Final answer:
The two critical residues in the O₂ binding site that do not change among three polypeptides are histidine and phenylalanine, both crucial for oxygen transport proteins' function. Serine, threonine, and tyrosine can be phosphorylated due to the hydroxyl group they contain, necessary for various biological functions.
Step-by-step explanation:
The critical residues in the O₂ binding site that do not change among the three polypeptides are histidine and phenylalanine. These amino acids are essential for the proper function of proteins involved in oxygen transportation, such as hemoglobin. Histidine plays a crucial role in binding to the iron of the heme group, while phenylalanine maintains the structural integrity necessary for high oxygen affinity.
Additionally, the property that enables the residues of the amino acids serine, threonine, and tyrosine to be phosphorylated is that they contain a hydroxyl group (option 'c'). This hydroxyl group can be enzymatically phosphorylated by ATP, which is significant in the context of signaling proteins and various biological processes.