Dissecting the conformational complexity and mechanism of a bacterial heme transporter.

Iron-bound cyclic tetrapyrroles (hemes) are redox-active cofactors in bioenergetic enzymes. However, the mechanisms of heme transport and insertion into respiratory chain complexes remain unclear. Here, we used cellular, biochemical, structural and computational methods to characterize the structure and function of the heterodimeric bacterial ABC transporter CydDC. We provide multi-level evidence that CydDC is a heme transporter required for functional maturation of cytochrome bd, a pharmaceutically relevant drug target. Our systematic single-particle cryogenic-electron microscopy approach combined with atomistic molecular dynamics simulations provides detailed insight into the conformational landscape of CydDC during substrate binding and occlusion. Our simulations reveal that heme binds laterally from the membrane space to the transmembrane region of CydDC, enabled by a highly asymmetrical inward-facing CydDC conformation. During the binding process, heme propionates interact with positively charged residues on the surface and later in the substrate-binding pocket of the transporter, causing the heme orientation to rotate 180°.

Analysis of propionate-degrading consortia from agricultural biogas plants.

In order to investigate the propionate-degrading community of agricultural biogas plants, four propionate-degrading consortia (Ap1a, N12, G12, and Wp2a) were established from different biogas plants which were fed with renewable resources. The consortia were cultivated in a batch for a period of 2-4 years and then analyzed in an 8-week batch experiment for microbial succession during propionate degradation. Community shifts showed considerable propagation of Syntrophobacter sulfatireducens, Cryptanaerobacter sp./Pelotomaculum sp., and "Candidatus Cloacamonas sp." in the course of decreasing propionate concentration. Methanogenic species belonged mainly to the genera Methanosarcina, Methanosaeta, and Methanoculleus. Due to the prevalent presence of the syntrophic acetate-oxidizing species Tepidanaerobacter acetatoxydans and potentially autotrophic homoacetogenic bacteria (Moorella sp., Thermacetogenium sp.), a theoretical involvement of syntrophic acetate oxidation and autotrophic homoacetogenesis in stable and efficient propionate degradation was indicated. Considering theoretical Gibbs free energy values at different hydrogen partial pressures, it is noticeable that syntrophic acetate oxidation and autotrophic homoacetogenesis have the potential to counterbalance adverse hydrogen partial pressure fluctuations, stabilizing most probably continuous and stable propionate degradation.