Scientists think they have identified core elements of the first proteins that made life possible. If they're right, it could open new doors to understanding the great question of how, and in what circumstances, life can emerge from an unliving world.
There are many lines of exploration and debate about where life began and whether DNA, RNA, or a mixture came first. Researchers at Rutgers University are exploring the question from a different angle, trying to identify the ancestral proteins from which we all came. They have provided some possible answers, published in the journal Science Advances.
Collecting and using energy are essential features for life, the researchers reasoned. Whatever the source of the energy, its chemical storage and use involves transferring electrons, and this must have been true from the beginning. When life was just getting started, it makes sense for it to have used the most readily available electron conductors, they continued. In the early ocean, this would have been the small subset of transition metals that were soluble under the conditions of the day.
Therefore, proteins that bind metals must have been original to life, with many subsequent biological functions performed by repurposed versions of these original proteins. Metal-binding remains crucial to life today, so the authors sought the structure of the original proteins by looking for common features in proteins that fulfill this role across the tree of life. They report commonalities in almost all transition metal-binding proteins, irrespective of their function, the organism they come from or the metal being processed.
"We saw that the metal-binding cores of existing proteins are indeed similar even though the proteins themselves may not be," said study author Professor Yana Bromberg in a statement.
"We also saw that these metal-binding cores are often made up of repeated substructures, kind of like LEGO blocks. Curiously, these blocks were also found in other regions of the proteins, not just metal-binding cores, and in many other proteins that were not considered in our study. Our observation suggests that rearrangements of these little building blocks may have had a single or a small number of common ancestors and given rise to the whole range of proteins and their functions that are currently available – that is, to life as we know it."
The near-universal structures are mostly oxidoreductases, enzymes that transfer electrons between molecules. The authors conclude existed more than 3.8 billion years ago.
Following the Great Oxidation Event, proteins diversified, folding in an abundance of new and more complex ways. The authors think this makes it too difficult to identify the original sequences, but consider possible to trace the evolution of protein components based on their structures. In the process, they identified distantly related peptides (short chains of amino acids that can form building blocks of proteins) using their structural alignments.
Bromberg noted that this, like any insight into how life emerged, could prove useful in searching for life beyond the Earth, as well as to the quest to create new living things through synthetic biology.