Journals

2016

Abstract: Recent advances in lipidomic analysis in combination with various physiological experiments set the stage for deciphering the structure-function of haloarchaeal membrane lipids. Here we focused primarily on changes in lipid composition of Haloferax volcanii, but also performed a comparative analysis with four other haloarchaeal species (Halobacterium salinarum, Halorubrum lacusprofundi, Halorubrum sodomense and Haloplanus natans) all representing distinctive cell morphologies and behaviors (i.e., rod shape vs. pleomorphic behavior). Common to all five haloarchaea, our data reveal an extraordinary high level of menaquinone, reaching up to 72% of the total lipids. This ubiquity suggests that menaquinones may function beyond their ordinary role as electron and proton transporter, acting simultaneously as ion permeability barriers and as powerful shield against oxidative stress. In addition, we aimed at understanding the role of cations interacting with the characteristic negatively charged surface of haloarchaealmembranes.Wepropose for instance that by bridging the negative charges of adjacent anionic phospholipids, Mg2+ acts as surrogate for cardiolipin, a molecule that is known to control curvature stress of membranes. This study further provides a bioenergetic perspective as to how haloarchaea evolved following oxygenation of Earth's atmosphere. The success of the aerobic lifestyle of haloarchaea includesmultiple membrane-based strategies that successfully balance the need for a robust bilayer structure with the need for high rates of electron transport – collectively representing the molecular basis to inhabit hypersaline water bodies around the planet.

Abstract: Proton bioenergetics provides the energy for growth and survival of most organisms in the biosphere ranging from unicellular marine phytoplankton to humans. Chloroplasts harvest light and generate a proton electrochemical gradient (proton motive force) that drives the production of ATP needed for carbon dioxide fixation and plant growth. Mitochondria, bacteria and archaea generate protonmotive force to energize growth and other physiologies. Energy transducing membranes are at the heart of proton bioenergetics and are responsible for catalyzing the conversion of energy held in high-energy electrons → electron transport chain → proton motive force → ATP. Whereas the electron transport chain is understood in great detail there are major gaps in understanding mechanisms of proton transfer or circulation during proton bioenergetics. This paper is built on the proposition that phospho- and glycoglycerolipids form proton transport circuitry at the membrane's surface. By this proposition, an emergent membrane property, termed the hyducton, confines active/unbound protons or hydronium ions to a region of low volume close to the membrane surface. In turn, a von Grotthuß mechanism rapidly moves proton substrate in accordance with nano-electrochemical poles on the membrane surface created by powerful proton pumps such as ATP synthase.

2007

Abstract: The three domains of life on Earth include the two prokaryotic groups, Archaea and Bacteria. The Archaea are distinguished from Bacteria based on phylogenetic and biochemical differences, but currently there is no unifying ecological principle to differentiate these groups. The ecology of the Archaea is reviewed here in terms of cellular bioenergetics. Adaptation to chronic energy stress is hypothesized to be the crucial factor that distinguishes the Archaea from Bacteria. The biochemical mechanisms that enable archaea to cope with chronic energy stress include low-permeability membranes and specific catabolic pathways. Based on the ecological unity and biochemical adaptations among archaea, I propose the hypothesis that chronic energy stress is the primary selective pressure governing the evolution of the Archaea.

2004

Abstract: The Omega-3 fatty acid DHA (docosahexaenoic acid, 22:6) and its sister molecule EPA (eicosapentaenoic acid, 20:5) are highlighted here. These highly unsaturated fatty acids are widespread in nature, especially in the marine environment, and are essential in membranes ranging from deep sea bacteria to human neurons. Studies of DHA/EPA in bacteria have led to a working model on the structural roles of these molecules and are described in this review. The main points are: (a) genomic analysis shows that genes encoding the DHA/EPA pathways are similar, supporting the idea that structural roles in bacteria might be similar, (b) biochemical analysis shows that DHA and EPA are produced in bacteria by a polyketide process distinct from the pathway of plants and animals; this allows DHA and EPA to be produced in anaerobic or oxygen-limited environments, (c) regulatory systems triggered by temperature and pressure have been identified and studied, and add to the understanding of the roles of these molecules, (d) DHA/EPA bacteria are located almost exclusively in the marine environment, raising the prospect of an important linkage between membrane processes and marine conditions, (e) physiological studies of an EPA recombinant of E. coli show that EPA phospholipids contribute essential fluidity to the bilayer and that an EPA-enriched membrane supports a respiratory lifestyle dependent on proton bioenergetics; the EPA recombinant displays other physiological properties likely attributed to high levels of EPA in the bilayer, and (f) chemical studies such as chemical dynamic modeling support the idea that DHA and presumably EPA contribute hyperfluidizing properties to the membrane. We hypothesize that DHA/EPA phospholipids contribute fluidity and other properties to the bilayer which distinguish these highly unsaturated chains from monounsaturates and polyunsaturates such as 18:2 and 18:3. We further hypothesize that the structural properties of DHA/EPA functioning in bacteria are also harnessed by higher organisms for enhancing crucial membrane processes including photosynthesis and energy transduction.