$Hb{O_2} + {H_3}{O^ + }C{O_2} \rightleftharpoons \left( {{H^ + } - Hb - C{O_2}} \right) + {O_2} + {H_2}O$
Release of $O_2$ is favoured when there is
$\begin{align}
\begin{matrix}
C{{H}_{3}}-CH-C{{O}_{2}}H \\
|\,\,\,\,\,\,\,\, \\
\underset{\oplus }{\mathop{N}}{{H}_{3}} \\
\end{matrix}\,\,\,\,\,\,p{{K}_{a}}=2.2 \\
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,p{{K}_{b}}=4.4 \\
\end{align}$