Revealing the structural origin of the redox-Bohr effect: The first solution structure of a cytochrome from Geobacter sulfurreducens

Gs (Geobacter sulfurreducens) can transfer electrons to the exterior of its cells, a property that makes it a preferential candidate for the development of biotechnological applications. Its genome encodes over 100 cytochromes and, despite their abundance and key functional roles, to date there is no structural information for these proteins in solution. The trihaem cytochrome PpcA might have a crucial role in the conversion of electronic energy into protonmotive force, a fundamental step for ATP synthesis in the presence of extracellular electron acceptors. In the present study, 15N-labelled PpcA was produced and NMR spectroscopy was used to determine its solution structure in the fully reduced state, its backbone dynamics and the pH-dependent conformational changes. The structure obtained is well defined, with an average pairwise rmsd (root mean square deviation) of 0.25 Å (1 Å=0.1 nm) for the backbone atoms and 0.99 Å for all heavy atoms, and constitutes the first solution structure of a Gs cytochrome. The redox-Bohr centre responsible for controlling the electron/proton transfer was identified, as well as the putative interacting regions between PpcA and its redox partners. The solution structure of PpcA will constitute the foundation for studies aimed at mapping out in detail these interacting regions. © The Authors Journal compilationPeer Reviewe

Unveiling the Structural Basis That Regulates the Energy Transduction Properties within a Family of Triheme Cytochromes from Geobacter sulfurreducens

A family of triheme cytochromes from Geobacter sulfurreducens plays an important role in extracellular electron transfer. In addition to their role in electron transfer pathways, two members of this family (PpcA and PpcD) were also found to be able to couple e/H transfer through the redox Bohr effect observed in the physiological pH range, a feature not observed for cytochromes PpcB and PpcE. In attempting to understand the molecular control of the redox Bohr effect in this family of cytochromes, which is highly homologous both in amino acid sequence and structures, it was observed that residue 6 is a conserved leucine in PpcA and PpcD, whereas in the other two characterized members (PpcB and PpcE) the equivalent residue is a phenylalanine. To determine the role of this residue located close to the redox Bohr center, we replaced Leu in PpcA with Phe and determined the redox properties of the mutant, as well as its solution structure in the fully reduced state. In contrast with the native form, the mutant PpcAL6F is not able to couple the e/H pathway. We carried out the reverse mutation in PpcB and PpcE (i.e., replacing Phe in these two proteins by leucine) and the mutated proteins showed an increased redox Bohr effect. The results clearly establish the role of residue 6 in the control of the redox Bohr effect in this family of cytochromes, a feature that could enable the rational design of G. sulfurreducens strains that carry mutant cytochromes with an optimal redox Bohr effect that would be suitable for various biotechnological applications.Peer Reviewe

Structural investigations of N-methylformamide-water mixtures at various concentrations

Structural investigations of N-methylformamide-water mixtures (NMF-water) are performed at room temperature and atmospheric pressure for two water molar fractions x w = 0.66 and x w = 0.75 . This paper extends our recent study on the equimolar system. H-bond networks are preferentially formed between NMF and water molecules. Among a large variety of DFT optimized models, X-ray scattering data shows that the local order of each mixture is better described by a tetramer where one NMF molecule is connected to three water molecules. No self-association is observed in the considered systems. The effect of hydration is compared to the temperature and pressure effects in some hydrogen-bonded liquids