Re: Metabolism first?


"Chrisantha" <ctf20@xxxxxxxxxxxx> wrote in message news:eqsprn$17dh$1@xxxxxxxxxxxxxxxxxxxxxx
Please take a look at our papers on the origin of metabolism in
compartments and tell us your views.

Szathmary, E, Santos, M, Fernando, C. (2005) Evolutionary Potential
and Requirements for Minimal Protocells. Topics in Current Chemistry
(2005). 23rd August. DOI 10.1007/tcc001 Springr-Verlag Berlin
Heidelberg. Download pdf.

and

Fernando, C., and Rowe, J. (2006) Natural Selection in Chemical
Evolution. (To appear in Journal of Theoretical Biology, accepted
January 2007). See supplementary material here. Submitted paper here


available at

http://www.cogs.susx.ac.uk/users/ctf20/dphil_2005/publications.htm


Thanks for the links. I will comment on the Szathmary review article
here, and on the Fernando and Rowe article in a later, separate posting.

It is a good review of the 'minimal cell' literature. If I were to
make a general complaint, it would be that it is too committed to the
'chemoton' model. Ganti showed lack of imagination in insisting that
metabolism takes place _within_ the enclosed aquaeous medium. Instead,
he should have simplified his system further by hypothesizing that
the metabolism takes place in the membrane itself. In fact, there are
two symbiotic metabolic systems here - one on the inner leaflet and
one on the outer. A 'minimal life' system consists simply of an
autocatalytic lipid bilayer, or even a mono-layer (oil slick). Ideas
like those of Deamer, that such a system could grow heterotrophically
(by assimilating pre-formed lipid molecules from the environment) are
just silly. Research should focus on identifying a lipid mediated
metabolism for making more lipid (from CO2, CO, or HCN and a source
of reducing power. The enclosed 'cytoplasm' should not be seen as
a store of organic metabolites, but rather as a pool of H+, NH4+, HCO3-,
HS-, and formate at different concentrations on the inside than on the
outside. All the interesting reactions take place on the two hydrophilic
surfaces of the membrane, catalyzed by the membrane molecules themselves.
At least, that is my opinion.

One more detailed comment: The review accepts uncritically Eigen's
assertion that there is an inherent limit on RNA replication fidelity
at around 1 error per 100 bases - in the absense of protein catalysts.
This is simply not so. In fact, there is no tangible limit to the
accuracies which can be achieved, even by simple catalysts, if you are
willing to waste some energy. The mechanism is known as 'kinetic
proofreading'. It involves the kinetic balance between two competing
chemical reactions - a productive one producing template-directed
polymerization from activated monomers; and a destructive one (exonuclease)
degrading polymers to non-activated monomers. Both processes take place
simultaneously. It is assumed that there is some slight preference for
the polymerization process to add the right nucleotide over the wrong one.
It is also assumed that the hydrolytic exonuclease reaction slightly
favors the removal of mispaired monomers. Finally, it is assumed that
the two processes are in a close balance with a slight bias toward net
polymerization. Under these circumstances, replication fidelity can
be arbitrarily high, even with simple catalysts - as long as you are
willing to pay the energy cost of repeatedly reactivating the monomers
which were degraded by the futile cycle. Therefore, I consider your
long review of proposed mechanisms to avert the 'error catastrophe' as
something of a 'red herring'.


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Relevant Pages

  • Re: Clay
    ... "cells with their genes on the ... Organisms needing monomers to reproduce, ... The container IS the contents. ... to invent metabolism or for metabolism to invent genes. ...
    (talk.origins)
  • Re: Clay
    ... "cells with their genes on the ... Organisms needing monomers to reproduce, ... somehow have containment without the usual kinds of containers. ... to invent metabolism or for metabolism to invent genes. ...
    (talk.origins)
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