A Possible Prebiotic Ancestry of Porphyrin-Type Protein Cofactors

Area of Science:

• Astrobiology and Geochemistry
• Origin of Life studies
• Prebiotic Chemistry

Background:

• Previous work demonstrated abiotic formation of hydrophobic porphyrins under simulated primordial volcanic island conditions.
• The role of metal ion sources in metalating these abiotically formed porphyrins remained an open question for understanding early biochemical pathways.

Purpose of the Study:

• To investigate the metalation of water-insoluble octaethylporphyrin (H₂oep) by prebiotically plausible metal ion sources.
• To simulate fluctuating wet-dry conditions in prebiotic rock pools to assess metalation efficiency.

Main Methods:

• Experiments used octaethylporphyrin (H₂oep) as a model compound under a nitrogen atmosphere.
• Cyclic wet-dry conditions were employed to mimic prebiotic rock pool environments.
• Metalation yields were quantified using various soluble metal salts (Mg, Fe, Co, Ni, Cu) and insoluble minerals (basalt, jaipurite, covellite) in freshwater, artificial seawater, and acidic media.

Main Results:

• Significant yields (20-78%) of metalloporphyrins were obtained from soluble metal salts (Fe, Co, Ni, Cu) and even insoluble minerals like basalt, jaipurite, and covellite.
• Metalation efficiency was influenced by the medium, with higher yields in artificial seawater for some iron sources and optimal yields in acidic conditions (pH 2.1) for iron sources, including iron meteorites.
• Magnesium porphyrin formation was unsuccessful in acidic media and unstable in the presence of Fe²⁺, questioning its prebiotic availability as a chlorophyll precursor.

Conclusions:

• Abiotic metalation of porphyrins is feasible under simulated early Earth volcanic island conditions, producing metalloporphyrins.
• Ancestors of essential cofactors like heme (Fe), B₁₂ (Co), and F₄₃₀ (Ni) could have been available for protometabolisms.
• The limited formation and stability of magnesium porphyrins suggest alternative pathways for early biological development, potentially bypassing chlorophyll-like precursors.