The Geobacter metallireducens bacterium can couple the oxidation of a wide range of compounds to the reduction of several extracellular electron acceptors, including pollutants or electrode surfaces for current production in microbial fuel cells. For these reasons, G. metallireducens are of interest for practical biotechnological applications. The use of such electron acceptors relies on a mechanism that permits electrons to be transferred to the cell exterior. The cytochrome PpcA from G. metallireducens is a member of a family composed of five periplasmic triheme cytochromes, which are important to bridge the electron transfer from the cytoplasmic donors to the extracellular acceptors. Using NMR and visible spectroscopic techniques, a detailed thermodynamic characterization of PpcA was obtained, including the determination of the heme reduction potentials and their redox and redox-Bohr interactions. These parameters revealed unique features for PpcA from G. metallireducens compared with other triheme cytochromes from different microorganisms, namely the less negative heme reduction potentials and concomitant functional working potential ranges. It was also shown that the order of oxidation of the hemes is pH-independent, but the protein is designed to couple e−/H+ transfer exclusively at physiological pH.
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In this issue of the Biochemical Journal, Tong et al. report how a sesquiterpene synthase generates eudesmane-type sesquiterpene diols in Tripterygium wilfordii and which residues are important for cyclization. The cover image shows the flower of T. wilfordii and the structure of cryptomeridiol synthase. The background depicts ancient Chinese text that recorded the medicinal benefits of T. wilfordii and the engineered manipulation in yeast to enhance the production of the eudesmane diol cryptomeridiol. For further details, see pages 2713–2725. Image kindly provided by Wei Gao (Capital Medical University, Beijing).
Thermodynamic and functional characterization of the periplasmic triheme cytochrome PpcA from Geobacter metallireducens
Tomás M. Fernandes, Leonor Morgado, Carlos A. Salgueiro; Thermodynamic and functional characterization of the periplasmic triheme cytochrome PpcA from Geobacter metallireducens. Biochem J 14 September 2018; 475 (17): 2861–2875. doi: https://doi.org/10.1042/BCJ20180457
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