from the primordial-soup dept.
From Quanta Magazine: Life's First Molecule Was Protein, Not RNA, New Model Suggests
Proteins have generally taken a back seat to RNA molecules in scientists' speculations about how life on Earth started. Yet a new computational model that describes how early biopolymers could have grown long enough to fold into useful shapes may change that. If it holds up, the model, which is now guiding laboratory experiments for confirmation, could re-establish the reputation of proteins as the original self-replicating biomolecule.
For scientists studying the origin of life, one of the greatest chicken-or-the-egg questions is: Which came first — proteins or nucleic acids like DNA and RNA? Four billion years ago or so, basic chemical building blocks gave rise to longer polymers that had a capacity to self-replicate and to perform functions essential to life: namely, storing information and catalyzing chemical reactions. For most of life's history, nucleic acids have handled the former job and proteins the latter one. Yet DNA and RNA carry the instructions for making proteins, and proteins extract and copy those instructions as DNA or RNA. Which one could have originally handled both jobs on its own?
For decades, the favored candidate has been RNA — particularly since the discovery in the 1980s that RNA can also fold up and catalyze reactions, much as proteins do. Later theoretical and experimental evidence further bolstered the "RNA world" hypothesis that life emerged out of RNA that could catalyze the formation of more RNA.
But RNA is also incredibly complex and sensitive, and some experts are skeptical that it could have arisen spontaneously under the harsh conditions of the prebiotic world. Moreover, both RNA molecules and proteins must take the form of long, folded chains to do their catalytic work, and the early environment would seemingly have prevented strings of either nucleic acids or amino acids from getting long enough.
Ken Dill, a biophysicist at Stony Brook University, has been studying protein folding for decades. He's now using that work to examine the chemistry-to-biology transition that took place four billion years ago.
Ken Dill and Elizaveta Guseva of Stony Brook University in New York, together with Ronald Zuckermann of the Lawrence Berkeley National Laboratory in California, presented a possible solution to the conundrum in the Proceedings of the National Academy of Sciences (PNAS) this summer. As models go, theirs is very simple. Dill developed it in 1985 to help tackle the "protein-folding problem," which concerns how the sequence of amino acids in a protein dictates its folded structure. His hydrophobic-polar (HP) protein-folding model treats the 20 amino acids as just two types of subunit, which he likened to different colored beads on a necklace: blue, water-loving beads (polar monomers) and red, water-hating ones (nonpolar monomers). The model can fold a chain of these beads in sequential order along the vertices of a two-dimensional lattice, much like placing them on contiguous squares of a checkerboard. Which square a given bead ends up occupying depends on the tendency for the red, hydrophobic beads to clump together so that they can better avoid water.
Is this a real world example of Auto-catalytic Sets?
I've always thought the proteins evolved first.
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Researchers from The Scripps Research Institute in California have identified a molecule capable of performing phosphorylation in water, making it a solid candidate for what has until now been a missing link in the chain from lifeless soup to evolving cells. In the classic chicken and egg conundrum of biology's origins, debate continues to rage over which process kicked off others in order to get to life. Was RNA was[sic] followed by protein structures? Did metabolism spark the whole shebang? And what about the lipids?
No matter what school of abiogenesis you hail from, the production of these various classes of organic molecules requires a process called phosphorylation – getting a group of three oxygens and a phosphorus to attach to other molecules.
Nobody has provided strong evidence in support of any particular agent that might have been responsible for making this happen to prebiotic compounds. Until now. "We suggest a phosphorylation chemistry that could have given rise, all in the same place, to oligonucleotides, oligopeptides, and the cell-like structures to enclose them," says researcher Ramanarayanan Krishnamurthy.
Enter diamidophosphate (DAP). Combined with imidazole acting as a catalyst, DAP could have bridged the critical gap from early compounds such as uridine and cytidine. That might not seem overly exciting, but phosphorylating nucleosides like these is a crucial step on the road to building the chains of RNA that could serve as the first primitive genes.
Phosphorylation, oligomerization and self-assembly in water under potential prebiotic conditions (DOI: 10.1038/nchem.2878) (DX)