Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT 06510.
Microbial Sciences Institute, Yale University, West Haven, CT 06516.
Proc Natl Acad Sci U S A. 2021 Jan 12;118(2). doi: 10.1073/pnas.2014139118.
Proteins are commonly known to transfer electrons over distances limited to a few nanometers. However, many biological processes require electron transport over far longer distances. For example, soil and sediment bacteria transport electrons, over hundreds of micrometers to even centimeters, via putative filamentous proteins rich in aromatic residues. However, measurements of true protein conductivity have been hampered by artifacts due to large contact resistances between proteins and electrodes. Using individual amyloid protein crystals with atomic-resolution structures as a model system, we perform contact-free measurements of intrinsic electronic conductivity using a four-electrode approach. We find hole transport through micrometer-long stacked tyrosines at physiologically relevant potentials. Notably, the transport rate through tyrosines (10 s) is comparable to cytochromes. Our studies therefore show that amyloid proteins can efficiently transport charges, under ordinary thermal conditions, without any need for redox-active metal cofactors, large driving force, or photosensitizers to generate a high oxidation state for charge injection. By measuring conductivity as a function of molecular length, voltage, and temperature, while eliminating the dominant contribution of contact resistances, we show that a multistep hopping mechanism (composed of multiple tunneling steps), not single-step tunneling, explains the measured conductivity. Combined experimental and computational studies reveal that proton-coupled electron transfer confers conductivity; both the energetics of the proton acceptor, a neighboring glutamine, and its proximity to tyrosine influence the hole transport rate through a proton rocking mechanism. Surprisingly, conductivity increases 200-fold upon cooling due to higher availability of the proton acceptor by increased hydrogen bonding.
蛋白质通常被认为只能在有限的几个纳米距离内传递电子。然而，许多生物过程需要电子在更远的距离上传输。例如，土壤和沉积物中的细菌通过富含芳香族残基的丝状蛋白，将电子在数百微米甚至厘米的距离上进行传递。然而，由于蛋白质和电极之间的大接触电阻，真正的蛋白质电导率的测量一直受到干扰。本研究使用具有原子分辨率结构的单个淀粉样蛋白晶体作为模型系统，采用四电极方法进行无接触的固有电子电导率测量。我们发现，在生理相关的电势下，通过微米级长的酪氨酸堆叠进行空穴传输。值得注意的是，酪氨酸的传输速率（10 秒）与细胞色素相当。因此，我们的研究表明，在普通热条件下，淀粉样蛋白可以在不需要氧化还原活性金属辅因子、大驱动力或光敏剂来产生高氧化态以进行电荷注入的情况下，有效地传输电荷。通过测量分子长度、电压和温度对电导率的影响，同时消除接触电阻的主要贡献，我们表明，多步跳跃机制（由多个隧道步骤组成）而不是单步隧道解释了测量的电导率。结合实验和计算研究揭示了质子耦合电子转移赋予了电导率；质子受体（相邻的谷氨酰胺）的能量及其与酪氨酸的接近程度通过质子摆动机制影响空穴传输速率。令人惊讶的是，由于氢键的增加增加了质子受体的可用性，冷却会使电导率增加 200 倍。
