Quantum
Chemistry Solves The Question of Why Life Needs So Many Amino Acids
A flexible approach to life.
DAVID NIELD
3 FEB 2018
One of the oldest and most fundamental
questions in biochemistry is why the 20 amino acids that support life are all
needed, when the original core of 13 would do – and quantum
chemistry might have just provided us with the answer.
According to new research, it's the extra
chemical reactivity of the newer seven amino acids that make them so vital to
life, even though they don't add anything different in terms of their spatial
structure.
Quantum chemistry is a way of taking some of
the principles of quantum mechanics – describing particles
according to probabilistic, wave-like properties – and applying them to the way
atoms behave in chemical reactions.
The international team of scientists behind
the new study used quantum chemistry techniques to compare amino acids found in
space (and left here by meteorite fragments) with amino acids supporting life today
on Earth.
"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," says one of the researchers, Bernd Moosmann
from Johannes Gutenberg University Mainz in Germany.
It's the job of amino acids to form proteins,
as instructed by our DNA. These acids were formed right after Earth
itself came into being, about 4.54 billion years ago,
and so represent one of the earliest building blocks of life.
However, why evolution decided that we needed
20 amino acids to handle this genetic encoding has never been clear, because the
first 13 that developed should have been enough for the task.
The greater "softness" of the extra
seven amino acids identified by the researchers means they are more readily
reactive and more flexible in terms of chemical changes.
If you were representing the amino acids as
circles, they could be drawn as multiple concentric circles representing
differing energy levels, rather than one single circle of the same chemical
hardness and energy level – kind of like in the photo below.
Having determined the hypothesis through
quantum chemistry calculations, the scientists were able to back up their ideas
with a series of biochemical experiments.
Along the way the team determined that the
extra amino acids – particularly methionine, tryptophan,
and selenocysteine – could well have evolved
as a response to increasing levels of oxygen in the biosphere in the planet's
youngest days.
Peering so far back in time is difficult, as
the first organic compounds never left fossils behind for us to analyse, but
this may have been part of the process that kicked off the formation of life on Earth.
As the very earliest living cells tried to
deal with the extra oxidative stress, it was a case of survival of the fittest.
The cells best able to cope with that additional oxygen – through the
protection of the new amino acids – were the ones that lived on and flourished.
"With this in view, we could characterise
oxygen as the author adding the very final touch to the genetic
code," says Moosmann.
The research has been published in PNAS.
--by Sciencealert.com
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