The chemical reactions behind the formation of common
metabolites in modern organisms could have formed spontaneously in
the earth's early oceans, according to new research that challenges
the events thought to have led to the origin of life.
The research was funded by the Wellcome Trust, and led by Dr
Markus Ralser of the University of Cambridge and the MRC's National
Institute for Medical Research (now part of the Francis Crick
Institute).
Dr Ralser's team at Cambridge reconstructed the chemical make-up
of the earth's earliest ocean in the laboratory. They found the
spontaneous occurrence of reaction sequences which in modern
organisms enable the formation of molecules essential for the
synthesis of metabolites such as amino acids, nucleic acids and
lipids. These organic molecules are critical for the cellular
metabolism seen in all living organisms.
The detection of one of the metabolites, ribose 5-phosphate, in
the reaction mixtures is particularly noteworthy, as RNA precursors
like this could in theory give rise to RNA molecules that encode
information, catalyze chemical reactions and replicate.
It was previously assumed that the complex metabolic reaction
sequences, known as metabolic pathways, occurring in modern cells
were only possible due to the presence of enzymes. Enzymes are
highly complex molecular machines that are thought to have come
into existence during the evolution of modern organisms. However,
the team's reconstruction reveals that metabolism-like reactions
could have occurred naturally in our early oceans, before the first
organisms evolved.
Almost four billion years ago, life on Earth began in iron-rich
oceans that dominated the surface of the planet. This was an
oxygen-free world, pre-dating photosynthesis, when the redox state
of iron was different and much more soluble to act as potential
catalysts. In the Archean sea, iron, other metals and phosphate,
facilitated a series of reactions which resemble the core of
cellular metabolism occurring in the absence of enzymes.
The findings suggests that metabolism predates the origin of
life and evolved through the chemical conditions that prevailed in
the worlds earliest oceans.
"Our results show that reaction sequences that resemble two
essential reaction cascades of metabolism, glycolysis and the
pentose-phosphate pathways, could have occurred spontaneously in
the earth's ancient oceans," said Dr Ralser.
"In our reconstructed version of the ancient Archean ocean,
these metabolic reactions were particularly sensitive to the
presence of ferrous iron which was abundant in the early oceans,
and accelerated many of the chemical reactions that we observe. We
were surprised by how specific these reactions were" he added.
The conditions of the prebiotic sea were reconstructed based on
the composition of various early sediments described in the
scientific literature which identify soluble forms of iron as one
of the most frequent molecules in the prebiotic oceans.
Alexandra Turchyn of the University of Cambridge, one of the
co-authors of the study, said: "We are quite certain that the
earliest oceans contained no oxygen, and so any iron present would
have been soluble in these oxygen-devoid oceans. It's therefore
possible that concentrations of iron could have been quite
high."
The different metabolites were incubated at temperatures of
50-90?C, similar to what might be expected close to the
hydrothermal vents of an oceanic volcano, and would not support the
activity of conventional protein enzymes. The chemical products
were separated and analyzed by liquid chromatography tandem mass
spectrometry.
Some of the observed reactions could also take place in water
but were accelerated by the presence of metals that served as
catalysts. "In the presence of iron and other compounds found in
the oceanic sediments, 29 metabolism-like chemical reactions were
observed, including those that produce some of the essential
chemicals of metabolism, for example precursors to the building
blocks of proteins or RNA," said Dr Ralser.
"These results indicate that the basic architecture of the
modern metabolic network could have originated from the chemical
and physical constraints that existed on the prebiotic Earth."
How the first enzymes adopted the metal-catalyzed reactions
described by the scientists remains to be established.
The paper, Non-enzymatic glycolysis and
pentose phosphate pathway-like reactions in a plausible Archean
ocean, is published in Molecular systems
Biology.