TABLE 7.1 Enantiomeric Enrichments for Amino Acids in the Murchison and Murray Meteorites

 

 

Enantiomeric

Enrichment (%)

Compound

On Eartha

Murchison

Murray

2-Amino-2,3-dimethyl-pentanoic acid

2S,3S/2R,3R

Unknown

7.6

1.0

2S,3S/2R,3S

Unknown

9.2

2.2

α-Methylnorleucine

Unknown

4.4

1.8

α-Methylnorvaline

Unknown

2.8

1.4

α-Methylvaline

Unknown

2.8

1.0

Isovaline

Rare

8.4

6.0

Norvaline

Rare

0.4

0.8

α-amino-n-butyric acid

Common

0.4

−0.4

Valine

Ubiquitous

2.2

−0.4

Alanine

Ubiquitous

1.2

0.4

aNatural abundance of the amino acid in Earth’s biosphere.

SOURCE: Data from Pizzarello, S., and Cronin, J.R. 2000. Non-racemic amino acids in the Murray and Murchison meteorites. Geochim. Cosmochim. Acta 64:329-338.

enriched (Table 7.1). Especially notable were enantiomeric enrichments of 7 to 9 percent in three specific amino acids not known in forms of life currently on Earth, thus making contamination from terrestrial sources unlikely.

If the amino acids in Murchison were not generated by living processes, it is possible that enantiomeric enrichment can be achieved by processes that are independent of life. Several such processes are known in the laboratory, some of which involve crystallization, or autocatalysis, and/or some multiple steps requiring human intervention. It is conceivable that some of these processes involve selective degradation of one of the two enantiomers. This weighs against the usual thinking that enantiomeric enrichment is a unique signature of life.

However, the data on enantiomeric excess are also consistent with a biotic origin, The larger asteroids may have stayed clement inside for several hundreds of millions of years, raising the possibility that life, or some borderline form, emerged there. This would be the case if the Murchison meteorite originated in an asteroid (or a comet) that supported life based on α-methyl amino acids that preferred one enantiomer to the other. While few would argue so, this result is consistent with life having emerged in the asteroid belt.

7.2
THERMODYNAMIC RELATION OF METABOLIC INTERMEDIATES AS A BIOSIGNATURE

For a metabolic sequence to convert substrates into products, the system in which they occur must not be at equilibrium. When they are in equilibrium, the overall reaction must be coupled to another reaction where substrates and products are at disequilibrium. This is frequently phrased as a requirement that a metabolic pathway be energetically “downhill,” or coupled to an energy source.

Many terran metabolic multistep pathways are run close to equilibrium for internal steps (Figure 7.1), with the first step being energetically downhill and a site of regulation. Required, however, is that the last step be energetically downhill, thereby pulling the reaction to completion.1 This feature may be universal in metabolic pathways simply because it exploits most economically a surrounding chemical disequilibrium. Having large drops in free energy at every step in a pathway is wasteful.

This characteristic energetic relationship between a set of compounds that are intermediates in an evolved metabolism may be a universal biosignature. If an inventory of the small molecules in a suspected living system (on Titan, for example) reveals this characteristic energetic relationship, this may be evidence of Darwinian evolution acting to create an optimal metabolism.



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