The redox potential values of cytochromes can be modulated by the protonation/deprotonation of neighbor groups (redox-Bohr effect), a mechanism that permits the proteins to couple electron/proton transfer. In the respiratory chains, this effect is particularly relevant if observed in the physiological pH range, as it may contribute to the electrochemical gradient for ATP synthesis. A constitutively produced family of five triheme cytochromes (PpcA−E) from the bacterium Geobacter sulfurreducens plays a crucial role in extracellular electron transfer, a hallmark that permits this bacterium to be explored for several biotechnological applications. Two members of this family (PpcA and PpcD) couple electron/proton transfer in the physiological pH range, a feature not shared with PpcB and PpcE. That ability is crucial for G. sulfurreducens’ growth in Fe(III)-reducing habitats since extra contributors to the electrochemical gradient are needed. It was postulated that the redox-Bohr effect is determined by the nature of residue 6, a leucine in PpcA/PpcD and a phenylalanine in PpcB/PpcE. To confirm this hypothesis, Phe6 was replaced by leucine in PpcB and PpcE. The functional properties of these mutants were investigated by NMR and UV–visible spectroscopy to assess their capability to couple electron/proton transfer in the physiological pH range. The results obtained showed that the mutants have an increased redox-Bohr effect and are now capable of coupling electron/proton transfer. This confirms the determinant role of the nature of residue 6 in the modulation of the redox-Bohr effect in this family of cytochromes, opening routes to engineer Geobacter cells with improved biomass production.

中文翻译：

---

外生电细菌Geobacter silverreducens电子/质子转移机制的合理设计

细胞色素的氧化还原电位值可以通过相邻基团的质子化/去质子化（氧化还原-玻尔效应）进行调节，这是一种允许蛋白质耦合电子/质子转移的机制。在呼吸链中，如果在生理 pH 范围内观察到这种效应，则这种效应特别重要，因为它可能有助于 ATP 合成的电化学梯度。来自细菌硫还原地杆菌的五个三血红素细胞色素 (PpcA-E) 的组成型家族在细胞外电子转移中起着至关重要的作用，这一标志使这种细菌可以用于多种生物技术应用。该家族的两个成员（PpcA 和 PpcD）在生理 pH 范围内耦合电子/质子转移，这是 PpcB 和 PpcE 不共享的特征。这种能力对 G 来说至关重要。由于需要额外的电化学梯度贡献者，因此硫还原菌在 Fe(III) 还原生境中的生长。据推测，氧化还原-玻尔效应由残基 6、PpcA/PpcD 中的亮氨酸和 PpcB/PpcE 中的苯丙氨酸的性质决定。为了证实这一假设，Phe6 被 PpcB 和 PpcE 中的亮氨酸取代。通过核磁共振和紫外-可见光谱研究了这些突变体的功能特性，以评估它们在生理 pH 范围内耦合电子/质子转移的能力。获得的结果表明突变体具有增强的氧化还原-玻尔效应，现在能够耦合电子/质子转移。这证实了残基 6 的性质在调节该细胞色素家族中氧化还原-玻尔效应中的决定性作用，