Quantum chemistry solves mystery why there are these 20 amino acids in the genetic code

All life on Earth is based on 20 amino acids, which are governed by the DNA to form proteins. In the inherited DNA, it is always three sequential DNA bases, or codons, which combine to "encode" one single of these 20 amino acids. The resultant grid of codons is what is known as the genetic code. "Researchers have been puzzled for decades why evolution has selected these 20 amino acids for genetic encoding," said Professor Bernd Moosmann. "The presence of the last and newest seven amino acids is particularly hard to explain, because suitable and functional proteins can be assembled using just the first and oldest 10 to 13 amino acids."

In a new approach, the researchers compared the quantum chemistry of all amino acids used by life on Earth with the quantum chemistry of amino acids from space, brought in on meteorites, as well as with that of modern reference biomolecules. They found that the newer amino acids had become systematically softer, i.e., more readily reactive or prone to undergo chemical changes. "The transition from the dead chemistry out there in space to our own biochemistry here today was marked by an increase in softness and thus an enhanced reactivity of the building blocks," explained Moosmann. The researchers were able to verify the results of their theoretical calculations in biochemical experiments. Functional aspects also must have played a significant role with regard to the more recent amino acids as these newcomers hardly exhibit particular advantages when it comes to building protein structures.

However, the problem remained of why the soft amino acids were added to the tool box in the first place. What exactly were these readily reactive amino acids supposed to react with? On the basis of their results, the researchers conclude that at least some of the new amino acids, especially methionine, tryptophan, and selenocysteine, were added as a consequence of the increase in the levels of oxygen in the biosphere. This oxygen promoted the formation of toxic free radicals, which exposes modern organisms and cells to massive oxidative stress. The new amino acids underwent chemical reactions with the free radicals and thus scavenged them in an efficient manner. The oxidized new amino acids, in turn, were easily repairable after oxidation, but they protected other and more valuable biological structures, which are not repairable, from oxygen-induced damage. Hence, the new amino acids provided the remote ancestors of all living cells with a very real survival advantage that allowed them to be successful in the more oxidizing, "brave" new world on Earth. "With this in view, we could characterize oxygen as the author adding the very final touch to the genetic code," stated Moosmann.