ENERGETICS OF FOLDING SUBTILISIN BPN'

Subtilisin is an unusual example of a monomeric protein with a substantial kinetic barrier to folding and unfolding. Here we document for the first time the in vitro folding of the mature form of subtilisin. Subtilisin was modified by site-directed mutagenesis to be proteolytically inactive, allowing the impediments to folding to be systematically examined. First, the thermodynamics and kinetics of calcium binding to the high-affinity calcium A-site have been measured by microcalorimetry and fluorescence spectroscopy. Binding is an enthalpically driven process with an association constant (K(a)) equal to 7 X 10(6) M-1. Furthermore, the kinetic barrier to calcium removal from the A-site (23 kcal/mol) is substantially larger than the standard free energy of binding (9.3 kcal/mol). The kinetics of calcium dissociation from subtilisin (e.g., in excess EDTA) are accordingly very slow (t1/2 = 1.3 h at 25-degrees-C). Second, to measure the kinetics of subtilisin folding independent of calcium binding, the high-affinity calcium binding site was deleted from the protein. At low ionic strength (I = 0.01) refolding of this mutant requires several days. The folding rate is accelerated almost 100-fold by a 10-fold increase in ionic strength, indicating that part of the free energy of activation may be electrostatic. At relatively high ionic strength (I = 0.5) refolding of the mutant subtilisin is complete in less than 1 h at 25-degrees-C. We suggest that part of the electrostatic contribution to the activation free energy for folding subtilisin is related to the highly charged region of the protein comprising the weak ion binding site (site B). The coalescing of the charged amino acids to form this ion binding site may present an electrostatic barrier to folding unless the charges can be shielded as they are brought into proximity during folding. A high kinetic barrier between the folded and unfolded states may be an essential characteristic of extracellular, microbial proteases to prevent autodegradation. Since this kinetic barrier is also an impediment to folding, the pro sequence may function in vivo to catalyze the folding reaction.