Cooperative binding refers to a process where the addition of a substance to one subunit increases the affinity of which type of other subunit?

Cooperative binding is a phenomenon commonly observed in multi-subunit proteins, such as hemoglobin, where the binding of a ligand (often a substrate) to one subunit influences the binding affinity of other subunits for the same ligand. When a ligand, such as oxygen, binds to one subunit of hemoglobin, it induces a conformational change in that subunit. This change, in turn, increases the affinity of the remaining unoccupied subunits for the same ligand. As a result, the addition of more of the same substance leads to an enhanced tendency for the remaining subunits to bind it as well, facilitating the process of saturation at higher ligand concentrations.

In the context of this question, the emphasis is on how the binding of the initial ligand positively influences the binding of subsequent molecules of the same type to the remaining subunits in the protein complex. This cooperativity is essential in biological systems, as it allows for more efficient uptake of ligands, particularly in oxygen transport in blood.