Autocatalyzed protein folding

Proline isomerization, an intrinsically slow process, kinetically traps intermediates in slow protein folding reactions. Thus, enzymes that catalyze proline isomerization (prolyl isomerases) often catalyze protein folding. We have investigated the folding kinetics of FKBP, a prolyl isomerase. The main conclusion is that FKBP catalyzes its own folding. Altogether, the FKBP refolding kinetics are resolved into three exponential phases: a fast phase, τ₃; an intermediate phase, τ₂; and a slow phase, τ₁. Unfolding occurs in a single phase, the unfolding branch of phase τ₂. In the presence of native FKBP, both the intermediate (τ₂) and slow (τ₁) phases are faster, suggesting that folding phases τ₁ and τ₂ involve proline cis-trans isomerization. In the absence of added native FKBP, autocatalytic folding of FKBP is detected. For refolding starting with all the FKBP unfolded initially, the slowest folding phase (τ₁) is almost 2-fold faster at a final concentration of 14 μM FKBP than at 2 μM FKBP, suggesting that catalytically active FKBP formed in the fast (τ₃) or intermediate (τ₂) folding phases catalyzes the slow folding phase (τ₁). Moreover, autocatalysis of folding is inhibited by FK506, an inhibitor of the FKBP prolyl isomerase activity. The results show that the slow phase in FKBP folding is an autocatalyzed formation of native FKBP from kinetically trapped species with non-native proline isomers. While the magnitude of the catalytic effects reported here are modest, FKBP folding may provide a prototype for autocatalysis of kinetically trapped macromolecular conformational changes in other systems.