Guberman-Pfeffer Matthew J, Herron Caleb L
Department of Chemistry and Biochemistry, Baylor University, Waco, TX, United States.
Front Chem. 2025 Mar 12;13:1549441. doi: 10.3389/fchem.2025.1549441. eCollection 2025.
Mineral-respiring microorganisms from hydrothermal vents to terrestrial soils express filaments that electrically connect intracellular respiration to extracellular geochemistry. Filaments dubbed "cytochrome nanowires" (CNs) have been resolved by CryoEM, but whether they are the two-decades-long sought-after physiological "nanowires" remains unproven. To assess their functional competence, we analyzed biological redox conduction in all CNs by computing driving forces in the presence of redox anti-cooperativities, reorganization energies with electronic polarizability, and Marcus rates for diffusive and protein-limited flux models. The chain of heme cofactors in any CN must be densely packed to realize weak (≤0.01 eV) electronic coupling for electron transfer, as evidenced by a single Soret band produced from coincidental absorptions on multiple hemes. Dense packing, in turn, has three consequences: (1) limited driving forces (≤|0.3| eV) due to shared electrostatic microenvironments, (2) strong (≤0.12 eV) redox anti-cooperativities that would accentuate the free energy landscape if the linear heme arrangement did not dictate a contra-thermodynamic oxidation order, and (3) an entropic penalty that is offset by thioether 'tethers' of the hemes to the protein backbone. These linkages physically necessitate the rate-throttling T-stacked motif (10-fold slower than the other highly conserved slip-stacked motif). If the sequence of slip- and T-stacked hemes in the CNs had the fastest known nanosecond rates at every step, a micron-long filament would carry a diffusive 0.02 pA current at a physiological 0.1 V, or a protein-limited current of 0.2 pA. Actual CNs have sub-optimal (≤10-fold lower), but sufficient conductivities for cellular respiration, with at most thousands of filaments needed for total cellular metabolic flux. Reported conductivities once used to argue for metallic-like pili against the cytochrome hypothesis and now attributed to CNs remain inconsistent by 10-10-fold with the physical constraints on biological redox conduction through multiheme architectures.
从热液喷口到陆地土壤的矿物呼吸微生物会表达出丝状结构，这些丝状结构将细胞内呼吸与细胞外地球化学进行电连接。被称为“细胞色素纳米线”（CNs）的丝状结构已通过冷冻电镜解析出来，但它们是否就是长达二十年一直寻找的生理“纳米线”仍未得到证实。为了评估它们的功能能力，我们通过计算存在氧化还原反协同性时的驱动力、具有电子极化率的重组能以及扩散和蛋白质限制通量模型的马库斯速率，来分析所有CNs中的生物氧化还原传导。任何CNs中的血红素辅因子链必须紧密堆积，以实现用于电子转移的弱（≤0.01电子伏特）电子耦合，这由多个血红素上的巧合吸收产生的单个索雷特带证明。紧密堆积反过来会产生三个结果：（1）由于共享静电微环境导致驱动力有限（≤|0.3|电子伏特），（2）强（≤0.12电子伏特）的氧化还原反协同性，如果线性血红素排列不规定反热力学氧化顺序，这将加剧自由能态势，（3）一种熵罚，由血红素与蛋白质主链的硫醚“系链”抵消。这些连接在物理上必然导致限速的T堆叠基序（比其他高度保守的滑动堆叠基序慢10倍）。如果CNs中滑动和T堆叠血红素的序列在每一步都具有已知最快的纳秒速率，那么一根微米长的细丝在生理0.1伏特下将携带扩散的0.02皮安电流，或蛋白质限制电流0.2皮安。实际的CNs具有次优（≤低10倍）但足以进行细胞呼吸的电导率，细胞总代谢通量最多需要数千根细丝。曾经用于支持类金属菌毛反对细胞色素假说、现在归因于CNs的报道电导率与通过多血红素结构进行生物氧化还原传导的物理限制相比，仍然相差10 - 10倍。
