"irr"  wrote in message
news:c8ol4d$2uj4$1{at}darwin.ediacara.org...
[...]
> There are a difficulties in using H2O as a reductant, that make
> photosynthetic water oxidation one of the most remarkable reactions in all
> of biochemistry.  To oxidize H2O through one electron steps requires an
> enormous initial energy -- around 2.75V -- that is roughly 2.5x more
energy
> than can be generated by chlorophyll oxidation (IIRC, photooxidized
> chlorophyll is the strongest oxidant known in biology).  On the other
hand,
> you can oxidize H2O to O2 with a paltry .82V, but the problem is this is a
> four electron process.  So the really amazing thing about oxygenic
> photosynthesis (or the so-called oxygen evolving complex) is this ability
to
> function as a capacitor, to turn four single-electron chlorophyll
> photooxidations into a concerted four electron "dump" onto H2O.
>
> There are some ideas on how two, two-electron transfer systems (manganese
> catalase, for example) might have come together to form an evolutionary
> precursor, but I'm not sure what the current state of things here is.
Maybe
> someone else can provide more insight on this question?
[...]

I don't have any facts, and probably not much insight, but I can offer
some wild-ass speculation.

IIRC, there is a UV reaction that goes like this:
(1)   2Fe+++  +  2OH-  +  UV photon  -->  2Fe++  +  HOOH
Earlier in this discussion, we speculated this reaction:
(2)  2Fe++  +  R  +  2H+  +  photon  -->  2Fe+++  +  RH2
You mentioned a catalase-like reaction:
(3)  HOOH  +  R'  -->  O2  +  R'H2
Add them all together and you get:
(4)  2H2O  +  R  + R'  +  photon  + UV photon  -->
          RH2  +  R'H2  +  O2

My speculation is that, before there was an ozone layer, there was
an organism that did oxygenic photosynthesis using (4).  The mixed
Fe++ and Fe+++ hydroxide slush served as both pigment and as a
cell wall.  Of course, much of the peroxide was lost, but the part
that did difuse through the membrane was utilized in (3).  But,
eventually, there was no more UV, so other, more efficient pigments
had to be substituted to replace the Fe+++ in (1).  At that point
we have a photolithoautotrophic iron oxidizer.  Still later, when the
Fe++ in the ocean was depleted,  (2) was replaced as well.  So,
in this speculation, the catalase function may be older that either
of the two chlorophyll photosystems.
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