Sohl J L, Jaswal S S, Agard D A
Graduate Group in Biophysics, Howard Hughes Medical Institute, University of California at San Francisco, 94143-0448, USA.
Nature. 1998 Oct 22;395(6704):817-9. doi: 10.1038/27470.
alpha-Lytic protease (alphaLP), an extracellular bacterial protease, is synthesized with a large amino-terminal pro-region that is essential for its folding in vivo and in vitro. In the absence of the pro-region, the protease folds to an inactive, partially folded state, designated 'I'. The pro-region catalyses protease folding by directly stabilizing the folding transition state (>26kcal mol(-1)) which separates the native state 'N' from I. Although a basic tenet of protein folding is that the native state of a protein is at the minimum free energy, we show here that both the I and fully unfolded states of alphaLP are lower in free energy than the native state. Native alphaLP is thus metastable: its apparent stability derives from a large barrier to unfolding. Consequently, the evolution of alphaLP has been distinct from most other proteins: it has not been constrained by the free-energy difference between the native and unfolded states, but instead by the size of its unfolding barrier.
α-裂解蛋白酶（αLP）是一种细胞外细菌蛋白酶，其合成时带有一个大的氨基末端前区，该前区对于其在体内和体外的折叠至关重要。在没有前区的情况下，蛋白酶折叠成一种无活性的、部分折叠的状态，称为“I”。前区通过直接稳定将天然状态“N”与I分开的折叠过渡态（>26千卡摩尔(-1)）来催化蛋白酶折叠。尽管蛋白质折叠的一个基本原理是蛋白质的天然状态处于最低自由能，但我们在此表明，αLP的I态和完全未折叠态的自由能都低于天然状态。因此，天然αLP是亚稳态的：其明显的稳定性源于展开的巨大障碍。因此，αLP的进化与大多数其他蛋白质不同：它不受天然态和未折叠态之间自由能差异的限制，而是受其展开障碍大小的限制。
