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Molecular Mechanisms of Microbial Extracellular Electron Transfer
Publication . Paquete, Catarina M.; Morgado, Leonor; Salgueiro, Carlos A.; Louro, Ricardo O.; Instituto de Tecnologia Química e Biológica António Xavier (ITQB); UCIBIO - Applied Molecular Biosciences Unit; DQ - Departamento de Química; IMR Press Limited
Extracellular electron transfer is a key metabolic process of many organisms that enables them to exchange electrons with extracellular electron donors/acceptors. The discovery of organisms with these abilities and the understanding of their electron transfer processes has become a priority for the scientific and industrial community, given the growing interest on the use of these organisms in sustainable biotechnological processes. For example, in bioelectrochemical systems electrochemical active organisms can exchange electrons with an electrode, allowing the production of energy and added-value compounds, among other processes. In these systems, electrochemical active organisms exchange electrons with an electrode through direct or indirect mechanisms, using, in most cases, multiheme cytochromes. In numerous electroactive organisms, these proteins form a conductive pathway that allows electrons produced from cellular metabolism to be transferred across the cell surface for the reduction of an electrode, or vice-versa. Here, the mechanisms by which the most promising electroactive bacteria perform extracellular electron transfer will be reviewed, emphasizing the proteins involved in these pathways. The ability of some of the organisms to perform bidirectional electron transfer and the pathways used will also be highlighted.
Uncovering the secrets of Geotalea uraniireducens: a study of key players in uranium bioremediation pathway
Publication . Almeida, Alexandre Santos Aires; Salgueiro, Carlos Alberto Gomes; Silva, Marta Alexandra Fernandes
The Geobacteraceae family has gained recognition for exhibiting remarkable respiratory versatility, which allow them to sustain their growth using different extracellular compounds (including hazardous metals) in contrast to conventional respiratory processes, in which soluble electron donors and acceptors are the protagonists. These extracellular electron transfer (EET) capabilities confer to bacteria the potential of creating impact on biotechnological applications, namely those related to the environment, such as bioremediation and bioenergy production. To optimize these applications, it is imperative to understand how this EET process occurs. Multiheme cytochromes, particularly periplasmic c-type triheme cytochromes, have been identified as essential components for EET mechanism. While significant progress has been made in the characterization of these components in G. sulfurreducens – the first Geobacteraceae bacterium with a sequenced genome and a genetic system developed – their characterization in other species remains unexplored.
This Thesis focuses on the study of the periplasmic triheme cytochromes family (PpcA, PpcB, PpcG, and PpcH), along with another cytochrome (Gura_0469) from Geotalea uraniireducens, an organism commonly found in uranium-contaminated environments that plays a key role in bioremediation by reducing the soluble hexavalent form of uranium (U(VI)) into less soluble forms (e.g., U(IV)). All cytochromes were successfully cloned and heterologously expressed in E. coli, although the expression of PpcH Gu is still in progress. PpcA Gu, PpcB Gu, PpcG Gu, and Gura_0469 were purified using two chromatographic steps and a detailed biochemical and biophysical characterization was performed on PpcA Gu and PpcB Gu. These cytochromes were characterized using UV-visible, NMR, and CD spectroscopy, enabling the determination of their heme core architecture, overall protein fold, reduction potential of each heme group, and intramolecular redox interactions network, as well as their secondary structural elements and thermal stability. Despite being structurally homologous to their counterparts from G. sulfurreducens, the results obtained indicate that they are functionally different. PpcA and PpcB from G. uraniireducens modulate their redox properties in such a way that all hemes exhibit similar reduction potential values at physiological pH. This suggests that the electron transfer does not require targeting a specific region of the protein, allowing an efficient uranium reduction by any of the heme groups.
Exploring oxidative stress pathways in Geobacter sulfurreducens
Publication . Portela, Pilar C.; Morgado, Leonor; Silva, Marta A.; Denkhaus, Lukas; Einsle, Oliver; Salgueiro, Carlos A.; UCIBIO - Applied Molecular Biosciences Unit; DQ - Departamento de Química; Frontiers Research Foundation
The recent reclassification of the strict anaerobe Geobacter sulfurreducens bacterium as aerotolerant brought attention for oxidative stress protection pathways. Although the electron transfer pathways for oxygen detoxification are not well established, evidence was obtained for the formation of a redox complex between the periplasmic triheme cytochrome PpcA and the diheme cytochrome peroxidase MacA. In the latter, the reduction of the high-potential heme triggers a conformational change that displaces the axial histidine of the low-potential heme with peroxidase activity. More recently, a possible involvement of the triheme periplasmic cytochrome family (PpcA-E) in the protection from oxidative stress in G. sulfurreducens was suggested. To evaluate this hypothesis, we investigated the electron transfer reaction and the biomolecular interaction between each PpcA-E cytochrome and MacA. Using a newly developed method that relies on the different NMR spectral signatures of the heme proteins, we directly monitored the electron transfer reaction from reduced PpcA-E cytochromes to oxidized MacA. The results obtained showed a complete electron transfer from the cytochromes to the high-potential heme of MacA. This highlights PpcA-E cytochromes’ efficient role in providing the necessary reducing power to mitigate oxidative stress situations, hence contributing to a better knowledge of oxidative stress protection pathways in G. sulfurreducens.
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Fundação para a Ciência e a Tecnologia
Programa de financiamento
3599-PPCDT
Número da atribuição
EXPL/BIA-BQM/0770/2021