Re: Lipids First!

Perplexed in Peoria wrote:
"Tom Hendricks" <tomhendricks474@xxxxxx> wrote in message news:dun7dh$ccs$1@xxxxxxxxxxxxxxxxxxxxxx
My chemistry is less than zero,

So do something about that.

Well perhaps less than zero was an understatement - and I agree
about textbooks - I've read many, and continue to read heavily in
all aspects.

but an intriguing idea is Iron catalysed UV photolysis. It could be an
energy source if the chemistry is there to capture the energy.
Otherwise, it probably destroys organic materials.

Yep! I pretty much guarantee to you that the first biological use
of solar energy involved absorption of uv by ferrous iron. And there
are 'molecular fossils' of this process in modern day life that
back up my claim. I refer to the heme iron of cytochrome C and the
non-heme iron of the iron-sulfur clusters which are still in use
in modern electron transport chains. There is no reason why evolution
would have selected these particular kinds of electron carriers unless
they were being preserved from primordial iron photochemistry.

De Duve mentions this in his book Vital Dust, he talks about
a process where a UV photon removes hydrogen from water
(with no intricate catalyst system needed) by exciting ferrous ions
which relinquish an electron and changes to the ferric form Fe3+ which
carries 3 positive charges.
The electron combines with a proton to give rise to hydrogen atom.
Electrons are transferred from ferrous iron, the donor, to protons, the
acceptor.

Right. But lets look at what is involved in making biological use of
this. (If you want to learn some chemistry, lets start now.)

Ferrous atoms just floating around in the soup absorb uv and give rise
to hydrogen atoms (not hydrogen molecules!). This is a single-electron
redox process. The hydrogen atom is a free-radical which is very reactive.
If it happens to run into some pre-biotic organic material, the results
will almost certainly be destructive. The reason is that the hydrogen
atom free radical is a single-electron source of reducing power. But
organics need two-electron reductions. Unless two hydrogen atoms happen
to bump into the same organic molecule at the right place at the right
time, the result will almost certainly not be what we want.

The best we can reasonably hope for is that this hydrogen atom free radical
runs into another hydrogen atom free radical and you end up with molecular
H2. The energy that came in the uv photons is almost completely wasted,
but now the hydrogen molecule constitutes a two-electron source, which
is just what we need. The only problem is that there is no obvious way
to make use of this reducing power - hydrogen molecules are not easy
things for organics to 'grab hold of'. And, besides, in an OOL scenario,
hydrogen molecules are probably not in short supply anyways.

But what if the ferrous atom is not just floating around? What if it
is already associated with biological material in some way. Now, it
is conceivable that the biology can make use of both the energy content
of the uv photon(s) and the reducing power of the Fe++ -> Fe+++
transition.

There are four serious problems that must be solved to make this work.
1. The organic molecule that is to be reduced must already be in contact
with the iron when the uv photon strikes.
2. We still need to have a two-electron reduction, so our molecule
needs to be in contact with two iron atoms.
3. We are not going to get both photons to hit at the same time, so
we need a way to store the first electron (and as much as possible
of the energy) in a safe place until we get the second photon.

You mean like a UV absorbing nucleotide?

4. Recycling and/or waste disposal. We started with oxidized organics
plus some (biological?) machinery in contact with reduced iron.
Two photons later, we have reduced organics plus machinery in
contact with oxidized iron. How do we restore the status quo ante,
so that we can reuse our machinery, and possibly reuse the iron?

I claim that lipid-world organisms solved all four problems, that no
other approach to OOL can even begin to solve these problems, and
furthermore, that lipid world organisms evolved through several different
ways of solving the four problems, all before the first nucleic acids and
proteins appeared.

