Speciation-Dependent Molecular Mechanism of Electron Transfer from the c-Type Cytochrome MtrC to U(VI)-Ligand Complexes

Earth Sciences Chemistry Geochemistry Inorganic Chemistry Environmental Chemistry Life Sciences Microbiology
c-type cytochrome electron transfer reduction kinetics NMR HR-XANES

Uranium (U) is a radioactive element widely present in the environment, primarily existing in its hexavalent (U(VI)) and tetravalent (U(IV)) oxidation states. Under oxidizing conditions, U(VI) is the dominant stable form, while under reducing conditions, U(VI) can be reduced to U(IV). This reduction process can occur through abiotic pathways (e.g., iron- or sulfide-bearing minerals) or biological pathways (e.g., bacteria). In particular, bacteria of the Shewanella genus can transfer electrons to metals and radionuclides, such as U(VI), via c-type cytochromes. Although the intracellular electron transfer mechanism has been extensively studied, the process by which electrons are delivered to external electron acceptors (e.g., U(VI)) remains unclear.

MtrC is a decaheme c-type cytochrome located on the outer membrane surface of Shewanella bacteria, capable of transferring electrons to U(VI). However, the mechanism of electron transfer between MtrC and U(VI), particularly the type of interaction between them and the parameters controlling electron transfer, has not been fully elucidated. To uncover this mechanism, researchers conducted an in-depth study on the reduction kinetics of U(VI) complexes bound to different ligands (e.g., carbonate, hydroxyl, citrate, nitrilotriacetic acid (NTA), and ethylenediaminetetraacetic acid (EDTA)).

Source of the Paper

This paper was co-authored by Margaux Molinas, Karin Lederballe Meibom, Ashley Brown, Luciano A. Abriata, Tim Prüßmann, and Rizlan Bernier-Latmani, affiliated with the Environmental Microbiology Laboratory, the Protein Production and Structure Core Facility at the Swiss Federal Institute of Technology Lausanne (EPFL), and the Institute for Nuclear Waste Disposal at the Karlsruhe Institute of Technology (KIT), Germany. The paper was published in 2025 in the journal Geo-Bio Interfaces, titled “Speciation-Dependent Molecular Mechanism of Electron Transfer from the c-Type Cytochrome MtrC to U(VI)-Ligand Complexes.”

Research Process and Results

1. Experimental Design and Workflow

The primary goal of the study was to reveal the electron transfer mechanism between MtrC and different U(VI)-ligand complexes. To this end, researchers designed a series of experiments, including reduction kinetics experiments, nuclear magnetic resonance (NMR) spectroscopy, and high-resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy.

a) Reduction Kinetics Experiments

Researchers first investigated the reduction kinetics of U(VI) complexes bound to five different ligands (carbonate, hydroxyl, citrate, NTA, and EDTA) with MtrC from Shewanella baltica. In the experiments, reduced MtrC was reacted with U(VI)-ligand complexes under anoxic conditions, and the reaction progress was monitored using ion-exchange chromatography (IEC) and inductively coupled plasma mass spectrometry (ICP-MS). The results showed significant differences in the reduction rates of U(VI) complexes bound to different ligands. Specifically, U(VI)-EDTA exhibited the fastest reduction rate, while U(VI)-carbonate and U(VI)-hydroxyl showed slower reduction rates.

b) Nuclear Magnetic Resonance (NMR) Spectroscopy

To further elucidate the interaction between MtrC and U(VI)-ligand complexes, researchers conducted NMR spectroscopy. By mixing oxidized MtrC with U(VI)-ligand complexes, researchers observed chemical shift changes in the heme region of MtrC. These changes indicated that different U(VI)-ligand complexes interacted with MtrC in distinct ways. For example, U(VI)-carbonate and U(VI)-hydroxyl may interact with MtrC through covalent or hydrogen bonding involving the U atom, while U(VI)-NTA and U(VI)-EDTA may interact with MtrC through electrostatic or hydrogen bonding involving the ligand.

c) High-Resolution X-ray Absorption Near-Edge Structure (HR-XANES) Spectroscopy

To determine the oxidation state changes during U(VI) reduction, researchers used HR-XANES spectroscopy. The results showed that in the U(VI)-carbonate system, the U(V) intermediate persisted during the reaction, suggesting that U(V) may play an important role in the electron transfer process. Additionally, U(IV) products were tightly bound to MtrC in the U(VI)-carbonate system, while in the U(VI)-NTA and U(VI)-EDTA systems, U(IV) products remained in a dissolved state.

2. Key Results and Conclusions

Through the above experiments, researchers drew the following main conclusions:

  1. Reduction Rate Depends on Ligand Type: The reduction rates of U(VI) complexes bound to different ligands varied significantly. U(VI)-EDTA exhibited the fastest reduction rate, while U(VI)-carbonate and U(VI)-hydroxyl showed slower reduction rates. This difference may be related to the interaction mode between the ligand and MtrC.

  2. Different Interaction Mechanisms: U(VI)-carbonate and U(VI)-hydroxyl may interact with MtrC through covalent or hydrogen bonding involving the U atom, while U(VI)-NTA and U(VI)-EDTA may interact with MtrC through electrostatic or hydrogen bonding involving the ligand.

  3. Role of the U(V) Intermediate: In the U(VI)-carbonate system, the U(V) intermediate persisted during the reaction, indicating that U(V) may play a crucial role in the electron transfer process.

  4. Form of U(IV) Products: In the U(VI)-carbonate system, U(IV) products were tightly bound to MtrC, while in the U(VI)-NTA and U(VI)-EDTA systems, U(IV) products remained in a dissolved state.

3. Significance and Value of the Study

This study is the first to systematically reveal the electron transfer mechanism between MtrC and different U(VI)-ligand complexes. The results demonstrate that ligand type not only affects the reduction rate of U(VI) but also determines the interaction mode between MtrC and U(VI) complexes. These findings are of great significance for understanding the bacterial uranium reduction process in the environment, particularly in applications such as nuclear waste treatment and environmental pollution remediation.

4. Highlights of the Study

  1. Multimethod Approach: This study combined reduction kinetics, NMR spectroscopy, and HR-XANES spectroscopy to comprehensively reveal the electron transfer mechanism between MtrC and U(VI)-ligand complexes.

  2. Discovery of the U(V) Intermediate: The study is the first to identify the role of the U(V) intermediate in the U(VI) reduction process, providing new insights into the uranium reduction mechanism.

  3. Application Potential: The results have significant application potential in nuclear waste treatment and environmental pollution remediation, particularly in the development of microbial uranium reduction technologies.

Summary

Through a multimethod approach, this study systematically revealed the electron transfer mechanism between MtrC and different U(VI)-ligand complexes. The results demonstrate that ligand type not only affects the reduction rate of U(VI) but also determines the interaction mode between MtrC and U(VI) complexes. These findings not only deepen our understanding of the bacterial uranium reduction mechanism but also provide new perspectives for nuclear waste treatment and environmental pollution remediation.