Chuck Bednar for redOrbit.com – @BednarChuck
According to the abiogenesis theory, the first amino acids formed into the proteins that served as the building blocks of all living cells nearly four billion years ago; and now a pair of studies published in the Proceedings of the National Academy of Sciences has revealed more about the processes behind this transition.
In the papers, University of North Carolina scientists Dr. Richard Wolfenden and Dr. Charles Carter, discuss two key stages of the process which brought life out of the primordial soup and ultimately resulted in the formation of plants and animals. These are the two primary stages through which the genetic code was first developed and how chemicals evolved into cells.
As Dr. Carter, a professor of biochemistry and biophysics at the UNC School of Medicine, explained in a statement, “Our work shows that the close linkage between the physical properties of amino acids, the genetic code, and protein folding was likely essential from the beginning, long before large, sophisticated molecules arrived on the scene. This close interaction was likely the key factor in the evolution from building blocks to organisms.”
Unlike the “RNA World” theory, which claims that the molecule that is currently involved in the coding, regulation, and expression of genes, elevated itself from the primordial soup before going on to form short proteins known as peptides (and eventually single-celled organisms), the authors of the new study argue that RNA did not act on its own. It had help.
Early interactions between amino acids and nucleotides
Approximately 3.6 billion years ago, there was a life form known as the last universal common ancestor (LUCA) of all living things that currently exist on Earth, the UNC scientists explained. This entity was likely a single-celled organism that had a few hundred genes, and it already had complete how-to guides for DNA replication, protein synthesis, and RNA transcription.
LUCA also already had all of the basic components that modern organisms possess (including lipids). From this point on, the evolutionary processes that resulted in modern-day life are easy to see. However, there is no concrete evidence detailing how LUCA’s predecessors formed from chemicals into amino acids, then into proteins, then into cells.
“We know a lot about LUCA and we are beginning to learn about the chemistry that produced building blocks like amino acids, but between the two there is a desert of knowledge,” Dr. Carter explained. He added that Dr. Wolfenden was able to determine the physical properties of the 20 amino acids, and found a link between those traits and the genetic code.
“That link suggests to us that there was a second, earlier code that made possible the peptide-RNA interactions necessary to launch a selection process that we can envision creating the first life on Earth,” Dr. Carter added. Thus, RNA did not invent itself out of the primordial soup. Instead, there were most likely early interactions between amino acids and nucleotides that resulted in the co-creation of proteins and RNA.
Protein folding differed during Earth’s primordial era
In one of the new PNAS studies, Dr. Wolfenden and his colleagues showed that the way amino acids distribute between water and oil (their polarities) and their sizes help explain the complex processes in which proteins fold themselves to function properly. They found that the polarities of amino acids consistently change based on temperature, but not in a way that disrupts the core relationship between genetic coding and protein folding.
This discovery was important because it shows that the hot temperatures that existed when the Earth was young and still forming would not have radically altered how amino acids behave. In a series of experiments, Dr. Wolfenden’s team showed how these amino acids would have acted in folded proteins that existed in the extreme temperatures of the planet four billion years ago.
In the other study, Dr. Carter and his colleagues examined how a specific type of enzyme called aminoacyl-tRNA synthetases (which translate the genetic code) was able to recognize tRNA, or transfer ribonucleic acid. According to Dr. Carter, tRNA is like “an adapter,” in that one end of it carries a specific amino acid and the other reads that amino acid’s genetic blueprint in messenger RNA. Each synthetase matches an amino acid with its own adapted to build proteins.
Their research revealed that the two different ends of the L-shaped tRNA molecule each had an independent set of rules that governed which amino acid to select, and that the end which carried the amino acid itself sorted them based on their size. The other end of this molecule is called the tRNA anticodon, and it reads codons (or sequences of three RNA nucleotides found in genetic messages that select amino acids based on their polarity), the study authors explained.
Together, the findings of these studies suggest that the link between tRNA and the physical traits of amino acids (size and polarity) played an essential role during the Earth’s primordial era. Combined with previous research, the results imply that selection by size preceded selection based on polarity, meaning that the very first proteins did not fold into specific shapes, and that the ability to form these unique structures evolved later.
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