The folding reaction of a stable monomeric variant of Cu/Zn superoxide dismutase (mSOD1), an enzyme responsible for the conversion of superoxide free radicals into hydrogen peroxide and oxygen, is known to be among the slowest folding processes that adhere to two-state behavior. The long lifetime, ∼10 s, of the unfolded state presents ample opportunities for the polypeptide chain to transiently sample nonnative structures before the formation of the productive folding transition state. We recently observed the formation of a nonnative structure in a peptide model of the C-terminus of SOD1, a sequence that might serve as a potential source of internal chain friction-limited folding. To test for friction-limited folding, we performed a comprehensive thermodynamic and kinetic analysis of the folding mechanism of mSOD1 in the presence of the viscogens glycerol and glucose. Using a, to our knowledge, novel analysis of the folding reactions, we found the disulfide-reduced form of the protein that exposes the C-terminal sequence, but not its disulfide-oxidized counterpart that protects it, experiences internal chain friction during folding. The sensitivity of the internal friction to the disulfide bond status suggests that one or both of the cross-linked regions play a critical role in driving the friction-limited folding. We speculate that the molecular mechanisms giving rise to the internal friction of disulfide-reduced mSOD1 might play a role in the amyotrophic lateral sclerosis-linked aggregation of SOD1.

中文翻译：

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二硫化物还原单体SOD1的摩擦限制折叠

众所周知，Cu/Zn 超氧化物歧化酶 (mSOD1) 是一种负责将超氧自由基转化为过氧化氢和氧气的酶，其稳定的单体变体的折叠反应是遵循两种状态行为的最慢折叠过程之一. 未折叠状态的长寿命（~10 s）为多肽链提供了充足的机会，可以在生产性折叠过渡态形成之前对非天然结构进行瞬时采样。我们最近观察到 SOD1 C 末端肽模型中非天然结构的形成，该序列可能作为内部链摩擦限制折叠的潜在来源。为了测试摩擦限制折叠，我们在粘胶原甘油和葡萄糖存在下对 mSOD1 的折叠机制进行了全面的热力学和动力学分析。据我们所知，使用对折叠反应的新分析，我们发现暴露 C 末端序列的蛋白质的二硫键还原形式，而不是保护它的二硫键氧化对应物，在折叠过程中经历内部链摩擦。内摩擦对二硫键状态的敏感性表明，一个或两个交联区域在驱动摩擦限制折叠中起关键作用。我们推测引起二硫键减少的mSOD1内摩擦的分子机制可能在肌萎缩侧索硬化相关的SOD1聚集中起作用。对折叠反应的新颖分析，我们发现暴露 C 末端序列的蛋白质的二硫键还原形式，但不是保护它的二硫键氧化对应物，在折叠过程中经历内部链摩擦。内摩擦对二硫键状态的敏感性表明，一个或两个交联区域在驱动摩擦限制折叠中起关键作用。我们推测引起二硫键减少的mSOD1内摩擦的分子机制可能在肌萎缩侧索硬化相关的SOD1聚集中起作用。对折叠反应的新颖分析，我们发现暴露 C 末端序列的蛋白质的二硫键还原形式，但不是保护它的二硫键氧化对应物，在折叠过程中经历内部链摩擦。内摩擦对二硫键状态的敏感性表明，一个或两个交联区域在驱动摩擦限制折叠中起关键作用。我们推测引起二硫键减少的mSOD1内摩擦的分子机制可能在肌萎缩侧索硬化相关的SOD1聚集中起作用。内摩擦对二硫键状态的敏感性表明，一个或两个交联区域在驱动摩擦限制折叠中起关键作用。我们推测引起二硫键减少的mSOD1内摩擦的分子机制可能在肌萎缩侧索硬化相关的SOD1聚集中起作用。内摩擦对二硫键状态的敏感性表明，一个或两个交联区域在驱动摩擦限制折叠中起关键作用。我们推测引起二硫键减少的mSOD1内摩擦的分子机制可能在肌萎缩侧索硬化相关的SOD1聚集中起作用。