Mutational analysis of acylphosphatase suggests the importance of topologyand contact order in protein folding

Muscle acylphosphatase (AcP) is a small protein that folds very slowly with two-state behavior. The conformational stability and the rates of folding and unfolding have been determined for a number of mutants of AcP in order to characterize the structure of the folding transition state. The results show that the transition state is an expanded version of the native protein , where most of the native interactions are partially established. The tran sition state of AcP turns out to be remarkably similar in structure to that of the activation domain of procarboxypeptidase A2 (ADA2h), a protein Ravi ng the same overall topology but sharing only 13% sequence identity with Ac P. This suggests that transition states are conserved between proteins with the same native fold. Comparison of the rates of folding of Acp and four o ther proteins with the same topology, including ADA2h, supports the concept that the average distance in sequence between interacting residues (that i s, the contact order) is an important determinant of the rate of protein fo lding.