in the total binding capacity of hemoglobin to oxygen (i.e. shifting the curve down, not just to the right) due to reduced pH is called the root effect . This is seen in bony fish . </P> <P> It is necessary for hemoglobin to release the oxygen that it binds; if not, there is no point in binding it . The sigmoidal curve of hemoglobin makes it efficient in binding (taking up O in lungs), and efficient in unloading (unloading O in tissues). </P>

The iron-containing portion of the hemoglobin molecule is called