Equilibrium partially folded states of B. licheniformis β -lactamase

β -Lactamases (penicillinases) facilitate bacterial resistance to antibiotics and are excellent theoretical and experimental models in protein structure, dynamics and evolution. Bacillus licheniformis exo-small penicillinase (ESP) is a Class A β -lactamase with three tryptophan residues located one in each of its two domains and one in the interface between domains. The conformational landscape of three well-characterized ESP Trp → Phe mutants was characterized in equilibrium unfolding experiments by measuring tryptophan fluorescence, far-UV CD, activity, hydrodynamic radius, and limited proteolysis. The Trp → Phe substitutions had little impact on the native conformation, but changed the properties of the partially folded states populated at equilibrium. The results were interpreted in the framework of modern theories of protein folding.