Final answer:
At lower pH levels, Hb has a reduced affinity for O2 due to the Bohr effect, which promotes oxygen dissociation and is beneficial for oxygen delivery to active tissues. The structural changes in Hb due to pH affect its oxygen-binding capabilities. At pH 7.4 Hb has higher affinity for oxygen than at pH 7.2.
Step-by-step explanation:
At a lower pH, hemoglobin (Hb) has a reduced affinity for oxygen, which is reflected in the right shift of the oxygen-hemoglobin dissociation curve due to the Bohr effect. This physiological response is beneficial because it allows more oxygen to be released to tissues where more carbon dioxide is being produced, and thus the pH is lower, such as in active muscles. The structural basis for this is the influence of pH on the hemoglobin molecule's shape and charge, which affects its ability to bind oxygen. Specifically, at a lower pH, the increased concentration of hydrogen ions (H+) promotes the release of oxygen from hemoglobin because hydrogen ions compete with oxygen for binding sites. At a pH of 7.4, hemoglobin has a higher affinity for oxygen compared to a pH of 7.2. The pKa related shift at 2.7 dictates that certain side chains within the protein structure are likely to be protonated and thus affect overall protein structure and function.