In terms of substrate binding, what effect describes the increase in binding affinity due to substrate binding to one subunit of a protein?

The concept described in the question pertains to hemoglobin and other multimeric proteins that exhibit a phenomenon known as "cooperative binding." When one subunit of a protein that can bind a substrate (like oxygen in the case of hemoglobin) binds to that substrate, it induces a conformational change in the protein. This change enhances the binding affinity for the substrate at the remaining subunits.

This behavior is characterized as a homotropic effect because it involves the same substrate molecule promoting enhanced binding at other sites within the same protein complex. Essentially, the binding of the substrate to one site increases the likelihood of binding at other sites, which is a hallmark of cooperative binding.

The other options refer to different concepts: the heterotropic effect involves a different ligand affecting the binding affinity of the substrate, while the allosteric effect generally describes changes in activity or affinity due to binding at regulatory sites rather than active sites. Thus, the correct answer is indeed the homotropic effect, as it specifically addresses the phenomenon of affinity increase due to substrate binding across multiple sites on the same protein.