Sojo Martinez, VR;
(2016)
Membrane bioenergetics at major transitions in evolution.
Doctoral thesis , UCL (University College London).
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Abstract
This thesis presents theoretical and computational studies of four major evolutionary transitions in which cellular membranes and their embedded proteins played crucial roles: 1)The divergence of archaea and bacteria Archaea and bacteria are the basal domains of life, so it is important to understand how they came to diverge. They share several core traits, such as transcription, translation, and the genetic code. They also share the chemiosmotic exploitation of ion gradients across membranes, yet they do not share the membranes themselves. Notably, the phospholipid backbone is sn-glycerol-1-phosphate in archaea but the enantiomer sn-glycerol-3-phosphate in bacteria. The synthesising enzymes are unrelated. I used mathematical modelling to propose an explanation for this divergence in the context of natural proton gradients in alkaline hydrothermal vents, plausible scenarios for an autotrophic origin of life. Results show that early membranes had to be leaky, so both pumping and glycerol-phosphate backbones (which drastically decrease permeability) evolved later, and independently, in archaea and bacteria. 2)The evolution of Homochirality The “dual homochirality” of lipids suggests that the stereospecificity of bioorganic catalysis itself, not prebiotic physics or chemistry, is behind the origin of handedness in life’s molecules (e.g. L-amino acids and D-sugars). 3)The evolution of membrane proteins I report that membrane proteins are less shared across the tree of life. Faster evolution of outside-facing regions and true gene losses point to a common cause: as cells adapt to new environments selective pressure is stronger on the outside, while the inside, subject to strong homeostasis, evolves more slowly. 4)The bacterial nature of eukaryotic membranes Eukaryotes arose from a merger of a bacterium into an archaeon, so the first eukaryote must have had an archaeal plasma membrane and bacterial (proto)mitochondrial membranes; yet all modern eukaryotes have exclusively bacterial membranes. I suggest that archaeal membranes were lost and bacterial ones kept because of the bioenergetic adaptation of mitochondrial proteins to the bacterial membrane.
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