Zhou Huan-Xiang
Department of Physics, Florida State University, Tallahassee, FL 32306, USA.
J Mol Biol. 2003 Sep 5;332(1):257-64. doi: 10.1016/s0022-2836(03)00886-6.
Circular versions of a large number of proteins have been designed by connecting the N and C termini via peptide linkers. A motivation for these designs is the assumed enhancement in folding stability, because backbone cyclization reduces the chain entropy of the unfolded state. Here, it is recognized that backbone cyclization also reduces the chain entropy of a flexible peptide linker in the folded state. Specifically, the end-to-end distance of the linker is restricted to fluctuations around the average displacement between the N and C termini of the folded protein. The balance of the chain-entropy reductions in the folded and unfolded states is used to predict the change in the unfolding free energy, deltadeltaG(cycl), by backbone cyclization. Predicted values of deltadeltaG(cycl) are in quantitative agreement with results of a careful study on cyclizing the 34 residue PIN1 WW domain by linkers with two to seen residues. The experimental results of an optimal linker length l=4 and a maximum stabilization of 1.7 kcal/mol are reproduced. Calculations of deltadeltaG(cycl) for a broad selection of circular proteins suggest that the stabilizing effect of backbone cyclization is modest, reflecting entropy reductions in both the unfolded and the folded states.
通过肽接头连接N端和C端，已经设计出了大量蛋白质的环状变体。这些设计的一个动机是假定折叠稳定性会增强，因为主链环化降低了未折叠状态的链熵。在此，人们认识到主链环化也会降低折叠状态下柔性肽接头的链熵。具体而言，接头的端到端距离被限制在折叠蛋白的N端和C端之间平均位移附近的波动范围内。利用折叠态和未折叠态链熵降低的平衡来预测主链环化引起的解折叠自由能变化ΔΔG(环化)。预测的ΔΔG(环化)值与通过含有2至7个残基的接头环化34个残基的PIN1 WW结构域的仔细研究结果在定量上一致。再现了最佳接头长度l = 4和最大稳定化1.7千卡/摩尔的实验结果。对多种环状蛋白质的ΔΔG(环化)计算表明，主链环化的稳定作用适中，反映了未折叠态和折叠态的熵降低。
