In 1996, an article was published in
Nature in which David Lee reports to have found a self replicating peptide.1
The title of the article is appropriately “A Self Replicating Peptide.”
Unfortunately, the investigator interference required for self replication is perhaps the
most extreme in the history of origins research. This experiment is not even relevant to the
origin of life.
The peptide of interest contains 32 amino acids. The sequence is as follows:
arg-met-lys-gln-lys-glu-glu-lys-val-tyr-glu-lys-lys-ser-lys-val-ala-
cys-leu-glu-tyr-glu-val-ala-arg-leu-lys-lys-leu-val-gly-glu.
The peptide does not self replicate using amino acids. Instead it uses a pool
of two peptides, one is 17 amino acids long and the other is 15 amino acids long. The
amino acid sequences of these two peptides are shown below. Notice that if a peptide bond
forms between ala (last amino acid on right in the peptide with 17 amino acids) and cys
(first amino acid on left in the peptide with 15 amino acids) then a replica of the self
replicating peptide results.
arg-met-lys-gln-lys-glu-glu-lys-val-tyr-glu-lys-lys-ser-lys-val-ala
cys-leu-glu-tyr-glu-val-ala-arg-leu-lys-lys-leu-val-gly-glu
Because the peptide with 32 amino acids facilitates the formation of this
single peptide bond, Lee claims that this peptide can self replicate. But is this really
true? To self replicate, this peptide requires a pool of two peptides. One of these
peptides has the same amino acid sequence as the first 15 amino acids in the self
replicating peptide, and the other has the same amino acid sequence as the next 17 amino
acids. Where do these peptides come from? In this case, they are supplied by the
investigator.
Chapter 9 discussed the difficulties of creating peptide chains under
plausible prebiotic conditions. Due to the difficulties, peptides with more than six amino
acids are expected to be very rare chemicals. Peptides composed of 15 to 17 amino acids
will be much more scarce. Yet to self replicate, this peptide requires an abundant supply
of both, and not just any peptide will do. One of these peptides must be identical to the
first half of the self replicator, and the other peptide must be identical the second half
of the self replicator.
This last requirement is particularly troublesome. Suppose the self
replicator comes into contact with two random peptide chains. One is 15 amino acids long
and the other is 17. How often will the two smaller peptides be an exact replica of the
self replicator? Answer 1 time in every 4x1041 tries (assuming that every amino
acid has a 1 in 20 chance of occurring at each position). Given the low concentration of
peptides in the primordial soup, the probability for such an encounter is zero.
The interference does not stop here. It is critical that the first amino acid
in the peptide with 15 amino acids be a cysteine. Cysteine has chemical properties that
facilitate peptide bond formation, and to make sure that the interference sets the record
for the most ever, the alanine (last amino acid on right in the peptide with 17 amino
acids) must be chemically altered to make it much more susceptible to attack by cysteine.
Finally, the self replicating peptide contains eight lysines. Lysine is
instrumental in its self replication as its charge plays a role in aligning the two small
peptides. Lysine is one of the amino acids that has yet to be synthesized under plausible
prebiotic conditions. So even if lysine was present in the soup, its concentration would
have been negligible.
Every possible strategy of interference is employed by this investigator to
promote replication. This mixture of peptides has almost no chance of existing on the
primitive earth. Even if it did, as soon as the supply of 15 and 17 amino acid peptides
runs out, the replication stops. Despite all of this interference, the claim of self
replication is not valid. Self replication involves a system that can duplicate all of its
components. In this system, the self replicating peptide is supplied with one peptide
containing 15 amino acids and one with 17 amino acids. A true self replicating molecule
could generate these two smaller peptides from the amino acids in the primordial soup.
The authors of this paper tried to use dynamite to blow up the door in figure
9.4, but the door withstood the blast and did not open. So the authors just said that it
was open.
Proteins do not self replicate, and this explains why most scientists
rejected the self replicating protein hypothesis in favor of the self replicating RNA
hypothesis.
Next: RNA self-replication
Previous: Origin of Life - Self Replication
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