Solution #1. Lipids adsorbed on the surface of FeS, hematite, or metallic
iron-nickel minerals. For our first solution, we let the mineral provide
the storage machinery. Our organics (fatty acids, say) are adsorbed
on the mineral surface. They are reduced (to aldehydes, say) and then
move away from contact with the surface, though still trapped by their
hydrophobic tails in the membrane or half-membrane. Assuming that the
mineral can conduct electricity, the two photon strikes can be some
distance apart, and still channel both electrons to the same lipid
molecule. We can recycle some of the ferric iron in a way that deDuve
would approve by reacting the aldehyde lipid with a sulfhydryl lipid
to form a thiohemiacetal which then reacts with the ferric iron to
produce a high-energy thioester. Or, we can bury some of the ferric
ions that we produced on the surface under some fresh ferrous ions
from solution - thus depositing hematite. At this stage of life's
existence, we don't necessarily have a lipid bilayer. A monolayer,
with the hydrophobic tails exposed to air would also work.

Solution #2. Lipid vesicles with 4Fe-4S clusters held at the surface
by sulfhydryl lipids. Same basic machinery for storing energy and
electrons between the first and second photon strike. But now the
machinery can be called biological, and the dynamics of lipid metabolism
must be well tuned (by lipid-world natural selection) to create these
clusters from ambient Fe++ and H2S. And we definitely need to recycle
the clusters. So we need to have the clusters participate in both
reductions of lipids (with a photoassist to provide the energy) and
oxidations of lipids (with no photoassist). Probably, there are some
lipid 'flips' between bilayers to be considered. A transmembrane
potential involving pH, explicit electrical voltage, and various ion
gradients will be produced inevitably. And natural selection will favor
those lipid composition patterns that can maintain homeostatic growth
while making use of the transmembrane potential.

Solution #3. Quinones and porphyrins. If lipids can be produced whose
head groups contain just the right pattern of amine, carbonyl, and
hydroxyl groups, the lipid head groups can condense into heterocyclic
rings which can then be easily oxidized to aromatics by recycling Fe+++
to Fe++. These lipid-attached rings can chelate iron to serve as antenna
complexes for the FeS clusters (porphyrins) or, as quinones, they can
enhance and simplify the machinery for storing energy and electrons
between photon strikes. In the early pre-protein world, there is only
one place where free radicals are not destructive - within the bulk
hydrophobic phase of a saturated lipid membrane (quinones). In addition,
the porphyrins may be used to cycle iron ions and reducing equivalents
between inner and outer leaflets.

All of these proposals could use additional detail. However, at the
present time, I don't have more detail to give. There is only one
thing I am relatively sure of. Autotrophy. If early life was making
use of uv light, it was doing so directly, and not by means of
intermediates like molecular hydrogen, formaldehyde, or Miller-Urey
racemates.

My problem is not with the details as such. If I accept that all this
can work - and I will for now - I would also accept that it can go
wrong or not work
somewhere along the line.
..
What driving force gives it the edge?
In my scenario the sun always forces each step. And each step results
in reactions that are stable (if not they would be destroyed and that
process would end - start again).

If you do not have the stability at every step OR if you don't have
that sun forced energy - what drives this, without misshap, in your
scenario?

Remember as de Duve says (though not directed at the origin here)
There was no goal ... beckoning from the distance future, inviting ...
to overcome hurdles and vanquish difficulties. Every step of this
extraordinary voyage was taken in its own present context,
.... that happened to confer an immediate benefit favoring the survival
and proliferation ... there and then."


You (Tom) seem to consider me to be a "ventist".

No - I really didn't think you had committed as of yet.


Well, it is true that
I think that Wachtershauser's pyrite idea is a good place for the
first lipid membranes to arise. But once you have the first membranes,
the slogan becomes "Every membrane from a membrane", and those membranes
are going to spread to any location on Earth where they can make a
living. Including the surface.


.

Relevant Pages

  • Re: Lipids First!
    ... of solar energy involved absorption of uv by ferrous iron. ... in modern electron transport chains. ... to bump into the same organic molecule at the right place at the right ... that lipid world organisms evolved through several different ...
    (sci.bio.evolution)
  • Re: Eggs and blood cholesterol
    ... > pubmed.com, and search for terms and phrases such as oxysterols, lipid ... say that lipid peroxidation is the problem is silly. ... lipid peroxidation that starts it off but the iron. ... with oxidative damage. ...
    (sci.med.nutrition)
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