Important aspects of proton transfer via hyducton include H-bonded lattices of lipid headgroups, protons as limiting substrates in energy transduction and the massive amounts of protons required for ATP production (e.g. around tens of million protons per second per E. coli cell growing aerobically). In this project we have studied four biological models, encompassing the three domains of life, to illustrate how hyductons may potentially enhance the efficiency in proton transport and gradient generation along membrane surfaces. This data supports the view that the hyducton is likely essential for all life forms, which have adapted their membrane lipid composition through Darwinian selection to meet the conditions imposed by the surrounding environment.

This study further provides a bioenergetic perspective as to how haloarchaea evolved following oxygenation of Earth’s atmosphere. For that we focused primarily on changes in lipid composition of five different haloarchaea all representing distinctive cell morphologies and behaviors (i.e., rod shape vs. pleomorphic behavior). Common to all five haloarchaea, this study revealed 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 haloarchaeal membranes. We propose for instance that by bridging the negative charges of adjacent anionic phospholipids, magnesium ion acts as surrogate for cardiolipin, a molecule that is known to control curvature stress of membranes.

This project explores the scientific marriage between bioenergetics and lipids throughout the evolutionary history of methanogens. We evaluate for instance the evolution of methanophenazine for electron transport at the membrane level in Methanosarcinales, which required a dramatic remodelling of their lipids as compared to the other methanogens. In addition, we depict the importance of the hyducton in these archaea, particularly relative to its function in proton transport coupling membrane-bound proton-pumping hydrogenases and ATP synthases. This hyducton-based mechanism for harnessing protonic energy generated by hydrogenases may be applicable to other prokaryotes, including gram-negative bacteria that might harness protons in the periplasmic space.

Advances in analytical techniques have been unraveling a vast structural diversity of lipid molecules estimated today at 10 to 100k species, with mostly unexplored biochemical functions. The tripartite membrane lipid code, originally developed to understand the biochemical functions of omega-3 fatty acids, unifies the importance of lipids in permeability, motional dynamics (fluidity) and oxidative stability of cellular membranes from all domains of life. In face of the technological progress in lipidomics research, the tripartite code may help to connect the dots between the massive diversity and the role of membrane lipids in major cellular functions such as bioenergetics, membrane domains and lipid-protein interactions.