Effect of a specific inhibitor on the unfolding and refolding kinetics of dimeric triosephosphate isomerase: establishing the dimeric and similarly structured nature of the main transition states on the forward and backward reactions.

2-Phosphoglycolate (PGA), a strong competitive inhibitor of the dimeric enzyme triosephosphate isomerase (TIM), brings about a large decrease in the unfolding rate constant of the protein. The data set of rate constants versus ligand concentration may be equally well explained by regarding either a monomeric or a dimeric transition state (TS). However, if the thermodynamics for binding of PGA to native TIM is taken into account, it becomes clear that a dimeric TS is the right assumption. Furthermore, by studying the effect of the ligand on the second-order refolding reaction, we found results indicating similar PGA-binding affinities to be present in the transition states for the rate-limiting steps of the forward and backward reactions. Most likely, therefore, both TS resemble each other in respect to the active site architecture. It should be mentioned, however, that our data do not rule out the possible occurrence of an unstable, (partially) folded monomeric intermediate, which would rapidly interconvert with the unfolded monomer.