27º Meeting of the Israel Society for Astrobiology and the Study of the Origins of Life Planetary evolution and biochemical adaptations: between contingency and determinism? Antonio Lazcano Facultad de Ciencias, UNAM MEXICO E-mail: [email protected]Weizmann Institute 2013
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27º Meeting of the Israel Society for Astrobiology and the Study of the Origins of Life Planetary evolution and biochemical adaptations: between contingency.
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27º Meeting of the Israel Society for Astrobiology and the Study of the Origins of Life
Planetary evolution and biochemical adaptations: between contingency and determinism?
At the time being, molecular phylogenies cannot be extrapolated to an evolutionary period prior to the emergence of ribosome-mediated protein synthesis.
There is nothing in molecular cladistics that supports the possibility of ancestral life with replicative and metabolic abilities based solely on RNA molecules.
Evidence for the RNA World comes from the catalytic versatility of ribozymes and is supported by the biological ubiquity of RNA and ribonucleotides. For instance, rRNA-mediated peptide bond formation is a strong indication that protein synthesis first evolved in an RNA world.
Moles (relative to glycine = 1) of the various amino acids detected in the volcanic apparatus vials. Amino acids underlined have not
been previously reported in spark discharge experiments.
Relative aa abundances (gly = 1) for the linear α-aa detected in the target ice sample no. 1 (grey) and target ice sample no. 2 (white)
Martins et al, 2013. Nature Geoscience
Synthesis of monomers under possible prebiotic conditions produce appreciable yields under a wide range of environmental settings*
Miller & Lazcano, 2002
H3C
CH3 NH2
OH
O
Leucine (Leu)
H3C
NH2
OH
O
Norvaline (Nva)
H3C
CH3
NH2
OH
O
Valine (Val)
H3CNH2
OH
O
Methionine (Met)
S
H3C
CH3
NH2
OH
O
Isoleucine (Ile)
H3CH3C NH2
OH
O
Isovaline (Iva)
Why do modern organisms use a small subset of the L -amino acids which are
structurally possible?
“The absence of -amino-n-butyric acid, norvaline and norleucine is most striking and a major challenge to any attempt to account for the selection of the twenty protein amino acids. Their abundance in prebiotic experiments and carbonaceous chondrites is comparable to or greater than the hydrophobic amino acids used in proteins”.
Weber & Miller (1981) Jour. Mol. Evol. 17: 273
Apostol et al. (1997) J. Biol. Chem. 272: 28980
Both norvaline (Nva) and norleucine (Nle) are incorporated in vivo in recombinant hemoglobin in E.coli
Propensity of a residue to be catalytic in each of the six classes of enzymes
Harold White’s hypothesis: is histidine an evolutionary remmant of a catalytic ribonucleotide?
prebiotic syntheses
lipidsnoncanonical basesimidazolidesracemic sugarsracemic nonprotein & (some) protein amino acids
catalytic RNA amino acids
lipids
cells with RNA,proteins & DNA
time
synthetic efficiency, physical stability, chemical natural selectionreactivity, self-assembly, self-organization
CONCLUSIONS….
Conclusions
1. There is very good evidence for a primitive soup, but the transition from it to an RNA World remains an open question;
2. Experimental simulations of prebiotic environments, analysis of meteorites and stability arguments suggest that RNA was preceded by a complex mixture of genetic polymers with different backbones and nucleobases;
3. The presence of twenty protein amino acids in extant biology does not means that they were essential for the origin of life nor that all of them were synthesized prebiotically; and
4. The key role of imidazolides in prebiotic chemistry and in biological catalysis suggests a connection between the chemical constituents of the RNA World and the emergence of the first proteins.