2018-09-30 21:31:07
Recently, I was talking about new data that strongly pointed to the existence of a universal ancestor, called LUCAfor its acronym in English. LUCA it could be the last population of identical organisms from which the three domains of life were derived: the bacteria and archaea, which are prokaryotes (cells without a nucleus), and the eukaryotes (cells with a nucleus).
Although the origin of eukaryotes seems to come from the symbiotic union of a bacterium with an archaea, much more obscure is the reason for the separation between them from LUCA, without which no eukaryotic plant and animal would exist. According to the most recent estimates, this separation occurred 3.5 billion years ago, about a billion years after the appearance of LUCA.
The separation of LUCA in two independent domains of life is far from being explained and some of the mysteries with which it is surrounded raise fundamental questions about the evolution and origin of life. One of these mysteries is the so-called lipid split between bacteria and archaea.
Lipids are fundamental components of cells, in fact, they allow the separation between life and non-life. Life takes place thanks to the biochemical processes that take place inside the lipid membrane, while everything that is outside that membrane constitutes the domain of the non-living. The generation of chemical energy, in the form of the adenosine triphosphate molecule (ATP), which enables metabolic processes, also takes place thanks to lipid membranes. These function, in this case, as hydraulic dams for hydrogen ions, and their passage in favor of the gradient through them is what provides the energy to move the molecular motors that manufacture the ATP.
Well, the lipids that make up the membranes of bacteria and archaea are very different, hence the lipid separation. Bacteria use the same class of lipid molecules as eukaryotes: the familiar triglycerides and phospholipids. To do this, they use one of the forms of the glycerol molecule (also called glycerin) to which they attach linear fatty acids (such as omega three) through ester-type chemical bonds. The archaea, on the other hand, use the other form of the glycerol molecule to which they attach atypical fat molecules, derived from isoprene, through ether bonds, which are more resistant. What is surprising is that the two different, but chemically identical, forms of glycerol used by bacteria and archaea are mirror images of each other. It is said, therefore, that the glycerol molecule is chiral, a word that derives from the Greek and that means “hand”.
left hand experiments
It is not known with certainty why living things have generally “chosen” one of the chiral forms of the fundamental molecules of life. Thus, in all living beings the amino acids are of the L variant (left), but the sugars are of the D variant (right). It could be the other way around, but it’s right.
However, bacteria and archaea are different in the glycerol molecule they use to make their membrane lipids. Bacteria use the D form, but archaea use the L form. This has left scientists studying molecular evolution and the origin of life dumbstruck.
It is not surprising, because using molecules of one or another chirality means having to have generated enzymes of one or another chirality in evolution as well. If I intend to cover my left hand, I need a left glove and, no matter how gloved it is, the right one won’t work for me. Indeed, bacteria and archaea have “glove” enzymes for the generation of lipids from their specific chiral variant of glycerol, which, beware, are not genetically related to each other.
This increases the mystery of its origin from LUCA. A hypothesis that was postulated to try to explain it was that LUCA it had all the enzymes to generate lipids with the two glycerol molecules, L and D. However, it was verified in the laboratory that the mixture of chiral lipids seems to generate unstable membranes. Therefore, the hypothesis maintains that finally LUCA spawned bacteria and archaea, each taking one of the original enzyme variants of LUCA to produce the lipids, which led to the generation of stable membranes.
Although it is difficult to understand how LUCA could survive a billion years with unstable membranes, this hypothesis is attractive, but very difficult to prove. We cannot travel to the past to check if LUCA could or could not generate both types of lipids. However, thanks to the tools of molecular biology, it is now possible to generate bacteria that also contain the genes of the archaea so that they can thus produce lipids of both kinds, and check whether or not these hybrid microorganisms are capable of growing and surviving. .
This is what a group of researchers from the University of Groningen, in the Netherlands, has done. The scientists introduce the archaeal genes for lipid generation into Escherichia coli bacteria and analyze the composition of their membranes and the ability to grow of this new hybrid organism.
The researchers find that these bacteria generate lipids of both kinds and form hybrid membranes that are no more fragile or unstable and, in fact, allow the bacteria to grow at the same rate as normal bacteria. Furthermore, the hybrid bacteria are more resistant, not less, to exposure to elevated temperatures and even freezing to minus 80ºC.
Thus, these interesting experiments do not support the hypothesis of the instability of the membranes of LUCA as a reason for the evolution of bacteria and archaea, although they indicate that LUCA it could have stable membranes made up of lipids of both kinds. We will have to wait for new and ingenious experiments to solve this interesting mystery, which is necessary if we want to understand one of the most fundamental aspects of the evolution of primordial life.
Reference: Antonella Caforio et al. (2018). Converting Escherichia coli into an archaebacterium with a hybrid heterochiral membrane. www.pnas.org/cgi/doi/10.1073/pnas.1721604115
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