Miller Shaylynn D, Ford Kathryne C, Gruenberg Cross Megan C, TerAvest Michaela A
Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI, USA.
Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI, USA.
Biotechnol Biofuels Bioprod. 2025 Jul 11;18(1):72. doi: 10.1186/s13068-025-02666-x.
As outlined by the Intergovernmental Panel on Climate Change, we need to approach global net zero CO emissions by approximately 2050 to prevent warming beyond 1.5 °C and the associated environmental tipping points. Future microbial electrosynthesis (MES) systems could decrease net CO emissions by capturing it from industrial sources. MES is a process where electroactive microorganisms convert the carbon from CO and reduction power from a cathode into reduced organic compounds. However, no MES system has attained an efficiency compatible with a financially feasible scale-up. To improve MES efficiency, we need to consider the energetic constraints of extracellular electron uptake (EEU) from an electrode to cytoplasmic electron carriers like NAD. In many microbes, EEU to the cytoplasm must pass through the respiratory quinone pool (Q-pool). However, electron transfer from the Q-pool to cytoplasmic NAD is thermodynamically unfavorable. Here, we model the thermodynamic barrier for Q-pool dependent EEU using the well-characterized bidirectional electron transfer pathway of Shewanella oneidensis, which has NADH dehydrogenases that are energetically coupled to proton-motive force (PMF), sodium-motive force (SMF), or uncoupled. We also tested our hypothesis that Q-pool dependent EEU to NAD is ion-motive force (IMF)-limited in S. oneidensis expressing butanediol dehydrogenase (Bdh), a heterologous NADH-dependent enzyme. We assessed membrane potential changes in S. oneidensis + Bdh on a cathode at the single-cell level pre to post injection with acetoin, the substrate of Bdh.
We modeled the Gibbs free energy change for electron transfer from respiratory quinones to NADH under conditions reflecting changes in membrane potential, pH, reactant to product ratio, and energetically coupled IMF. Of the 40 conditions modeled for each method of energetic coupling (PMF, SMF, and uncoupled), none were thermodynamically favorable without PMF or SMF. We also found that membrane potential decreased upon initiation of EEU to NAD for S. oneidensis on a cathode.
Our results suggest that Q-pool-dependent EEU is both IMF-dependent and is IMF-limited in a proof-of-concept system. Because microbes that rely on Q-pool-dependent EEU are among the most genetically tractable and metabolically flexible options for MES systems, it is important that we account for this thermodynamic bottleneck in future MES platform designs.
正如政府间气候变化专门委员会所概述的那样，我们需要在2050年左右实现全球二氧化碳净零排放，以防止气温上升超过1.5摄氏度以及相关的环境临界点。未来的微生物电合成（MES）系统可以通过从工业源捕获二氧化碳来减少净二氧化碳排放。MES是一个电活性微生物将二氧化碳中的碳和来自阴极的还原力转化为还原有机化合物的过程。然而，没有一个MES系统达到了与经济上可行的扩大规模相兼容的效率。为了提高MES效率，我们需要考虑从电极到细胞质电子载体（如NAD）的细胞外电子摄取（EEU）的能量限制。在许多微生物中，EEU进入细胞质必须通过呼吸醌池（Q池）。然而，从Q池到细胞质NAD的电子转移在热力学上是不利的。在这里，我们使用特征明确的双向电子转移途径——奥奈达希瓦氏菌的双向电子转移途径来模拟Q池依赖性EEU的热力学屏障，该菌具有与质子动力（PMF）、钠动力（SMF）能量耦合或未耦合的NADH脱氢酶。我们还测试了我们的假设，即在表达丁二醇脱氢酶（Bdh）（一种异源NADH依赖性酶）的奥奈达希瓦氏菌中，Q池依赖性EEU到NAD受离子动力（IMF）限制。我们在单细胞水平上评估了在向奥奈达希瓦氏菌+Bdh注入Bdh的底物乙偶姻前后，其在阴极上的膜电位变化。
我们模拟了在反映膜电位、pH值、反应物与产物比例以及能量耦合的IMF变化的条件下，从呼吸醌到NADH的电子转移的吉布斯自由能变化。对于每种能量耦合方法（PMF、SMF和未耦合）所模拟的40种条件中，没有一种在没有PMF或SMF的情况下在热力学上是有利的。我们还发现，在阴极上，奥奈达希瓦氏菌向NAD的EEU开始时，膜电位会降低。
我们的结果表明，在一个概念验证系统中，Q池依赖性EEU既依赖于IMF，又受IMF限制。由于依赖Q池依赖性EEU的微生物是MES系统中遗传上最易处理且代谢最灵活的选择之一很重要，我们在未来的MES平台设计中考虑这个热力学瓶颈。
