Analysis of the stability of multimeric proteins by effective DeltaG and effective m-values.

Analyzing the stability of a multimeric protein is challenging because of the intrinsic difficulty in handling the mathematical model for the folded multimer-unfolded monomer equilibrium. To circumvent this problem, we introduce the concept of effective stability, DeltaGeff (= -RTlnKeff), where Keff is the equilibrium constant expressed in monomer units. Analysis of the denaturant effect on DeltaGeff gives new insight into the stability of multimeric proteins. When a multimeric protein is mostly folded, the dependence of effective stability on denaturant concentration (effective m-value) is simply the m-value of its monomeric unit. However, when the protein is mostly unfolded, its stability depends on denaturant concentration with the m-value of its multimeric form. We also find that the effective m-value at the Cm is a good approximation of the apparent m-value determined by fitting the equilibrium unfolding data from multimeric proteins with a two-state monomer model. Moreover, when the m-value of a monomeric unit is estimated from its size, the effective stability of a multimeric protein can be determined simply from Cm and this estimated m-value. These simple and intuitive approaches will allow a facile analysis of the stability of multimeric proteins. These analyses are also applicable for high-throughput analysis of protein stability on a proteomic scale.