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Folding Mechanism of an Extremely Thermostable (βα)8-Barrel Enzyme: A High Kinetic Barrier Protects the Protein from Denaturation

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journal contribution
posted on 24.04.2012, 00:00 by Linn Carstensen, Gabriel Zoldák, Franz-Xaver Schmid, Reinhard Sterner
HisF, the cyclase subunit of imidazole glycerol phosphate synthase (ImGPS) from Thermotoga maritima, is an extremely thermostable (βα)8-barrel protein. We elucidated the unfolding and refolding mechanism of HisF. Its unfolding transition is reversible and adequately described by the two-state model, but 6 weeks is necessary to reach equilibrium (at 25 °C). During refolding, initially a burst-phase off-pathway intermediate is formed. The subsequent productive folding occurs in two kinetic phases with time constants of ∼3 and ∼20 s. They reflect a sequential process via an on-pathway intermediate, as revealed by stopped-flow double-mixing experiments. The final step leads to native HisF, which associates with the glutaminase subunit HisH to form the functional ImGPS complex. The conversion of the on-pathway intermediate to the native protein results in a 106-fold increase of the time constant for unfolding from 89 ms to 35 h (at 4.0 M GdmCl) and thus establishes a high energy barrier to denaturation. We conclude that the extra stability of HisF is used for kinetic protection against unfolding. In its refolding mechanism, HisF resembles other (βα)8-barrel proteins.