Posted: Feb 26, 2019 3:49 pm
Remnants of an Ancient Metabolism without Phosphate: Cell - proposes a predecessor to the RNA world: what might be called a sulfur-metabolism world.
The RNA world successfully accounts for several molecular-level features of our biota:
So the authors decided to consider what metabolic pathways do not use phosphate, and they had a surprisingly impressive haul. They found the Krebs cycle (citric-acid cycle, tricarboxylic-acid cycle), biosynthesis pathways for several protein-forming amino acids, and several others.
Some of these reactions use thioesters. A thioester is
R1 - (C = O) - S - R2
where R1 and R2 are the rest of the molecule. Note the sulfur instead of one of the oxygens. A notable thioester is acetyl coenzyme A, an important metabolic intermediate:
CH3 - (C = O) - S - CoA
Another notable feature of these pathways is that they have a lot of enzymes with iron-sulfur groups and metal ions in them, especially zinc ions. This sort of feature is often considered a relic of prebiotic origins, from mineral catalysts causing reactions that were later taken over by enzymes.
All this suggests that the RNA world had a predecessor, a predecessor likely with its own kind of replicator molecule. This replicator could be something like peptide nucleic acid (PNA), but whatever it was, it became displaced by RNA. But this replicator could have been what origin-of-life researchers are looking for: a replicator without a predecessor replicator.
The RNA world successfully accounts for several molecular-level features of our biota:
- DNA as modified RNA
- Protein-assembly apparatus: messenger, transfer, and ribosomal RNA's
- RNA enzymes (ribozymes)
- RNA cofactors (ATP, B vitamins, etc.)
So the authors decided to consider what metabolic pathways do not use phosphate, and they had a surprisingly impressive haul. They found the Krebs cycle (citric-acid cycle, tricarboxylic-acid cycle), biosynthesis pathways for several protein-forming amino acids, and several others.
Some of these reactions use thioesters. A thioester is
R1 - (C = O) - S - R2
where R1 and R2 are the rest of the molecule. Note the sulfur instead of one of the oxygens. A notable thioester is acetyl coenzyme A, an important metabolic intermediate:
CH3 - (C = O) - S - CoA
Another notable feature of these pathways is that they have a lot of enzymes with iron-sulfur groups and metal ions in them, especially zinc ions. This sort of feature is often considered a relic of prebiotic origins, from mineral catalysts causing reactions that were later taken over by enzymes.
All this suggests that the RNA world had a predecessor, a predecessor likely with its own kind of replicator molecule. This replicator could be something like peptide nucleic acid (PNA), but whatever it was, it became displaced by RNA. But this replicator could have been what origin-of-life researchers are looking for: a replicator without a predecessor replicator.