1. under appropriate conditions, hemoglobin dissociates into its four subunits. the isolated α subunit binds oxygen, but the o2-saturation curve is hyperbolic rather than sigmoid. in addition, the binding of oxygen to the isolated α subunit is not affected by the presence of h+, co2, or bpg. what do these observations indicate about the source of the cooperativity in hemoglobin?

Haemoglobin is an oxygen-binding protein comprising four peptide subunits: two α subunits and two β subunits. The classic sigmoidal oxygen disassociation curve shows haemoglobin’s percentage saturation at different oxygen concentrations (partial pressures). The equilibrium between oxyhaemoglobin (haemoglobin in its oxygen bound state and haemoglobin (free of oxygen molecules). Each molecule of haemoglobin has the ability to bind up to four oxygen molecules at a time; this occurs via cooperative binding-  meaning that with increased haemoglobin- oxygen binding at the protein haemoglobin’s subunits (where oxygen is a ligand) there is an increase in its affinity for oxygen.

This is because there is a conformational change in the haemoglobin molecule due to the altered orientation of the protein’s secondary structures, making it easier for a second molecule of oxygen to successively followed by more -Haem binding until the molecule is saturated- at this point the curve plateaus, The saturation curve for the disassociated α haemoglobin illustrates a rapid oxygen-haemoglobin binding and saturation. concentration-dependent saturation of this monomeric oxygen-binding protein.

The production of a hyperbolic curve illustrates that the subunit has been saturated with oxygen, i.e. it has reached its carrying limit. It is unaffected by the other molecules because the enzyme is showing specificity towards its substrate, oxygen. Additionally, this illustrates Haemoglobin’s allosteric ability to cooperatively bind to O2 stems from its quarterly structure; the oxygen interacts with the iron atom within its prosthetic groups. The conformational change in the haemoglobin molecule due to the altered orientation of the protein’s secondary structures via interactions between its histidine residues and heme rings; these result in new subunit binding interactions including new hydrogen bonding, hydrophobic interactions which together contribute to an altered quaternary structure.

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