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NADH: ubiquinone oxidoreductase (complex I) is the first enzyme complex of the respiratory chain. Complex I is a redox-driven proton pump that contributes to the proton motive force that drives ATP synthase. The structure of complex I has been analyzed by x-ray crystallography and electron cryo-microscopy and is now well-described. The ubiquinone (Q) reduction site of complex I is buried in the peripheral arm and a tunnel-like structure is thought to provide access for the hydrophobic substrate from the membrane. Several intermediate binding positions for Q in the tunnel were identified in molecular simulations. Structural data showed the binding of native Q molecules and short chain analogs and inhibitors in the access pathway and in the Q reduction site, respectively. We here review the current knowledge on the interaction of complex I with Q and discuss recent hypothetical models for the coupling mechanism.
In times of global warming caused by the extensive use of fossil fuels, the need to capture gaseous carbon compounds is growing bigger. Several groups of microorganisms can fix the greenhouse gas CO2. Out of these, acetogenic bacteria are role models in their ability to reduce CO2 with hydrogen to acetate, which makes acetogens prime candidates for genetic modification towards biotechnological production of value-added compounds from CO2, such as biofuels. However, growth of acetogens on gaseous substrates is strongly energy-limited, and successful metabolic engineering requires a detailed knowledge of the bioenergetics. In 1939, Clostridium aceticum was the first acetogen to be described. A recent genomic study revealed that this organism contains cytochromes and therefore may use a proton gradient in its respiratory chain. We have followed up these studies and will present data that C. aceticum does not use a H+ but a Na+ gradient for ATP synthesis, established by a Na+-Rnf. Experimental data and in silico analyses enabled us to propose the biochemistry and bioenergetics of acetogenesis from H2 + CO2 in C. aceticum.
NADH:ubiquinone-oxidoreductase (complex I) is the largest membrane protein complex of the respiratory chain. Complex I couples electron transfer to vectorial proton translocation across the inner mitochondrial membrane. The L shaped structure of complex I is divided into a membrane arm and a matrix arm. Fourteen central subunits are conserved throughout species, while some 30 accessory subunits are typically found in eukaryotes. Complex I dysfunction is associated with mutations in the nuclear and mitochondrial genome, resulting in a broad spectrum of neuromuscular and neurodegenerative diseases. Accessory subunit NDUFS4 in the matrix arm is a hot spot for mutations causing Leigh or Leigh-like syndrome. In this review, we focus on accessory subunits of the matrix arm and discuss recent reports on the function of accessory subunit NDUFS4 and its interplay with NDUFS6, NDUFA12, and assembly factor NDUFAF2 in complex I assembly.