Cellular energy generation uses membrane-localized electron transfer chains for ATP synthesis. Formed ATP in turn is consumed for the biosynthesis of cellular building blocks. In contrast, heme cofactor biosynthesis was found driving ATP generation via electron transport after initial ATP consumption. The FMN enzyme protoporphyrinogen IX oxidase (HemG) of Escherichia coli abstracts six electrons from its substrate and transfers them via ubiquinone, cytochrome bo 3 (Cyo) and cytochrome bd (Cyd) oxidase to oxygen. Under anaerobic conditions electrons are transferred via menaquinone, fumarate (Frd) and nitrate reductase (Nar). Cyo, Cyd and Nar contribute to the proton motive force that drives ATP formation. Four electron transport chains from HemG via diverse quinones to Cyo, Cyd, Nar, and Frd were reconstituted in vitro from purified components. Characterization of E. coli mutants deficient in nar, frd, cyo, cyd provided in vivo evidence for a detailed model of heme biosynthesis coupled energy generation.anabolism coupled catabolism | protoporphyrinogen IX oxidase | HemG | tetrapyrrole | respiration

The structural and functional integrity of biological membranes is vital to life. The interplay of lipids and membrane proteins is crucial for numerous fundamental processes ranging from respiration, photosynthesis, signal transduction, solute transport to motility. Evidence is accumulating that specific lipids play important roles in membrane proteins, but how specific lipids interact with and enable membrane proteins to achieve their full functionality remains unclear. X-ray structures of membrane proteins have revealed tight and specific binding of lipids. For instance, cardiolipin, an anionic phospholipid, has been found to be associated to a number of eukaryotic and prokaryotic respiratory complexes. Moreover, polar and septal accumulation of cardiolipin in a number of prokaryotes may ensure proper spatial segregation and/or activity of proteins. In this review, we describe current knowledge of the functions associated with cardiolipin binding to respiratory complexes in prokaryotes as a frame to discuss how specific lipid binding may tune their reactivity towards quinone and participate to supercomplex formation of both aerobic and anaerobic respiratory chains. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

In bacteria, chromosome dynamics and gene expression are modulated by nucleoid-associated proteins (NAPs), but little is known about how NAP activity is coupled to cell cycle progression. Using genomic techniques, quantitative cell imaging, and mathematical modeling, our study in Caulobacter crescentus identifies a novel NAP (GapR) whose activity over the cell cycle is shaped by DNA replication. GapR activity is critical for cellular function, as loss of GapR causes severe, pleiotropic defects in growth, cell division, DNA replication, and chromosome segregation. GapR also affects global gene expression with a chromosomal bias from origin to terminus, which is associated with a similar general bias in GapR binding activity along the chromosome. Strikingly, this asymmetric localization cannot be explained by the distribution of GapR binding sites on the chromosome. Instead, we present a mechanistic model in which the spatiotemporal dynamics of GapR are primarily driven by the progression of the replication forks. This model represents a simple mechanism of cell cycle regulation, in which DNA-binding activity is intimately linked to the action of DNA replication.

Anionic lipids play a variety of key roles in membrane function, including functional and structural effects on respiratory complexes. However, little is known about the molecular basis of these lipid-protein interactions. In this study, NarGHI, an anaerobic respiratory complex of Escherichia coli, has been used to investigate the relations in between membrane-bound proteins with phospholipids. Activity of the NarGHI complex is enhanced by anionic phospholipids both in vivo and in vitro. The anionic cardiolipin tightly associates with the NarGHI complex and is the most effective phospholipid to restore functionality of a nearly inactive detergent-solubilized enzyme complex. A specific cardiolipin-binding site is identified on the basis of the available X-ray diffraction data and of site-directed mutagenesis experiment. One acyl chain of cardiolipin is in close proximity to the heme b D center and is responsible for structural adjustments of b D and of the adjacent quinol substrate binding site. Finally, cardiolipin binding tunes the interaction with the quinol substrate. Together, our results provide a molecular basis for the activation of a bacterial respiratory complex by cardiolipin.bioenergetics | EPR spectroscopy | metalloprotein | molybdenum