26. Origin of Life on Earth   Previous PageNext Page
       Coacervate Drops and "Protobionts"

Even more interesting are coacervates prepared with enzymes inside. These can absorb substrate molecules from solution, catalyze chemical reactions, and let the products diffuse out.

If coacervates containing the enzyme phosphorylase are prepared, and glucose-1-phosphate is added to the bulk solution, the primed glucose molecules will diffuse into the coacervate droplets and be polymerized there into a starch polymer.

If the coacervates also contain the enzyme amylase, then the starch produced by the first enzyme is chopped back to disaccharide molecules of maltose, which diffuse into the bulk solution again. Coacervates with these two enzymes are miniature factories for turning glucose-1-phosphate into maltose, using the energy of the phosphate bond in the starting molecules.

In another experiment, coacervates were prepared that contained mitochondrial NADH dehydrogenase, the flavoprotein enzyme found at the beginning of the respiratory chain. These droplets could absorb NADH and a reducible dye from the solution, reduce the dye molecules, and release dye and NAD+ back into solution.

In the most spectacular model experiment of all, coacervate drops containing chlorophyll were allowed to absorb ascorbic acid and an oxidized dye that could not be reduced spontaneously by ascorbate alone.

When the droplets were kept in the dark, nothing happened; but when they were illuminated by light the dye became reduced. In a very close parallel to the single-center photosynthesis of bacteria, chlorophyll molecules absorbed light energy, and used their excited electrons to reduce the dye molecules.

Ascorbate merely played the role of H2S in the electron-transfer chain by providing the electrons required to restore the electron deficit in the chlorophyll molecules. Reduction occurred with a net increase in free energy, the increased energy coming from the absorbed light.


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